Surface‐enhanced Raman Spectroscopy and Density Functional Theory Studies of Riboflavin, Lumiflavin, and Lumichrome Adsorbed on Silver Colloids

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Abstract
Spectroscopic investigation of riboflavin, lumiflavin, and lumichrome has been proceeded by means of surface‐enhanced Raman scattering (SERS) using silver colloids. While the SERS spectra of these molecules are somewhat alike to their Raman spectra, some of the bands show clear enhancements while others diminish. Weakening of the SERS spectra in acidic solution is greatly affected by the substituent on the isoalloxazine ring. The higher acidity of lumichrome leads to the stability of the molecule coordinated to silver colloid. Density functional theory (DFT) calculations have been performed for the plausible configurations of the molecules coordinated to a silver adatom. Our results indicate that deprotonation precedes adsorption, and the molecules coordinate to an Ag+ adatom via a single N atom between the two carbonyl groups. The calculated spectra well reproduce the observed spectra, and they have been used to assign the observed vibrational bands. The uniqueness of lumichrome arises from the absence of substituent and resulting tautomerism.

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