Raman and surface-enhanced Raman study of thiamine at different pH values

Raman spectroscopy was employed for evidencing the protonated and unprotonated thiamine molecular species in aqueous solutions at different acid and basic pH. The recorded Raman spectra were assigned on the basis of density functional theory (DFT) calculations. Also, at different pH values of the gold sol, the adsorption of the two molecular species to colloidal metal particles was monitored by means of surface-enhanced Raman scattering (SERS). At pH values of 3, 4 and 5, the SERS spectra revealed the coexistence of unprotonated and protonated thiamine molecules adsorbed on the gold surface. The adsorption was concluded taking place through the N3′ and N1′ atoms from the pyrimidine moiety of thiamine. The orientation to the gold surface of the protonated and unprotonated thiamine molecular species was discussed.