Circular Dichroic Properties and Conformation of Thionicotinamide Dinucleotides

Abstract

The spectroscopic properties of the 3-thioamide analogues of coenzymes NAD and NADH (sNAD and sNADH) were investigated in order to obtain information about their conformational properties. In particular, UV absorption and circular dichroism [CD] properties of solutions in phosphate buffer pH 7 and ethanol were studied. Equimolar mixtures of AMP and sNMN(H), obtained by cleaving the coenzymes with phosphodiesterase, were investigated using the same solvents. The appearance of a couplet aroung 260 nm, which is not present for the mixture of sNMN and AMP, suggest a stacking interaction of the 2 aromatic moieties in sNAD. This is further substantiated by a hyperchroism of the UV absorption band in the 260 nm region in both sNAD and sNADH. The comparison of the UV and CD properties of intact and cleaved cyenzymes in the different solvent systems makes it possible to single out the bands which are more sensitive to conformational changes (i.e., to open-stacking equilibrium) and those which are sensitive to solvent effects only.