The 220 MHz Nuclear Magnetic Resonance Study of a Solvent-induced Conformational Change in Flavin Adenine Dinucleotide

Proton magnetic resonance studies at 220 MHz have shown that the conformation of flavin adenine dinucleotide in D2O-dioxane-d-8 (70:30, v/v) solutions is in agreement with the model proposed by Sarma, Dannies, and Kaplan (Biochemistry, 7, 4359 (1968)) for D2O solution. The determination of the temperature dependence of the FAD, flavin mononucleotide, and adenosine diphosphoribose proton shifts were found to be necessary for a correct assignment of the isoalloxazine protons in FAD and hence for determining the conformation. The FAD proton shifts in D2O solutions have a sigmoid dependence on temperature which is discussed in terms of a two-state model, where an equilibrium exists between the folded and unfolded states of the molecule. Sigmoid curves are not observed in D2O-dioxane solutions, suggesting that dioxane lowers the transition temperature. Dioxane decreases the fraction of folded FAD molecules in solution by lowering the energy barrier between the two states and also reduces the intermolecular stacking of FAD and FMN. Line broadening of the adenine AC8H proton in ADP-ribose is observed and may be interpreted in terms of rotational isomers in solution.