zyme surface would be expected to increase the rate of the reaction. This may be one of the principal ways in which the enzyme increases the rate relative to that of a model system, he says. Further, Dr. Hamilton believes that this general type of mechanism is applicable to many other enzymic redox reactions as well, not only those involving flavin coenzymes. Interactions. Perhaps one of the first prerequisites, however, to understanding how flavin coenzymes function is determination of the site and nature of flavin coenzyme binding to amino acid residues within the holoenzyme. Cornell University chemist Donald B. McCormick and his coworkers are currently studying the nature of the nonionic association between flavins and amino acid residues in flavoprotein enzymes. The importance of strong ionic interactions between anionic flavin coenzymes and the specific cationic sites in proteins with which they function as flavoproteins has long been recognized by biochemists, Dr. McCormick says. But in addition to ionic associations, the binding and oxidation-reduction function of flavin coenzymes involves the association of the isoalloxazine ring system of such flavin coenzymes as FAD. Dr. McCormick, like Dr. Hamilton, deals largely with model systems in which he studies synthetic flavin derivatives in order to get a handle on what is actually occurring within the catalytically functional enzyme. Dr. McCormick and his colleagues, Dr. W. Fory, Dr. R. E. MacKenzie, and Dr. S. Y.-H. Wu, have investigated the properties of model flavinyl peptides in solution in inter- and intramolecular association with aromatic amino acid residues such as tyrosine and tryptophan. Using physical techniques including measurements of light absorption and emission, infrared spectroscopy, and proton magnetic resonance, they have delineated many of the features of such interactionsfirst in their model synthetic flavinyl peptides and more recently in naturally occurring flavoproteins. They find that in more aqueous environments the flavin association with the aromatic amino acids is mainly through coplanar stacking with much of the benzenoid portion of the flavin shielded. As the environment becomes more nonpolar the pyrimidinoid part of the flavin ring system increasingly participates through hydrogen bonding to amino acids such as tyrosine or tryptophan. For example, quite recently Dr. McCormick, Dr. Wu, Dr. C.-W. Wu, and S. C. Tu have observed that the fluorescence of a given apoenzyme can be quenched and/or shifted upon binding a flavin coenzyme. The blue shift ob-
Cornell's Dr. McCormick studies nonionic interactions in flavinyl peptides
served upon reconstitution of D-amino acid apooxidase with FAD indicates a likely interaction whereby tryptophan residues become more masked in a nonpolar environment. From the model systems it is only a step to observing the same interactions in naturally occurring systems, Dr. McCormick asserts. This is important because the associations of flavin
coenzymes with particular amino acid residues of proteins markedly affect the redox behavior of the two-partner system and confer stereochemical specificity for the substrate, he adds. Finally, an understanding of flavinamino acid interaction is becoming helpful in photochemical studies aimed at elucidating certain of the active-site residues in flavoproteins.
Nuclear decay rates assist chemical rate measurements Chemical nuclear rate coupling can be a big help in studying the chemical dynamics of fast chemical processes by various experimental techniques, such as crossed molecular beams. The method consists of using nuclear decay rates to measure chemical rates under controlled conditions. In describing the concept to the Division of Nuclear Chemistry and Technology, Dr. J. Robb Grover of Brookhaven National Laboratory emphasized that the technique differs from radioactive tracer applications, which make use of radioactive decay only for efficient detection. It differs also from hot atom chemistry, which is concerned only with energetic species that result from the dynamics of nuclear transitions. Chemical nuclear rate coupling makes use of nuclear decay rates by incorporating a short-lived radioactive nuclide in the reaction. The main idea, Dr. Grover says, is to arrange apparatus so that the observed signal depends on time-dependent processes, such as translation and rotation, of the reaction under study.
^CHICAGO
The nuclear technique is unlike many detection methods, such as those using short light pulses for detecting product species following a sudden disturbance. In these, signal-to-noise ratio is dependent on resolution time. With the nuclear method, the ratio is independent of time, since at steady state, birth rate of nuclei equals decay rate, and the signal is thus independent of the mean life of the nuclei. High. This independence and the strong suppression of noise because of rapid nuclear decay lead to one of the method's main advantages, a high signal-to-noise ratio. The technique can be widely applicable. Dr. Grover notes that there are more than 130 nuclides of 69 elements available with half-lives between 1 _ 1 and 10~5 second. A prototype experiment being carried out by Dr. Grover and coworkers is a good example. In the experiment, a molecular beam of HAt made with astatine-217 (0.032-second half-life) is crossed with a beam of bromine atoms, with detection of reaction product astatine and scattered HAt being carried out with chemically selective detectors. SEPT. 28, 1970 C&EN 33
