Interactions between flavin mononucleotide (FMN) and apoprotein have been investigated in the reduced and oxidized states of the flavodoxin isolated from Anabaena 7120 (Mr approximately 21,000). 1H, 15N, and 31P NMR have been used to characterize the FMN-protein interactions in both redox states. These are compared with those seen in other flavodoxins. Uniformly enriched [15N]flavodoxin (greater than 95% isotopic purity) was isolated from Anabaena 7120 grown on K15NO3 as the sole nitrogen source. 15N insensitive nucleus enhanced by polarization transfer (INEPT) and nuclear Overhauser effect (NOE) studies of this sample provided information regarding protein structure and dynamics. A 1H-detected 15N experiment allowed the correlation of nitrogen resonances to those of their attached protons. Over 90% of the expected N-H cross peaks could be resolved in this experiment.

The proton magnetic resonance spectra of actinomycin solutions with mixtures of deoxynucleotides have been investigated to determine the relative preference for the binding of guanine and adenine nucleotides to the two nucleotide binding sites of actinomycin D. An analysis of the chemical shifts of the actinomycin D resonances shows that adenine and guanine nucleotides competitively bind to the benzenoid portion of the phenoxazone ring of actinomycin D while guanine nucleotides bind stronger than adenine nucleotides to the quinoid portion of the phenoxazone ring. The chemical shift data for the titrations of actinomycin D with pdG-dG, pdC-dC, and an equimolar mixture of these complementary deoxydinucleotides show that: (1) pdG-dG forms a stacked complex much like dGMP; (2) pdC-dC does not bind to actinomycin D under the conditions used in these experiments; (3) in the titration of actinomycin D with the equimolar mixture of pdG-dG + pdC-dC, a miniature intercalated complex is formed.

Multinuclear two-dimensional NMR techniques were used to assign nearly all diamagnetic 13C and 15N resonances of the plant-type 2Fe.2S* ferredoxin from Anabaena sp. strain PCC 7120. Since a 13C spin system directed strategy had been used to identify the 1H spin systems [Oh, B.-H., Westler, W. M., & Markley, J. L. (1989) J. Am. Chem. Soc. 111, 3083-3085], the sequence-specific 1H assignments [Oh, B.-H., & Markley, J. L. (1990) Biochemistry (first paper of three in this issue)] also provided sequence-specific 13C assignments. Several resonances from 1H-13C groups were assigned independently of the 1H assignments by considering the distances between these nuclei and the paramagnetic 2Fe.2S* center. A 13C-15N correlation data set was used to assign additional carbonyl carbons and to analyze overlapping regions of the 13C-13C correlation spectrum. Sequence-specific assignments of backbone and side-chain nitrogens were based on 1H-15N and 13C-15N correlations obtained from various two-dimensional NMR experiments.

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. 61, 558 (1983).The linewidth of the "~a resonance in sodium tetraethylaluminate in benzene has been measured as a function of the ratio of [donor]: [Na+] using the strong complexing agent hexalnethylphosphoramide. In this system changes in the sodium linewidth arise primarily from changes in the nuclear quadrupole relaxation at the sodiuni nucleus which in turn arise from changes in the electric field gradient. As the ratio of hexamethylphosphoramide to sodiuni ion is increased up to 3: 1 the 2 3~a linewidth progressively decreases due to an increase in cation-anion distance in the ion pair. After a ratio of 4 : 1 is reached, essentially no llnewidth change is noted, indicating a stable solvation number of 4 for hexamethylphosphoramide. The results of this study indicate that this is a new and promising approach to the determination of solvation numbers of the sodium ion.JAMES HAROLD MEDLEY, EDDIE STEVEN MOOBERRY et MARION CLYDE DAY. Can. J. Chem. 61, 558 (1983).On a mesuri. la largeur des raies de la rCsonance du "Na du tCtraCthylalulninate de sodium dans le benzkne en fonction du rapport [donneur]:[Nai] en utilisant un puissant agent complexant: 1'hexamCthylphosphoramide. Dans ce systkme, les variations de la largeur des raies du sodium proviennent principalement des variations de la relaxation quadrupolaire nuclCaire au niveau du noyau du sodium qui a leur tour proviennent des variations du gradient de champ Clectrique. Quand le rapport de I'hexamCthylphosphoramide sur I'ion sodium augmente jusqu'a 3 : 1, la largeur des raies du "Na diminue progressivement ii cause d'une augmentation de la distance cation-anion de la paire d'ions. Lorsqu'on atteint un rapport de 4: 1 , on n'observe plus de variation de la largeur des raies ce qui indique un nombre de solvatation stable de 4 pour I'hexamCthylphosphoramide.Les rksultats de cette etude indiquent que c'est une approche nouvelle et prometteuse pour la determination des nombres de solvatation de I'ion sodium.[ Traduit par Ie journal]Introduction With the commercial availability of high resolution multinuclear nmr spectrometers, there has been a corresponding upsurge in the interest in nmr spectroscopy of the less commonly observed nucleii (1-4). Of particular interest here is the study of the solvation of the sodium ion by means of "Na nmr (5-lo), a system that is of considerable interest biologically as well as in terms of fundamental solvation theory.Because of its spin of 312, the relaxation time of the '%a nucleus is dominated by the coupling of the nuclear quadrupole moment with the time-dependent electric field gradient. Presently there is no single unambiguous theory for the origin of the fluctuating electric field gradient at the nucleus that covers the full range of ion-ion and ion-solvent interactions encountered by the sodium ion in solution (1 1-14). Rather there are two fundanientally different theoretical models. The electrostatic approaches formulated by Valiev (1 1) and by Hertz (12) describe a static electric field gradient in terms of the actio...