Surface-enhanced Raman spectroscopy of NAD+ and related compounds

Abstract

Surface-enhanced Raman spectra (SERS) are reported for nicotinamide adenine dinucleotide (NAD⁺, oxidised form) adsorbed on colloidal silver at concentrations between 10^–3 and 10^–5 mol dm^–3, and using various excitation wavelengths. Adenosine diphosphate (ADP) gave an SER spectrum similar to that of NAD⁺, except for the absence of the nicotinamide SER band observed for NAD⁺ at 1030 cm^–1. NADH (reduced form of NAD⁺) did not give an SER spectrum at the alkaline pHs necessary to prevent decomposition. No SER spectrum was obtained for NAD⁺ when bound to the enzyme GAPDH; the NAD⁺ is too deeply buried in the protein for effective surface enhancement. The applicability of the surface selection rules arising from the electromagnetic enhancement mechanism to determine NAD⁺ orientation at the silver surface is discussed in the light of the probable contribution to SERS intensity from a charge-transfer mechanism. While the selection rules do not, in this case, appear useful for determining NAD⁺ orientation at silver, wavenumber shifts do indicate surface interaction with N₁ of adenine. The general surface geometry of NAD⁺ adsorbed on silver at low ( < 10^–4 mol dm^–3) concentrations is deduced as being extended, with phosphate groups binding directly to silver, and ribose, nicotinamide and adenine moieties all in close proximity to the surface.