Investigation on the electronic structures of thiamine and related compounds: Free base or salt?

Abstract

We report the simulated photoelectron spectra of thiamine free base, thiamine and thiamine diphosphate (ThDP) in the gas phase. The structures of eighty conformers of thiamine (vitamin B1) were optimized by DFT method at the B3LYP/6-31+G(d) level. Then population ratios (PRS) with Boltzmann distribution equation were calculated using Gibbs free energy and the stable conformers were obtained. The ionization energies of these conformers were determined by two high-level computational methods. General-R-symmetry adapted cluster-configuration interaction (general-R-SAC-CI) method and electron-propagator theory (EPT) method were used to obtain vertical valence ionization energies (IP). To characterize the nature of ionization processes about different spectral bands, the natural bonding orbital (NBO) method was used to obtain orbital electron densities. The electron correlation was taken into account with SAC-CI method and MOs shapes and ionization assignments given for each spectral band. Results for ThDP show that spectra of two conformers are different: oxygen lone pair ionizations appear at higher ionization energies in the spectrum of conformer II while giving rise to the first ionization band in conformer I. The validity of Koopmans’s approximation was considered for each ionization band and the role of electron correlation was attributed to breaking down of this approximation. The implications of electronic structure results for biological activity of thiamin are discussed. Comparison with the earlier work using aerosol thermodesorption method suggests that free base was present in their photoionization spectra in addition to the deprotonated cation.