Science on View

Book review of "Envisioning Science: The Design and Craft of the Science Image," by Felice Frankel, MIT Press, 2002

Assessing Pauling's Wide-Ranging Life

Book Review of ‘Linus Pauling in his own words’ edited by Barbara Marinacci, Simon & Schuster,1995, "Linus Pauling: A Life in Science and Politics," by Ted Goertzel and Ben Goertzel, Basic Books, 1995, "Force of Nature: The Life of Linus Pauling," by Thomas Hager, Simon & Schuster, 1995

Pauling: Charismatic, Controversial, and Tough

Book Review of "Linus Pauling: A Man and His Science" by Anthony Serafini, Paragon House, 1989

Science For Sale - Interactions between academia and businesses could affect the future of scientific research

Book review of "The Perils, Rewards, and Delusions of Campus Capitalism," by Daniel S. Greenberg, University of Chicago Press, 2007

A Perspective Distilled from Seventy Years of Research

Physical organic chemistry might be regarded as officially recognized as a distinct discipline through the publication of L. P. Hammett’s book of that title, although substantial earlier work can be traced back to the turn of the 20th century. Many of the instrumental tools that helped the discipline develop in so many different ways began to appear in the late thirties and during World War II and were soon built to be increasingly operated in the “hands-on” mode. This development became very popular in academia, where instruments are not operated for you by an expert, but even if you are an undergraduate, you can more or less be the expert yourself and take many varieties of data on instruments usually available on a 24 h basis. It has been my privilege and joy to begin research in chemistry just as these waves of change began to grow and to savor the great contribution that the new methods, such as measurement of 14C, UV−vis, IR, NMR, and hands-on use of computers, made in facilitating our research programs at MIT and later at Caltech. Among those programs, which will be discussed, were 14C tracing of carbocation rearrangements and benzyne formation, electrical effects of substituents, Grignard reagents, synthesis of small-ring compounds, (2 + 2) cycloaddition reactions of halogenated ethylenes, assisting in development of ^(19)F, ^(13)C, and ^(15)N NMR for conformational analysis, other structural, kinetic, and tracer studies, as well as helping through textbooks to bring Hückel MO theory and the elements of NMR to familiarity for organic chemists. From the very beginning of my research career, I have been the beneficiary of personal mentoring which has been very crucial to my success in research and is an important theme in what follows

Nuclear magnetic resonance spectroscopy. A stereospecific ^3J_(CF) coupling in the low-temperature ^(13)C nmr spectrum of 1,1-difluorocyclohexane

The proton-decoupled ^(13)C nmr spectrum of 1,1-difluorocyclohexane has been examined at room temperature and at -90 degrees C. There are only minor changes in the one-bond and two-bond carbon-fluorine scalar coupling constants at the lower temperature; however, the triplet observed for C-3 (^3J_(CF) = 4.7 Hz) collapses to a doublet (3JCF = 9.5 Hz) at -90 °C. It is proposed that only the equatorial fluorine is coupled with the C-3 carbon as the result of operation of a back-lobe orbital interaction

Ammonia assimilation in Bacillus polymyxa. 15N NMR and enzymatic studies

Pathways of ammonia assimilation into glutamic acid and alanine in Bacillus polymyxa were investigated by 15N NMR spectroscopy in combination with measurements of the specific activities of glutamate dehydrogenase, glutamine synthetase, glutamate synthetase, alanine dehydrogenase, and glutamic-alanine transaminase. Ammonia was found to be assimilated into glutamic acid predominantly by NADPH-dependent glutamate dehydrogenase with a Km of 2.9 mM for NH4+ not only in ammonia-grown cells but also in nitrate-grown and nitrogen-fixing cells in which the intracellular NH4+ concentrations were 11.2, 1.04, and 1.5 mM, respectively. In ammonia-grown cells, the specific activity of alanine dehydrogenase was higher than that of glutamic-alanine transaminase, but the glutamate dehydrogenase/glutamic-alanine transaminase pathway was found to be the major pathway of 15NH4+ assimilation into [15N]alanine. The in vitro specific activities of glutamate dehydrogenase and glutamine synthetase, which represent the rates of synthesis of glutamic acid and glutamine, respectively, in the presence of enzyme-saturating concentrations of substrates and coenzymes are compared with the in vivo rates of biosynthesis of [15N]glutamic acid and [alpha,gamma-15N]glutamine observed by NMR, and implications of the results for factors limiting the rates of their biosynthesis in ammonia- and nitrate-grown cells are discussed

Glutamate biosynthesis in Bacillus azotofixans. 15N NMR and enzymatic studies

Pathways of ammonia assimilation into glutamic acid in Bacillus azotofixans, a recently characterized nitrogen-fixing species of Bacillus, were investigated through observation by NMR spectroscopy of in vivo incorporation of 15N into glutamine and glutamic acid in the absence and presence of inhibitors of ammonia-assimilating enzymes, in combination with measurements of the specific activities of glutamate dehydrogenase, glutamine synthetase, glutamate synthase, and alanine dehydrogenase. In ammonia-grown cells, both the glutamine synthetase/glutamate synthase and the glutamate dehydrogenase pathways contribute to the assimilation of ammonia into glutamic acid. In nitrate-grown and nitrogen-fixing cells, the glutamine synthetase/glutamate synthase pathway was found to be predominant. NADPH-dependent glutamate dehydrogenase activity was detectable at low levels only in ammonia-grown and glutamate-grown cells. Thus, B. azotofixans differs from Bacillus polymyxa and Bacillus macerans, but resembles other N2-fixing prokaryotes studied previously, as to the pathway of ammonia assimilation during ammonia limitation. Implications of the results for an emerging pattern of ammonia assimilation by alternative pathways among nitrogen-fixing prokaryotes are discussed, as well as the utility of 15N NMR for measuring in vivo glutamate synthase activity in the cell

Carbon-13 nuclear magnetic resonance spectra and mechanism of bridge–terminal carbonyl exchange in di-µ-carbonyl-bis[carbonyl(η-cyclopentadienyl)iron](Fe–Fe)[{(η-C5H5)Fe(CO)2}2]; cd-di-µ-carbonyl-f-carbonyl-ae-di(η-cyclopentadienyl)-b-(triethyl -phosphite)di-iron(Fe–Fe)[(η-C5H5)2Fe2(CO)3P(OEt)3], and some related complexes

A mechanism involving carbonyl-bridge breaking, rotation about the Fe–Fe bond, and bridge reformation is shown to account qualitatively for changes in the carbonyl region of the 13C n.m.r spectrum of the complex [(cp)(OC)[graphic omitted]e(cp){P(OEt)3}] and quantitatively for [(cp)(OC)[graphic omitted]e(CO)(cp)](cp =η-cyclopentadienyl).The activation energy for this process, 49.0 ± 4 kJ mol^–1(11.7 ± 1 kcal mol^–1), is close to that reported for cis–trans-isomerization of the cp groups, in accord with this mechanism. Variable-temperature 13C n.m.r. spectra of the complexes [(cp)(OC)[graphic omitted]u(CO)(cp)] and [(cp)(OC)[graphic omitted]i(cp)] are also reported

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N-arylguanidinium chlorides

The 13C and 15N chemical shifts of five N-arylguanidinium chlorides carrying polar substituents, ranging in character from 4-methoxy to 4-nitro groups, have been determined by NMR spectroscopy at the natural-abundance level of 13C and 15N in dimethyl sulfoxide solution. Comparison of the 13C shifts of these salts with those of monosubstituted benzenes shows that the guanidinium group induces an average downfield shift of -5.8 ppm of the resonance of the aryl carbon to which it is attached (C1), an average upfield shift of +4.2 ppm for C2 and C6, and a small upfield shift of +1.9 ppm for C4. The shifts of C3 and C5 are small and erratic relative to the corresponding carbons in monosubstituted benzenes. The 15N resonances of the guanidinium nitrogens are quite sensitive to electric effects resulting from substitution of polar groups at C4. The 15N shift of the ==NAr nitrogen relative to that of the salts suggests that the predominant tautomer for N-arylguanidines is (H2N)2C==NAr. The 15N shifts of the (NH2) 2 nitrogens correlate rather well with σp- parameters, whereas the shifts of the -NHAr nitrogens seem to correlate only with R values derived from the σp- substituent constants