934 assessment of individual factors so that the more complex interactions between nucleic acids and amino acids can be understood. Clearly, even on the monomer level the interac- tions between amino acids and nucleotides are sufficiently quantifiable;& if polar interactions are included, a rudimentary "preferential" scheme can be seen: the stability of the adducts decreases within the series (ATP)(H(~~P))~- zyxwvuts > (ATP)(H- (leu)) zyxwvutsrq 4- > (ATP) (H( ala))4-. In addition, the stability of the ternary M(ATP)(Aa)3- complexes differs not very much for a given metal ion (Table zyxwvut I), but the results of Table V show that the ability to form specific and distinct structures differs; especially the differences for the ternary zn2+ complexes are quite in 10-3 M solution at pH 7 the "closed" form of Zn(ATP)(trp)3- is favored by a factor of about 30 Over the "Closed" isomer of Zn(ATP)(leu)'. Hence, these data suggest that evolutionary selectivity in nucleotide/metal ion/amino acid systems is probably not so much achieved, at least for a given metal ion, by differences in complex stability but rather by the ability Inorg. zyxwvut Chem. 1983, 22, 934-939 to form specific and distinct structures. In summary, via the formation of mixed-ligand complexes certain ligand-ligand associations may be favored and thus distinct structures may be created in a way that involves only small changes from an energetic point of view. Regarding the specificity and selectivity observed in nature, this seems to be the most fascinating point of the present results. Acknowledgment. We thank Rita Baumbusch for technical assistance, K* Aegerter for recording the lH NMR spectra, and the CIBA-Stiftung Basel for support toward the costs of these spectra. The computers were made available by the Rechenzentrum der Universitat Base1 (UNIVAC 1100/81). These SUPPO& and a research Pant from the swiss National Science Foundation are gratefully acknowledged. Registry No. ATP, 56-65-5; Mn, 7439-96-5; Cu, 7440-50-8;Zn, 7440-66-6; a, 7440.43-9;pb, 7439.92-1; leucine, 61-965; norleucine, 327-57-1; norvaline, 6600-40-4; a-aminobutyric acid, 80-60-4;iso- leucine, 73-32-5; alanine, 56-41-7; tryptophan, 73-22-3. Contribution from the Departments of Chemistry, University of Denver, Denver, Colorado 80208, and University of Colorado at Denver, Denver, Colorado 80202 Metal-Nitroxyl Interactions. 27. Comparison of Electron-Electron Spin-Spin Coupling Constants for Urea and Amide Linkages in Spin-Labeled Copper Porphyrins KUNDALIKA M. MORE, GARETH R. EATON,* and SANDRA S. EATON Received May 19, 1982 A series of spin-labeled copper porphyrins has been prepared with urea linkages between the porphyrin ring and the nitroxyl ring. The electron-electron coupling constants, zyxwvutsr J, have been obtained from the solution EPR spectra. The observed values of J are lower by factors of about 2 to >40 for the compounds with urea linkages than for analogous compounds with amide linkages. The decreased values of Jfor the urea linkages are consistent with a largely u-bonding pathway for the coppepnitroxyl interaction. In several cases two components were observed in the EPR spectra. These are attributed to different conformations of the porphyrin-nitroxyl linkage. Introduction The ability to determine patterns and pathways of electron spin delocalization is important in areas of chemistry ranging from molecular orbital theory and electronic structure to mechanisms of reaction. Recent results from these laboratories have demonstrated that EPR studies of electron-electron spin-spin coupling constants, J, in spin-labeled metal com- plexes can be used to monitor changes in electron spin delo- calization due to changes in coordinated metal,'a2 changes in metal-nitroxyl and changes in ligand conforma- tion.4-6 In a study of spin-labeled pyridine adducts of vanadyl and copper( 11) bis( hexafluoroacetylacetonates) it was found that the change in J that resulted when an amide linkage between the pyridine and nitroxyl rings was replaced by a urea linkage correlated with the relative importance of u- and r-delocalization pathways in the metal-nitroxyl interaction.' Our previous studies of the values of J for the amide-linked spin-labeled copper porphyrins I-IV indicated that a-bonding pathways were largely responsible for the electron-electron interactiom6 We have therefore prepared the analogous urea-linked complexes V-VI11 to further explore the inter- action pathway in these porphyrins. The spectral data for the To whom correspondence should be addressed at the University of Den- ver. 0020-166918311322-0934%01 zyxwvut SO10 -C-N-NH2 XV H VI VI1 Vlll IX X XI1 XI Xlll -C02Et XIV Me Et amideliiked porphyrins X and XI indicated little ?r interaction between the porphyrin ring and the amide side chain.4 Com- 0 1983 American Chemical Society