EXTENT OF INTRAMOLECULAR AROMATIC-RING STACKING IN TERNARY CU2-BIPYRIDYL OR 1,10-PHENANTHROLINE AND FLAVIN MONONUCLEOTIDE (FMN(2-))( COMPLEXES FORMED BY 2,2')

The stability constants of the 1:1 complexes formed between Cu(Arm)(2), where Arm = 2,2'-bipyridyl or 1,10-phenanthroline, and flavin monon ucleotide (=FMN(2-) = riboflavin 5'-phosphate) were determined by pote ntiometric pH titrations in aqueous solution at 25 degrees C and I = 0 .1 M (NaNO3). The experimental conditions were carefully selected such that only the monomeric complex species formed. On the basis of previ ously established log K versus pK(a) straight-line plots (Chen, D.; et al. J. Chem. Soc., Dalton Trans. 1993, 1537-1546) for the correspondi ng ternary complexes of simple phosphate monoesters and phosphonate de rivatives, R-PO32-, where R is a noncoordinating residue, it is shown that the stability of the ternary Cu(Bpy)(FMN) and Cu(Phen)(FMN) compl exes is considerably higher than is expected on the basis of the basic ity of the phosphate group of FMN(2-). By comparison with the stabilit y of the ternary Cu(Arm)(G1P) complexes, where G1P = glycerol 1-phosph ate, which had previously been studied (Liang, G.; et al. J. Am. Chem. Soc. 1992, 114, 7780-7785) and in which the coordination sphere of Cu 2+ is identical with the one in Cu(Arm)(FMN), it can unequivocally be shown that the mentioned enhanced stability of the Cu(Arm)(FMN) comple xes is solely due to the formation of intramolecular stacks; their for mation degree reaches for Cu(Bpy)(FMN) and Cu(Phen)(FMN) about 80 and 90%, respectively. These, as well as recent results regarding the self -stacking of FMN(2-) (Bastian, M.; Sigel, H. Biophys. Chem. in press) show that the flavin moiety is ideally suited for stacking and charge- transfer interactions, which are so important for the flavin coenzymes in nature.