Science
Steps in nitrogen fixation clarified Studies indicate that nitrogen, bonded to molybdenum-containing enzyme, interacts with iron atoms of ferredoxin A better understanding of the chemical pathway of biological nitrogen fixation is being gleaned from experiments at Stanford University. Nitrogen fixation by soil bacteria is a well-established biological process of key importance in the nitrogen cycle. However, scientists have not clearly understood the precise steps involved at the molecular level during conversion of nitrogen to ammonia. "It seems likely that nitrogen, bonded to a molybdenum-containing enzyme, interacts with the iron atoms of cubelike ferredoxin," Stanford's Dr. Eugene E. van Tamelen told attendees at the 24th Congress of the International Union of Pure and Applied Chemistry in Hamburg, West Germany, early this month. He bases his idea on evidence arising from the use of model reactions that he and his coworkers, Dr. Christa Brulet and John Gladysz, are studying. "This work," Dr. van Tamelen points out, "could provide the first genuine chemical understanding of what's happening in nature." The route whereby soil organisms convert molecular nitrogen to ammonia has been worked out in broad outline. In the case of Azobacter, for instance, nitrogenase, the enzyme that captures nitrogen, is in equilibrium with two constituent proteins. One of these contains molybdenum and iron. A third, ferredoxin, functions as an electron transfer agent, presumably acting on the nitrogenase-bound nitrogen. Ferredoxin has a cubelike structure of alternate iron and sulfur atoms. Polypeptide chains are attached to each of the four iron atoms through cysteine moieties. The most recent model system that Dr. van Tamelen and his associates devised consists of three components. One is a complex of molybdenum and diphenylphosphoethylene that Dr. J. Chatt and his group prepared at the Agricultural Research Council's nitrogen fixation unit at Sussex University in the U.K. This molecule forms a coordination complex with molecular nitrogen. Another component is a compound first synthesized by Dr. R. W.
Holm and coworkers at Massachusetts single electron pair or the pi-electrons Institute of Technology. Like ferredoxin, (or both) of the nitrogen ligand are init is made up of a central cube lattice of volved isn't certain, although precedent alternating iron and sulfur atoms with favors the [single electron pair]." What also happens during the interthioethyl groups linked to each iron atom. The third component is a radical action, he postulates, is that electrons flow from the Fe-S cluster molecule to ion reducing agent. In a typical experiment, the Stanford the bound nitrogen, breaking the nitroworkers add the nitrogen-bound molyb- gen's triple bond. This would facilitate denum complex, in a mixture of tetra- further reduction and uptake of protons hydrofuran and methanol, to a solution with concomitant formation of amcontaining the Fe-S cluster molecule monia. The new chemical evidence, coupled bearing a double negative charge. To this reaction mixture they add excess with low energy states of the reactants radical anion reducing agent such as so- being involved, leads Dr. van Tamelen dium anthracene or sodium fluoran- to believe that a similar reaction sethene. On stirring at room temperature, quence might well occur in nature. He ammonia is generated in yields ranging visualizes that nitrogen, bound to the from 0.03 to 0.05 mole per mole of the molybdenum-containing enzyme, would cluster molecule. Even in the absence of interact with ferredoxin's cube moleradical ion reducing agent, similar cules in a similar way, thereby creating the condition for reduction and protonaamounts of ammonia are produced. Through a series of screening experi- tion of the nitrogen and ultimate rements, Dr. van Tamelen's coworkers lease of ammonia. Nitrogen fixation in the model system have shown that the relatively weak re2 ducing agents sodium anthracene and involving [Fe(S)SC 2 H 5 ]4 ~ is a onesodium fluoranthene alone don't reduce step reaction. It contrasts with a twothe nitrogen ligand in the molybdenum- step reaction that occurs in an earlier nitrogen complex, nor do they bring model system that Dr. van Tamelen about nitrogen fixation by the Fe-S and his associates have studied. That cluster molecule in the presence of ni- system involves a diphenyldithiolenesubstituted Fe-S cluster compound first trogen. "Through such screening, our results synthesized by workers at the University point to [Fe(S)SC 2 H 5 ] 4 2 - (but not the of California, San Diego, led by Dr. 3~ or 4' forms) as being an essential G. N. Schrauzer. Dr. A. L. Balch at the participating species in the reduction University of California, Davis, has of nitrogen to ammonia in our model since shown that the compound has system," Dr. van Tamelen notes. "And a central cube structure. because of the available iron coordinaWhat appears to happen in this earlier tion site in this double negatively model reaction system is that molecular charged complex, it seems likely that nitrogen is first released by the molybthe molybdenum-bound nitrogen would denum-nitrogen complex. The nitrogen interact at a corner, rather than at a then is captured by the diphenyldiface, of the cluster cube. Whether the thiolene Fe-S cluster and reduced.
New system models nitrogen reduction
Sept. 24, 1973 C&EN
15
Societies ponder role in public issues Drawing any firm conclusions from a three-day conference earlier this month at Alta, Utah, on the role of professional societies in the public interest is no easy task. The gathering attracted more than 60 deeply involved individuals. They came from Government, the universities, activist groups, and professional societies. Each had his own strong views on this complex topic. And each had his own political philosophy. These ranged from the right of Barry Goldwater to the left of Fidel Castro. However, a few trends did emerge for the disinterested observer: • Professional societies seem reasonably at ease when their involvement in public interest science supports, or at least runs parallel to, the established values and policies of the powers-thatbe—Government, industry, academia. • Professional societies are certainly not yet at ease with public interest science issues that might involve them in controversy and put them in an adversary position with the establishment. To date such issues have been more successfully tackled by either inspired and highly motivated scientists working essentially alone or in small, grass-roots public interest groups. • Many professional societies seem anxious to improve the status and job security of their members. One of the motivations behind this is to give their members greater freedom to involve themselves individually in public interest science matters. These trends were brought out by the resolutions unanimously adopted at the close of the conference. None called for professional societies to become involved in specific public interest issues, although several such issues of major concern to the public health and welfare had been discussed at length at the gathering. What the resolutions did call for was: • The setting up of an intersociety system to help in selecting members for science advisory panels to the Government and other organizations. • The establishment of guidelines for the involvement of professional societies in public interest issues. • The enlargement of the current Congressional fellowship program. • The greater involvement of professional societies in appropriate science study projects. • The enhancement of mechanisms to protect professional society members from discrimination resulting from carrying out their professional responsibilities. Dr. Peter H. von Hippel of the University of Oregon had the most specific suggestion of how societies can help in the selection of advisory panel members. He is president of the Biophysical Society—a relatively small organization of about 2500 members. However, 16
C&ENSept. 24, 1973
the society has already established a roster of the expertise of the roughly 600 of its members who are willing to serve either on such advisory panels or in other public interest science work. The society will act essentially as a marriage broker, putting clients and appropriately qualified scientists in touch with one another. The system will be administered by an "editorial board" of experts in various technical areas. Dr. von Hippel feels the system is flexible enough to be expanded to include similar rosters drawn up by other societies. And he would like to see it become an intersociety affair. He and others at the conference admit, however, that the ethical considerations of the system might present problems. For instance, there may be some organizations that the societies might not wish to aid with scientific advisers. There may also be projects with which they might not wish to become involved. And there would be the difficulties, particularly for an intersociety system, of just how advisers are selected and of exactly how the selectors themselves will be selected. Dr. Charles Schwartz, a physicist at the University of California, Berkeley, is particularly concerned about these ethical issues. He told the conference that he defines public interest science in terms of working to oppose the centralization of power and of trying to put the resources of scientists at the service of the large number of people farthest away from the centers of control. He proposes guidelines for professional society activity in the public interest area. These include a ban on involvement in classified work, strict procedures to avoid any conflict of interest among advisers and study group members, and a system to assure a complete spectrum of views on any study or advisory panel. The Congressional fellow programs already under way were generally applauded by the conferees at Alta. Currently there are six fellows sponsored by scientific societies—three by the American Association for the Advancement of Science, two by the American Physical Society, and one by the American Society of Mechanical Engineers. The ASME fellow, Dr. Barry Hyman of George Washington University, has been working with the Senate Commerce Committee since early this year. The AAAS and APS fellows are just getting started. The idea behind these programs is to place scientists and engineers with Congressional staffs for about a year so that they can bring their very much needed expertise to the legislative process while gaining enormous insight into the workings of Government. Richard A. Scribner, who is involved with the AAAS program, reported the
conference consensus on these programs. He said that the effort should be enlarged to a coordinated program involving about 15 fellows and about six societies. The American Chemical Society is among other societies giving very serious thought to sponsoring a Congressional fellow. The ACS Board has approved the concept in principle. However, funding and other critical details are yet to be worked out. The direct involvement of professional societies in specific public interest science projects proved a difficult topic for the conferees to come firmly to grips with. Such efforts were seen as an appropriate and important function of such societies. But any direct involvement was sidestepped at the conference. For instance, an appeal by Thomas B. Cochran of the Natural Resources Defense Council for society aid in assuring that an upcoming Atomic Energy Commission environmental impact statement on its controversial liquidmetal fast-breeder reactor program (LMFBR) will cover all responsible views was essentially ignored. However, the American Physical Society may be close to entering into just such a controversial area. Next month the society's council will decide if APS will sponsor a study of the safety aspects of today's nuclear power reactors. ACS's major contribution to the Alta conference concerned the critical matter of defending the status, security, and responsibility of professional society members—an area in which it appears to be well ahead of other scientific societies. In summing up the views of the conference, ACS President Alan C. Nixon pointed out that there are many cases of scientists being in serious trouble when their professional positions conflict with the organizations for which they work. Dr. Nixon outlined to the conference a strong program of action by which a professional society can move aggressively to help counteract such abuses and to upgrade the status of its members generally. This program, most of which ACS already has well under way, includes: • Establishment of a professional relations committee—a committee that must have the full support of the society's governing body. • Establishment of a legal aid fund to assist members trying to obtain redress in the courts. • Involvement of local sections in investigation of mass layoffs of members. • Use of sanctions against employers. • Establishment of standards such as guidelines for employers. • Continued legislative action to try to establish, among other things, laws banning malicious discharge. • Establishment of procedures to follow up and aid jobless members.
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Sept. 24, 1973 C&EN
17
Scientists determine polypeptide structure The structure of subtilin, a peptide an tibiotic, has been determined by Dr. Erhard Gross and John E. Morrel, of the National Institute of Child Health and Human Development, Bethesda, Md. It has been found to be very simi lar to that of nisin, also a peptide anti biotic, determined two years ago by the same scientists. Dr. Gross points out that it is very unusual for polypeptides from two different microorganisms to have so many structural features in common. Subtilin is produced by Ba cillus subtilis, and nisin by Streptococ cus lactis. The structures of the peptides are in themselves unusual. Generally, some 21 amino acids are commonly found in proteins and peptides; numerous other amino acids occur less frequently. There are 29 amino acids in the nisin chain and 27 in the subtilin chain. Eight of these are rarely found in na ture. Each chain contains one residue of lanthionine, four of /j-methyllanthionine, and three «,/i-unsaturated amino acids. The lanthionine molecule consists of two alanines bonded to a sulfur atom at their β-carbons. Each of these ala nine (or β-methylalanine) residues is part of the peptide chain, so the sulfur atoms that join them form heterodetic
(other than amino acids alone) cyclic units along the peptide chain. At the time the structure of nisin was delineated, the three of the cyclic units that contained only 13 atoms were the smallest sulfur-containing rings of amino acids ever found in pep tides. Another unusual feature of these chains is the presence of D-isomers. Most naturally occurring amino acids are L-amino acids; only in a very few cases does the η-isomer occur natural ly. However, in both the nisin and sub tilin chains, the «-carbons of the ala nine residue at position 3 and the aminobutyric acid residues at positions 8, 13, 23, and 25 are all in the D config uration. Nisin was the first peptide shown to contain «^-unsaturated amino acids as part of its chain. These same amino acids have been found in subtilin. Of the three «,β-unsaturated amino acids present, two are dehydroalanine resi dues and the other dehydrobutyrine (β-methyldehydroalanine). The two dehydroalanine residues are in identi cal locations in both chains. However, the dehydrobutyrine residue is shifted from position 2 in the nisin chain to position 18 in the subtilin chain. Dr. Gross theorizes that at one time there may have been other «,β-unsaturated
Subtilin resembles previously sequenced nisin
HZN -{ILB
Nisîn
Subtilin
H*N
ABA = aminobutyric acid (β-methylalanine) ABA—S—ALA = β-methyllanthionine ALA - alanine ALA—S—ALA = lanthionine AN = asparagine DHA = dehydroalanine DHB = dehydrobutyrine (β-methyldehydroalanine) GLU = glutamic acid GLY = glycine GN = glutamine
18
C&EN Sept. 24, 1973
HIS = histidine ILEU = isoleucine LEU - leucine LYS - lysine MET = methionine PHE = phenylalanine PRO = proline SER = serine TRY = tryptophan VAL = valine
amino acids present in the chain but that somehow sulfur was picked up by the amino acids—as in the reaction of cysteine and dehydroalanine to form lanthionine—to form the heterodetic cyclic units seen in the chains. In their antibiotic activity, nisin and subtilin act on dividing bacteria and on bacterial spores undergoing germi nation. They are also effective against food-poisoning Staphylococci. Although these antibiotic effects of nisin and subtilin are well known, Dr. Gross feels that the double bonds of the unsaturated amino acids present in the chains are potentially in a position to react in vivo with the sulfhydryl groups of enzymes or other biochemicals. Thus, the double bonds might hold the key to other biological activi ties of nisin and subtilin. This theory was partly substantiated by the ability of nisin to limit the growth of malarial parasites in mice. This inhibitory effect may be explained by the known sensitivity of the para sites to coenzyme-Α, which has a ter minal sulfhydryl group. Work has also been done on the effect of nisin on neo natal development with rats and mice. Most of the experimental work has been done with nisin because subtilin is in very short supply. However, Dr. Gross expects that subtilin's biological activity will be very similar to that of nisin.
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Bifluoride ion C&EN Talks With... asymmetry uncovered Discovery of an asymmetric bifluoride ion raises some interesting questions for theoretical chemists, says Dr. Jack M. Williams of Argonne National Laboratory's chemistry division. Existence of the lopsided bifluoride ion has been confirmed by single-crystal neutron diffraction studies of p-toluidinium bifluoride, carried out at Argonne by Dr. Williams and associate Lynn F. Schneemeyer. Dr. Williams notes that the bifluoride ion has been the prototype for theoretical studies of very strong hydrogen bonds. And in virtually every experimental study of bifluoride ions made in the past 30 years, the same two conclusions were reached: The anions contain the shortest, strongest hydrogen bonds known, and the bonds are centered. Thus, the symmetrical nature of the hydrogen bond became accepted as intrinsic to the system. The Argonne chemist points out, however, that earlier experiments had dealt mainly with alkali metal and ammonium bifluorides, in which the anions are in a symmetrical environment. Dr. Williams chose p-toluidinium bifluoride because, he says, its crystalline structure is such that it should leave the bifluoride ion free to assume a configuration unobscured by crystalimposed symmetry. The studies snowed the crystal to consist of discrete CH 3 C6H 4 NH 3 ^ cations and (F-H—F)~ anions. The latter had F-H distances of 1.025 A. and 1.235 A.—according to Dr. Williams, the first known example of a bifluoride ion with an asymmetric hydrogen bond. The distances and the 178.1° (F-H—F) bond angle indicated that the anion was essentially linear. Dr. Williams believes that the unusual hydrogen bond configuration is a consequence of its asymmetric nearneighbor hydrogen bonding environment. He notes that each of the two fluorine atoms has two near-neighbor hydrogen atoms (from - N H 3 + groups of the cations), but that the F---H(N) distances are significantly different. The findings suggest, he says, that the symmetrical hydrogen bond is actually a special case that arises whenever the combined electronic and crystal environment about an ion lacks sufficient asymmetry to distort the potential surface "seen" by the bridging hydrogen atom, and that regardless of the "shortness" of the X - - - X bond in the (X-H-X) ± moiety, the molecular geometry of the hydrogen bond is strongly dependent on its near-neighbor environment. The job now is to find out whether all bifluoride ions have the same bond energies, Dr. Williams says. He adds that the findings will have an important bearing on the theory of the hydrogen bond. 20
C&EN Sept. 24, 1973
Clayton Kirkpatrick A perennial concern of scientists is improving public awareness and understanding of science and technology. It's often suggested that better understanding will result from giving more information to the mass media. However, this remedy doesn't always work. One reason may be that scientists and engineers are guilty of information overkill. Another reason is that the information offered by scientists isn't always the information desired by the media. Clayton Kirkpatrick, editor of the Chicago Tribune, tells C&EN that he is less concerned about public awareness of science and technology than scientists apparently are. He is convinced that an active, independent newspaper will automatically equip itself to report on science just as it does on politics, economics, sports, and crime. It doesn't want or need predigested information packages to pass on to readers. This would amount to abrogation of a paper's editorial independence. Editorial independence is a subject about which Mr. Kirkpatrick, like most editors, has very strong convictions. One thing that he doesn't like at all is an attempt to "spoon f e e d " information to his paper. Scientists, he observes, seem prone to do this and not always because of parochial attempts to manipulate the news. More often than not it stems from inordinate concern over technical minutiae that would probably be lost on the public anyway. Mr. Kirkpatrick suggests that the public's appreciation of science could be vastly improved if members of the press were granted greater access to the scientists themselves. Scientists tend to isolate themselves, sometimes for reasons of security or proprietary considerations, behind bland official statements. Although this isolation may satisfy security requirements, it tends to make science more remote. Another problem that troubles scientists more than editors is the allocation of space for science news. In any given issue of a newspaper, science news must compete for space with all other news. The ultimate decision on what to print and when to print it lies with the editor. Mr. Kirkpatrick regards this as another manifestation of editorial independence. As for the Tribune, Mr. Kirkpatrick, as one might suspect, thinks that he has every reason to be proud of its science coverage. One reason for the pride is science editor Ronald M. Kotulak, who has won numerous awards during his 10 years with the Tribune. With a twinkle in his eye, Mr. Kirkpatrick observes that, apparently, chemists agree
with him because the American Chemical Society has added another award to Mr. Kotulak's collection. At the ACS meeting next spring in Los Angeles, Mr. Kotulak will receive the 1974 James T. Grady Award for Interpreting Chemistry for the Public. Another thing that suggests that mass media coverage of science is greater than some scientists might believe is the drift away from compartmentalization. Hard news about science has been dispersed with the rise of multidisciplines. Consequently there are fewer science news stories specifically labeled as such. This trend has also produced new kinds of editors whose departments are at least pseudoscientific in scope. The Tribune has an environmental editor, for instance. Although much of the environmental editor's work deals with the politics of environmental regulation, he also gets highly involved with environmental sciences. Other papers have appointed energy editors, industrial editors, or technology editors, who function in concert with or in place of a science editor. Like more conventional businesses, the Tribune has had its share of economic problems, Mr. Kirkpatrick says. The latest is the newsprint shortage. Also like other businesses, newspapers will have to change out of enlightened self-interest as well as for better public service. The Tribune, says Mr. Kirkpatrick, will continue to accord science and technology their share of news coverage, as independently decided by the editorial staff. At the same time the hope of both editors and scientists is that there will be greater individual and collective cooperation from scientists and engineers. The ultimate beneficiaries will be the public, one part of which is the scientific c o m munity itself.
