Interaction between metal ions and NAD(P) coenzymes. 1H, 31P, 13C and 59Co NMR Spectroscopy and conformational analysis

Abstract

The interaction of the coenzymes NAD(P) with metal ions Mg²⁺, Ca²⁺, Zn²⁺, Li⁺, [Cr(NH₃)₆]³⁺, [Ru(NH₃)₆]³⁺ and [Co(NH₃)₆]³⁺ have been studied by ¹H, ¹³C, ³¹P and ⁵⁹Co NMR spectroscopy. The complexes with Mg²⁺ have been examined in detail, by performing the conformational analysis of the ribose rings, of the glycosidic moieties, of the chain fragments from C(5′) to P(5′) atoms, by using H–H and H–P coupling constant values and ¹H NOESY experiments. The stability constants of the complexes with diamagnetic metal ions have been obtained from titration experiments, following the chemical shift of the two (three) phosphorus atoms. The effects of association on the coupling constant J(P–O–P) and on the ³¹P chemical shifts were compared for the different ions. The ligands [M(NH₃)₆]³⁺ have been utilized as mimics of [Mg(H₂O)₆]²⁺ and the paramagnetic ion provided structural information through the measurements of the line-width variation of ³¹P and ¹³C signals. The ⁵⁹Co experiments allowed us to exclude dimeric structures such as (NADP)₂–Co. Models of (1:1) complexes of NADPH with [Mg(H₂O)₆]²⁺ and [Cr(NH₃)₆]³⁺ are proposed. The metal ion binds to the three phosphates with interactions of electrostatic nature, through the mediation of NH₃ or H₂O molecules. The hydrogen-bonding of NH₃ and H₂O to the phosphate oxygens contributes to the stabilization of the complexes.