Document eKQKp8oZjEq5r0yxz1G86qw4

Lead concentration in city air increases WATER, AIR & WASTE Like many other air pollution issues, the facts about leaded gasoline and its impact on air and public health have been little known and are often contradictory when they are known. Some light was shed on an otherwise cloudy subject, however, when Dr. Tsaihwa J. Chow told participants that he, John L. Earl, Carrie Snyder, and coworkers at Scripps Institution of Oceanography and University of California San Diego, La Jolla, Calif., have isotopically identified leaded gasoline as the overwhelming con tributor of lead pollutants to the atmosphere. This conclusion stems from Dr. Chow's three-year study of lead con centrations in the San Diego, Calif., area sponsored by the U.S. Public Health Service. Other conclusions: In the San Diego region, the at mospheric lead concentrations show an annual cycle, with a winter high and a summer low. A long-term increasing trend of lead content in the air is observed as compared with 1957 and 1965 data. Geographically there is a logarith mic increase of atmospheric lead con tent from midocean to remote high mountains and from seashore to subur ban and urban locations. During the temperature inversion phenomenon, lead content of down town San Diego air approaches 8 mi crograms per cubic meter of air, nearly that of the tentative national air qual ity limit. Lead aerosols constitute as much as 7% of total suspended particulate material in the air. Midpacific marine air with lead concentration of 0.001 micrograms per cubic meter is the least polluted of northern temperate atmospheres. Sampling. Dr. Chow's team has collected air samples at four stations representing various environmental conditions in the San Diego area. The Mount Laguna station, elevation 6100 feet, located at the Astronomical Ob servatory on the crest of the peninsu lar range, is remote from any industrial sources of pollution. The San Diego metropolitan area is about 45 miles west of this site. Automotive traffic within a half mile of the station is less than a dozen cars per week. Lead concentration at this location, Dr. Chow finds, is uniformly low throughout the year with an average of 0.05 micro grams per cubic meter. n C&EN MARCH 9, 1970 Lead was introduced into gasoline to raise its octane rating 47 years ago. Each gallon of today's gasoline has about 4 grams of lead in it, which adds up to 700 million pounds of lead consumed each year by gasoline sales. While there is not hard agree ment about how much lead gets exhausted into the air, most experts say that about 70% of the lead in gasoline comes out of a car's tailpipe. A conservative estimate is that half the exhausted lead gets airborne, meaning that at least 250 million pounds of lead swirl into the air every year from automobile ex hausts. Scripps Institution's sampling site is located at the end of an observation pier which extends 1000 feet into the Pacific Ocean. This site shows rela tively low lead content, about 0.4 micrograms per cubic meter, especially during summer months. The Mission Valley station is about 10 miles inland from the ocean in a broad flat-floored valley transecting the heavily populated San Diego mesa. The valley floor is being increasingly developed with major shopping cen ters, hotels, and sports facilities. Sev eral major freeways and highways either follow the valley or transect it. Traffic flow in the vicinity of the sta tion is about 175,000 cars per day. Not surprisingly, lead content in this area has been increasing over the years of the study, with a 1969 mean of more than 2 micrograms per cubic meter. The downtown San Diego station is in a commercial area; however, traffic flow downtown has not increased as fast as in most other parts of the city. Dr. Chow concludes, based on three years of sampling in this area and on some earlier work by Bureau of Air Sanitation and National Air Sampling Network, that lead concentration ap pears to increasing at a rate of about 5% a year. Filters. Sampling operations in volve continuous pumping of air through filters for periods of seven days. Air sampling gear consists of vacuum pumps which draw air through MF-Millipore cellulose ester membranes, type HAWP, with a 0.45micron mean pore size and a retention efficiency of more than 95% for 0.05micron particles. The filter holder is made of ano dized aluminum with a gold plated screen and a clamp ring to support the filter. Filter holders are housed in inverted polyethylene cups for pro tection from rain, dust fall, and sea spray and are suspended from wooden poles extending from the roof of the building. Dr. Chow's group dissolves the ex posed filters in concentrated nitric and perchloric acids, spiked with lead-206 tracer. The team isolates lead by standard ion exchange techniques, purifies it by several dithizone extrac tions, and then converts it to a sulfide. It then analyzes the samples by mass spectrometry to determine lead iso tope ratios. This technique is known as isotopic dilution. From thousands of air samples col lected over more than three years the California group is able to determine seasonal trends of weekly lead con centrations at San Diego, Mission Val ley, and Scripps Institution. It finds that atmospheric lead concentrations show an annual cycle with a winter high, summer low. Dr. Chow attrib utes this pattern of seasonal variation to local meterological conditions. Summer months, explains Dr. Chow, in southern California are usually John Earl (left) and Dr. Chow analyze lead aerosols by mass spectrometry DSW 552112 STLCOPCB4090529 Dr. Story (left) and Dr. Busch analyze products of peroxide fragmentation fraction of the cost of present methods, Dr. Story says. Mixed peroxides. The most im portant new development since Dr. Story's original 1968 publication of his procedure is his discovery of methods of generating mixed trimeric peroxides in good yield. The decomposition of symmetrical dimeric and trimeric per oxides was previously accomplished by the Georgia scientist. But to make the method truly general and extend it to synthesis of all size rings, substituted and unsubstituted, it is necessary to synthesize unsymmetrical peroxides. The key intermediate in the synthe sis of mixed trimeric peroxides is 1,1'dihydroperoxydicyclohexyl peroxide, prepared by a new method giving quantitative yields. In a typical reac tion, this dihydroperoxide is stirred with cyclopentanone and anhydrous cupric sulfate at room temperature for 13 days. Addition of a large excess of water and recrystallization of the crude product from methanol give a 58% yield of the 6-6-5 trimeric peroxide. Decomposition of the mixed 6-6-5 trimer by refluxing in decane yields cyclotetradecane and 15-pentadecanolide, in 17 and 22% yields, respec tively. These rings are not otherwise available, except through cyclooctanone diperoxide--which is obtainable only in low yield. Many mixed tricycloalkylidene per oxides have been synthesized and con verted to the corresponding macrocyclic compounds. The 12-12-7 trimeric peroxide gives a C28 hydrocarbon ring and a C29 lactone on thermal decom position, in 30% and 10 to 15% yields, respectively. Some of the macrocyclic compounds obtainable from the mixed peroxides cannot be synthesized from any readily available symmetrical per oxide. The utility of the method depends considerably on the availability of the dihydroperoxide. Dr. Story and Dr. Busch have recently developed a new procedure for preparing it in very high yield. Typically, it is synthesized by stirring for five hours, at room tempera ture in an open crystallization dish, a mixture of 90% hydrogen peroxide and cyclohexanone in 2:1 molar propor tions, together with several drops of 70% perchloric acid catalyst in aceto nitrile. The mixture becomes a crys talline mass, is washed with water, and recrystallized from hexane, giving a quantitative yield of 94% pine dihy droperoxide. Besides the 6-6 dihydro peroxide, Dr. Story has also prepared 5-5, 7-7, 8-8, 12-12, and other perox ides in good yield. Substituted macrocyclics. Perhaps even more important than other appli cations, monosubstituted macrocyclic compounds can be synthesized by the ketone peroxide fragmentation method. For example, methoxycyclopentadecane can be prepared in 35% yield, by reacting l,l'-dihydroperoxydicyclohexyl peroxide with 4-methoxycyclohexanone for two to three hours at -- 10 C. in propionic acid solvent, us ing a perchloric acid catalyst, and then decomposing the methoxytricyclohexylidene peroxide product. Monosubstituted macrocyclic com pounds have been prepared with alkyl, aryl, acetoxy, benzoyloxy, halogen, and amido substituents. The decomposition of the ketone peroxides is hypothesized by Dr. Story to proceed through homolysis of an oxygen-oxygen bond. The cleavage should be analogous to that of alkyl peroxides and ozonides, he believes, with a double yS scission following ho molysis of the oxygen-oxygen bond, producing alkyl radicals--which un dergo a very efficient cage recombina tion to form a new carbon-carbon bond. An intermediate acyl peroxide is formed, and the cyclic hydrocarbon results from its homolytic decomposi tion, loss of carbon dioxide, and cage recombination of die resulting alkyl radicals. Preliminary data obtained on the kinetics, solvent effects, and activ ity parameters of the reaction are in support of this scheme. The ketone peroxide fragmentation procedure already constitutes the most generally useful synthesis of macrocy clic systems. Dr. Story and his col leagues are currently seeking to extend the utility of the method to synthesis of very large ring systems--up to Cioo> if possible--opening the door to study of many interesting conformational problems, and possible use of C30 to C40 rings as selective ion-chelating agents. They are also planning to ap ply their techniques to synthesis of heterocyclic ring systems and macrolide antibiotics. A further possible ap plication is preparation of new mono meric systems for polymerization--us ing small or large rings as polymer monomers--yielding elastomeric ma terials which are elastic on a molecu lar basis because of ring rigidity. one of a group of Evans Mercapto Acids... THIOLACTIC ACID (2-Mercaptopropionic Acid) HS-CH-COOH Other Evans Mercapto Acids commercially available include: Thioglycolic Acid (Mer- captoacetic Acid), 3-Mercaptopropionic Acid, Thiomalic Acid (Mercaptosuccinic Acid), Thiosaiicylic Acid (o-Mercaptobenzoic Acid). Inquiries are invited regarding your eom- mericai requirements for other Organic Di-Valent Sulfur Compounds. Data Sheets and Samples available on request. CHemerics, inc 90 Tokeneke Road Darien. Connecticut QM2Q Phone: 203-655-5741 Cable: EVANSCHEM DSW 552113 MARCH 9, 1970 C&EN 41 STLCOPCB4090530 ' Lead aerosol concentrations show seasonal variations .' Yearly Jan.-March April-June. Jufy-Sept Oct.rDec. mean Purity determination by DSC gain in accuracy Concentrations in niicrograms/cubic meter ANALYTICAL 1967 Downtown Mission Valley 3.50 1.16 1.04 2.95 2.16 -- -- 1.07 1.87 -- Careful control of experimental vari ables can greatly improve the accuracy Scripps Pier -- -- 0.27" 0.61 -- of purity determinations with dif 1968 ferential scanning calorimetry (dsc). Downtown Mission Valley Scripps Pier 3.27 1.13 1.33 3.65 2.38 1.76 1.27 1.55 2.94 1.90 0.58 0.24 0.25 0.61 0.42 Using Perkin Elmer's dsc-1b instru ment, Dr. Edward M. Barrall, II, and Richard D. Differ of ibm Research Laboratory, San Jose, Calif., have Laguna Mountain -- -- -- 0.04 -- found that when they properly con 1969 trol all variables, they can obtain an Downtown Mission Valley Scripps Pier 2.57 1.05 1.07 4.05 2.20 2.05 1.48 1.52 3.14 2.06 0.40 0.21 0.23 0.63 0.37 QC/, centration component. Dr. Barrall spoke at a symposium on recent ad vances in thermal analysis. Laguna Mountain 0.01 0.06 0.07 0.05 0.05 In differential thermal analysis B Last six weeks of quarter. (dta), a sample's actual temperature change is measured (during an exo thermic or endothermic reaction) as the sample is being heated or cooled. In dsc, the power required to pre cloudy with overcast mornings along and his coworkers purchased various vent such a temperature change is the coast. Temperatures increase in brands of gasoline from San Diego ser measured. land where skies are clear, creating a vice stations and simultaneously In the operation of a Perkin-Elmer thermal low-pressure region in the in gathered lead aerosol samples. They dsc instrument (c&en, Aug. 18, terior and setting in motion summer also obtained gasoline and aerosol 1969, page 46), an average tempera sea breeze circulation. Therefore, lead samples from other cities. They then ture loop compares the average tem concentration in the inland atmosphere extracted the lead in the gasolines and perature of the sample and reference is lessened by dilution with clean aerosols and determined their isotopic holders to what the linear program marine air. During winter months compositions. They also determined ing unit directs. The loop controls there is little difference between land the isotopic composition of lead in the power to compensate for any de and sea temperature and so sea breezes marine sediments and water. viation the sample and reference are diminished; the reduced wind ve They find that the isotopic composi holders show from the heating pro locity can't effectively dissipate pol tion of lead in gasoline additives and gram. As the temperature of the two lutants, and lead aerosols accumulate in aerosols represents a typical tertiary, holders is programed up or down, in inland air. or older age, mined ore lead which is the program temperature analog is Furthermore, during early mornings less radiogenic than that of quaternary recorded on the x-axis of a strip-chart of winter months inversions develop lead (which typifies the results of recorder. when the wind is light and air layers natural weathering). Marine sedi Meanwhile, a differential tempera above ground are dry, a situation con ments are also quartemary lead. This ture compares the temperature of the tributing to rapid heat losses from the means that lead aerosols couldn't be sample holder and the reference holder ground into the overlying air. Under derived from the surface soils by and proportions power to the heater in these conditions the lowest tempera natural weathering, says Dr. Chow. each holder so that the temperatures ture is found near the ground, and Comparison of the isotopic composi remain equal. When a sample under upward diffusion is inhibited. As a tion of lead in the southern California goes an endothermic transition and result, pollutants are trapped in a aerosols with that of the averages of melts, a differential temperature am limited air space. During such in local gasoline leads shows that the two plifier senses the heat lost by the version phenomena, the Chow group kinds of lead are identical within ex "sample holder and increases power to encountered high atmospheric lead perimental error. This is true for the sample holder to maintain thermal i content, especially at the downtown other cities in the study, says Dr. balance with the reference holder. A station. Weekly average lead concen Chow, indicating that excess lead in signal proportional to the power dif trations as high as 8 micrograms per the aerosols can be attributed only to ference is plotted along the y-axis. In cubic meter have been observed down automotive exhausts. the resulting thermogram, the area town, says Dr. Chow. The tentative What happens to lead aerosols in under the curve measures the heat of national air quality limit for lead is 10 the atmosphere? In other studies. transition directly in calories. micrograms per cubic meter as set Dr. Chow finds that coarse fractions Applications. Purity determina forth last year by the American Indus settle out along roadsides and account tions using dsc have been demon trial Hygiene Association. for high lead concentrations in soil and strated by other workers, Dr. Barrall Isotopes. Dr. Chow stresses that vegetation. Fine lead aerosols are points out. For example, Dr. G. L. each lead ore deposit has its character able to travel long distances and are Driscoll, Dr. I. N. Duling, and Dr. F. istic isotopic composition, which is often washed out of the atmosphere Magnotta of Sun Oil Co., Marcus fixed during mineral genesis, and it is in rain and polar snow. Part of the Hook, Pa., have used the technique this unique chemical property which pollutants has been discharged di to determine the mole fraction of im he uses in identifying the source of in rectly or through runoff into the purity in several synthetic mixtures. dustrial lead pollutants. Dr. Chow oceans. And Dr. N. "J. DeAngelis and Dr. G. DSW 552114 MARCH 9, 1970 C&EN 43 \ STLCOPCB4090531 J. Papariello of Wyeth Laboratories, Randor, Pa., have tested the method on known samples of certain pharma ceutical materials, but found that it has certain limitations. Although the dsc method shows promise in purity determination, little work has been done in examining the systematic variables and optimizing the conditions. Dr. Barrall says. Also, he and Mr. Diller recognize the need to compare analytical results obtained from the dsc method with other es tablished analytical techniques. In a purity determination, the dsc instrument is used to obtain a heat uptake-temperature curve in the heat ing mode. Given the proper condi tions the melting curve has all the information needed to calculate the mole fraction of impurity using the van't Holf equation. The van't Hoff equation, which relates melting point depression to impurity content, can be written as Ts = T0 - RT^X/aII 1/F where T3 is the instantaneous tem perature of the sample in K., T0 is the melting point of the infinitely pure sample (solvent) in K., R is the gas constant (1.987 cal. per mole-K.), AH is the heat of fusion of the sam ple (solvent) in cal. per mole, X is the mole fraction of impurity, and F is the fraction of the total sample melted at Ts. A plot of Ts versus 1/F theoretically gives a straight line with a slope of -- RT^X/aH with an intercept of T0. AH can be obtained from the area under the dsc curve. Ts is measured from the dsc curve. 1/F is the re ciprocal of the partial area of the dsc curve up to Ts divided by the total area of the curve. This particular correction has been recognized by many workers. To use the van't Hoff equation in dsc purity determinations, a chemist must make sure that the impurity is insoluble in the predominant solid phase. Also, - the impurity must be completely soluble (form an ideal so lution) in the molten or liquid phase. The equation can't be used for co crystals (impurity crystallizing in the host lattice), cases where the impurity forms a nonideal solution in the melt by association or chemical reaction, and cases where the impurity is totally insoluble in the liquid phase. Corrections. To get accurate purity determinations with dsc, two cor rections must be made to account for instrumental variables, the ibm chem ists find. The first of these variables is thermal resistance of the instru ment. This causes an error in the instrument's temperature reading. To M C&EN MARCH 9, 1970 Dr. Edward Barrall (left) and Richard Diller use DSC for purity determinations correct for this, they melt a material of very high purity (greater than 99.999%) and record the melting curve. The pure material must melt in or near the range for which it is being used as a correction if the ef fective thermal resistance is to be evaluated completely. The curve's leading edge slope is direcdy propor tional to the thermal resistance. To correct any temperature in a re corded transition it's necessary to superimpose this slope on the curve and extrapolate to the isothermal base line. The standard curve must be ob tained at the same chart speed, sensi tivity, encapsulation, and heating rate as the purity determination of the sample. The second instrumental variable for which a correction must be made is undetected melting. If the dsc is set at a sensitivity low enough to keep the whole record on the chart, the low temperature melting will not be detected by the recorder. This means that area will be lost, and the initial small fraction melted, F, will be too small. This small amount of heat can be recovered by making the Ts versus 1/F plot linear, the ibm pair has shown. They do this by computer trial and error using progressively larger area increments to be added and determining the effect on the plot. The increment added to produce a linear plot is selected for the correla tion. In addition, it's necessary to consider the entire endothermal area up to the vertex. This follows from the van't Hoff equation which de scribes the entropy of solution and mixing in the total liquid phase. It isn't necessary to have some arbitrary cutoff point as thought before. Experimental. In the ibm studies on purity determination using the Perkin-Elmer dsc instrument, the data were reduced with a polar planimeter. The results were calculated with a computed program developed at ibm. In their study. Dr. Barrall and Mr. Diller used samples in which the im purity could be analyzed by an alter nate method. Among them were leadtin, indium-lead, and tin-indium metal mixtures which were analyzed by atomic absorption spectrometry. The scientists studied organic mixtures which were analyzed by ultraviolet ab sorption spectrometry, fluorescence photometry, and thin-layer chroma tography. These analyses were done on the sample that was thermographed. Serious impurity gradients can exist in a bulk sample, they found. To obtain accurate, reproducible results in purity determinations, cer tain experimental precautions must be taken, the ibm chemists emphasize. Care must be taken that the sample doesn't relocate in the cell during fusion and freezing. This alters the thermal resistance between . sample encapsulation and heater, changing the shape of the dsc curve inde- DSW552115 STLCOPCB4090532 pendent of purity. To keep the thermal resistance constant it's also necessary to position the sample pan in the same location in the calorimeter for each run. The temperature of the sample is not a direct but a relative measure ment in the dsc-1b instrument. Thus, it's necessary to calibrate the tem perature axis. This is done with .pure samples of metals or organic compounds melting in the range studied. The calibration is different for each heating range because the instrument's thermal resistance varies with temperature. Equilibrium condition. Since the van't Hoff equation describes an equi librium condition, the sample must be in equilibrium both physically and thermally at all times during the melt ing process in the purity determina tion. Physical equilibrium is promoted by using very small samples. Slow heating rates provide thermal equi librium. With samples weighing 3 mg. or less and with a heating rate of 1.25 C. per minute, Dr. Barrall and Mr. Differ can get very good purity results. With a lead-tin mix ture containing 0.419 mole % lead, they get an error of only about 1.4%. When heating rates are above 1.25 C. per minute, or when sample mass is greater than 3 mg., the sample isn't at equilibrium during the entire melting process. Once melting ex ceeds a certain rate, Dr. Barrall ex plains, the calorimeter can't respond rapidly enough (because of the ther mal resistance of the sample-encapsu lation-cell combination) to record die process accurately. Thus, errors be come more serious. In high-impurity samples, eutectic formation can cause errors in purity determination by dsc. For example, in a sample containing 3.06 mole % stilbene in triphenylmethane, the ibm chemists found only 1.71 mole %. By contrast, with a sample containing only 0.71 mole % stilbene, they ob tained 0.640 mole %, about a 10% error but still acceptable for most analyses. If the eutectic point is well removed from the melting points of either component, the error is small. However, if the eutectic melts within the low melting tail of the impure prin cipal component, a serious error in the purity determination can result. Materials which form cocrystals can be studied by dsc in some cases, the ibm workers say. But the van't Hoff method can't be used effectively with these materials. They find that they can construct a calibration curve based on endothermal minimum temperature as a function of the minor component concentration. This curve is very sensitive to purity at concentrations less than 5 mole % of impurity. roblems get m NAA of air Tecfimque determines composition of 33 elements in solid particles from air ANALYTICAL. Neutron activation analysis (naa) of particles carried by air will give in vestigators of air pollution an effective tool for understanding mechanisms of solids transport and discovering sources of pollution. These expecta tions were expressed by Dr. Richard Dams of Nuclear Research Center, Ghent, Belgium, speaking of work with Dr. John W. Winchester, Dr. John A. Robbins, and Kenneth A. Rahn done at University of Michigan, Ann Arbor. The techniques used by the Ann Arbor men determine composition of 33 elements in solid particles collected from air. Applicability to arsenic, an timony, mercury, and selenium gives information vital to judging health dangers of air polluted by sizable amounts of these elements, says Dr. Dams. The method would have come in handy in the New York City-New Jersey area last month as officials sought frantically to find the source of a highly irritating miasma which wafted in no apparent pattern over New York City and several New Jersey communities. Dr. Dams sees the avail ability of profiles on so many elements as an aid to locating pollution sources by their compositional fingerprints. The Michigan team collects particu late matter by drawing air at high flow rates through 25-mm. polystyrene fil ters. Purity of filter materials is im portant, since trace impurities give rise to high blanks in supersensitive neu tron activation analysis. Even in their best work, however, chlorine is stiff a problem, and they report no results in a trial run of the method. Irradiated. Placed in polyethylene, the filter is sent into the core of a nu clear reactor, where it is irradiated at a flux of 2 X 1012 neutrons per cm.2-sec. for five minutes. The filter is re moved from the polyethylene package and counted after three minutes with a Ge(Li) detector coupled to a 4096channel analyzer. These instruments detect gamma rays at energies charac teristic of each element. The count after three minutes gives quantities of isotopes with very short half-lives, such as those from calcium, titanium, va nadium, copper, aluminum, and sulfur. A second count on the same filter after 15 minutes, when very' hot radioactiv ity has died away, allows determina tion of sodium, magnesium, manga nese, indium, chlorine, bromine, and iodine. A second irradiation at 1.5 X 1013 neutrons per cm.2-sec. on the same sample gives analyses of longer half life isotopes. A count after 20 to 30 hours gives potassium, lanthanum, sa marium, europium, copper, zinc, tung sten, gold, gallium, arsenic, antimony, and bromine, whereas another count on the same sample after 20 to 30 hours assays scandium, cerium, thorium, chromium, iron, cobalt, nickel, silver, zinc, mercury, antimony, and selenium. The Ge(Li) detector is a germa nium crystal doped with gallium, which is then treated with lithium to allow that metal to diffuse into crystal interstices. The detector acts like a solid-state Geiger counter with gamma rays triggering generation of electrons and positive electron-holes, which then travel to the appropriate electrode at tached to the crystal for detector cur rent glow. The 4096 channels of the analyzer are stepped 1 k.e.v. apart in terms of gamma ray energies, and the rise and faff of a peak in the spectrum is interpreted by an ibm system 360/67 computer, which reads the intensity in each channel. Routine. The fortran IV pro gram enables the computer to analyze peak shapes for discrepancies, do qual itative and quantitative analysis for the 33 elements based on counting rates in each channel--stored on seven-track magnetic tape--and subtract analytical blanks. The reactor and computer sys tems are quite sophisticated, but the Michigan scientists see the collection of samples as easily teachable routine. The only disquiet the scientists ex press is dwindling of sources of pure polystyrene filter materials. Their present stocks were obtained from a firm in Germany which has since stopped making material in such high purity. Dr. Winchester is presently trying to generate interest in com mercial production of high-purity poly styrene filters for air monitoring. DSW 552116 MARCH 9, 1970 C&EN 49 STLCOPCB4090533 Dr. Andrews (front) and Dr. Boss study algebraic structure of genetic code DNA code may transmit "superinformation" Redundancy enables codons to do more than pick amino acid sequence in proteins, Florida team says Scientists should look for "superinfor mation" in the algebraic group struc ture of the genetic code, according to Dr. Donald H. Andrews and Dr. Man ley L. Boss of Florida Atlantic Univer sity, Boca Raton, Fla. In codes used for electrical transmission of engineer ing signals, they note, group structure is imposed to increase efficiency and reduce error. Similarly, they reason, the group characteristics of codon re dundancy could serve to transmit ad ditional information superimposed on the messages directing amino acid order in protein synthesis. Dr. Andrews, a chemist, is distin guished professor of biophysics at Florida Atlantic. He and Dr. Boss, chairman of the biology department at the same school, are studying alge braic group correlations of activation entropies controlling chemical trans ference of information. Discussing the team's findings. Dr. Andrews notes that the molecules that serve as carriers of genetic in formation control the reaction involved in protein synthesis by acting as cata lysts. But, he maintains, the function of these catalysts is not primarily to change the activation energy; rather, they influence the rates of reactions by changing the activation entropy. Barrier. The relation between en tropy and chemical transference of in formation can be shown in the simpler types of state change found in crystal growth, Dr. Andrews explains. The simplest type of crystal growth takes place when molecules in a supercooled liquid are oriented and attached to the surface of a crystal immersed in the liquid. If there is no seed crystal, the supercooled liquid can remain liquid for months. The kinetic barrier is high enough to keep the crystallization rate infinitesimally small. Dr. Andrews observes. The barrier--the activation free energy for the reaction--is much higher than the barrier for normal growth because of the absence of any crystal surface. A large positive value for activation free energy means that there is a large negative value for activation entropy. Dr. Andrews continues. Because of the relation between entropy and in formation, he adds, one can say that a large amount of "information" is needed for the reaction to proceed when the activation entropy has a large negative value. The "ignorance barrier" is very high, Dr. Andrews comments. But when a single seed crystal is added, the supercooled liquid starts to crystallize on the surface of the seed crystal. Addition of the seed crystal provides the "information" which lowers the "ignorance barrier," Dr. Andrews says. The probability of the reaction increases, activation entropy is raised, the free energy barrier is lowered, and growth proceeds rapidly. Crystallization can often be induced by seeding with a different substance or even by scratching the side of the container. In such cases, Dr. Andrews says, the equivalent of introducing a smaller amount of information. 50 C&EN MARCH 9, 1970 Similarly, entropy of activation is also directly related to molecular sym metry, Dr. Andrews continues. For ex ample, benzene has a symmetry num ber of 12: There are 12 equivalent positions as the molecule is rotated and inverted, 12 ways for the molecule to fit into a crystal lattice. Benzene's melting point is about 5 C. In con trast, the structurally similar but asym metric 1,3-cyclohexadiene (symmetry number = 1) has about the same heat of fusion as benzene, but melts at --89 C. The difference in melting points--and entropy--is functionally related to the difference in symmetry, Dr. Andrews states. Group. Transformations of the position of a benzene molecule from any one position to any of the 12 possi ble equivalent positions constitute an algebraic group. The symmetry num ber is the number of elements in the group. Since the symmetry number affects activation entropy, and since the order of the group is numerically equal to the number of elements in it, Dr. Andrews says, the activation en tropies for a set of similar molecules having different symmetries are func tionally related to the order of the groups involved. As in crystal growth, a major proc ess in biological growth is the transfer of molecules from a state resembling a liquid to a state resembling a solid. If algebraic group relations play a role in crystal growth, Dr. Andrews asserts, it should not be surprising to find them also playing a role in biological growth. Protein synthesis, the basic process in biological growth, is controlled by the genetic code expressed by the order of nucleotides in deoxyribonu cleic acid (dna). The genetic code is redundant and, Dr. Andrews says, the redundancy has group character istics. In the case of a symmetric molecule, the algebraic group consists of all possible transformations of posi tion. In the genetic code, the "posi tions" consist of all possible forms of the codons which code for the different amino acids. Codons. Consider, Dr. Andrews says, one of the codons found in mes senger ribonucleic acid (kirna), an intermediate between dna and pro tein. uuu is one of the codons for phenylalanine. This codon can be changed into all the other possible codons, he explains, by inserting into each of the three places in the codon one of the four symbols u, a, g, or G (corresponding to the kna bases uracil, adenine, cytosine, and gua nine). Since there are four possi bilities for each of three places, there are 4x4x4 possible codons. When the Florida workers arranged the 64 codons to show their algebraic group relations, they also uncovered DSW 552117 STLCOPCB4090534