Charge transfer complexing of NAD with phenol and influence of phenol substituents on complex stability

Abstract

Mixture of NAD with phenols in aqueous solution results in the appearance of a broad absorption band in the 300–500 nm range. Increased absorbancy of NAD-phenol mixtures with a decrease in temperature indicates that a stable charge transfer complex between NAD and phenol is primarily responsible for the absorption band rather than contact charge transfer reactions. This complex absorbancy was used to determine the association constants of the charge transfer complexes of an array of substituted phenols with NAD. Analysis of the absorption data revealed a 1:1 stoichiometry for the complexes. Changes in absorbancy of NAD-phenol solutions with pH are due predominantly to a shift in the λ max or a change in the extinction coefficient of the complex with pH rather than to substantial changes in the concentration of complex present. Complex formation between nicotinamide mononucleotide and phenol was observed, but no evidence of complexing was found with mixtures of adenine or AMP with phenol. This supports previous observations (7, 13) indicating that the pyridinium rather than the adenine moiety of NAD is the electron acceptor in charge transfer interactions between NAD and phenols. Pyridinium ion-phenol and NAD-anisole complex formation indicate that hydrogen ion bonding involving the hydroxyl groups of phenol or the carboxamide moiety of the pyridinium ring of NAD is not essential to complex formation. Fitting of the experimentally determined association constants for a group of phenols to their electronic and physical parameters indicates that NAD-phenol complex formation is significantly correlated with the hydrophobic binding constant π, the Hammet σ function, and the size of the phenol substituent. The importance of the hydrophobicity of a phenol to its complex forming ability is suggested to arise from a solvent cage effect on complex formation.