Concentration dependent conformation of inosine on colloidal silver nanoparticles: A study by Raman, SERS and DFT calculation

Surface enhanced Raman spectroscopic (SERS) study of biologically important inosine molecules adsorbed in Ag-sol is reported for the first time. Possible adsorption mechanism is discussed. The enhancement is significantly influenced by the concentration of inosine in Ag-sols. The in plane base ring breathing mode appears at 715 cm−1 in solid phase and is shifted to 724 cm−1 in aqueous solution. Appearance of this band at 736 cm−1 in the adsorbed state suggests C(2′) endo-anti conformation at concentrations up to 5 × 10−7 M at which monomolecular layer is formed on the silver surface. However, C(3′) endo-syn conformation is suggested at a concentration of 5 × 10−11 M. Inosine molecule, possibly, interact with the silver nano-particles through C5′ atom at higher concentration. The π system of electrons of the hypoxanthine ring may be involved in the adsorption process at lower concentration. Monomolecular layer is formed at concentrations of 5 × 10−7 M and 5 × 10−11 M indicating perpendicular and parallel adsorption respectively. Density functional calculations are performed for the explanation of the experimental results.