The Conformation of Pyridine Dinucleotides in Solution

The dependence of chemical shifts in the nuclear magnetic resonance spectra of pyridine nucleotides on concentration, pH, and temperature has been examined. The data indicate that all of the dinucleotides exist predominantly in a folded conformation, with the planes of the two rings stacked in parallel. The free energy of formation of the intramolecular complexes has been estimated to be of the order of -0.5 to -1.0 Kcal per mole. The present data do not allow a distinction between a single complex with a unique geometrical configuration and a multiplicity of complexes undergoing rapid exchange. However, the relative instability of the complexes, together with a detailed examination of the 64 alternative structures with the aid of molecular models, leads us to prefer the latter possibility.