1H-, 13C-, 31P-NMR studies and conformational analysis of NADP+, NADPH coenzymes and of dimers from electrochemical reduction of NADP+

All H,H, H,P and several C,P coupling constants, including those between C-4′ and the vicinal phosphorus atom, have been determined for NADP⁺, NADPH coenzymes and for a 4,4-dimer obtained from one-electron electrochemical reduction of NADP⁺. From these data the preferred conformation of the ribose, that of the 1,4-dihydronicotinamide rings, and the conformation about bonds C(4′)C(5′) and C(5′)O(5′) were deduced. The preferred form of the 1,4- and 1,6-dihydropyridine rings and the conformation about the ring-ring junction were also obtained for all the other 4,4- and 4,6-dimers formed in the same reduction. All the dimers show a puckered structure, i.e., a boat form for the 1,4- and a twist-boat for the 1,6-dihydronicotinamide ring; both protons at the ring-ring junctions are equatorial and have preferred gauche orientation. On the contrary, the reduced coenzyme NADPH displays a planar or highly flexible conformation, rapidly flipping between two limiting boat structures. The conformation of the ribose rings, already suggested for the NADP coenzymes to be an equilibrium mixture of C(2′)-endo (S-type) and C(3′)-endo (N-type) puckering modes, has been reexamined by using the Altona procedure and the relative proportion of the two modes has been obtained. The S and N families of conformers have almost equal population for the adenine-ribose, whereas for the nicotinamide-ribose rings the S-type reaches the 90%. The rotation about the ester bond C(5′)O(5′) and about C(4′)C(5′), defined by torsion angles β and γ respectively, displays a constant high preference for the trans conformer β^t (75–80%), whereas the rotamers γ are spread out in a range of different populations. The values are distributed between the gauche γ⁺ (48–69%) and the rans γ^t forms (28–73%). The γ⁺ conformer reaches a 90% values in the case of NADP⁺ and NMN⁺. The conformations of the mononucleotides 5′-AMP, NMN⁺ and NMNH were also calculated from the experimental coupling constant values of the literature.