Ionization energies and photoelectron spectra of fat-soluble vitamins in the gas phase: a theoretical study

Abstract

The electronic structures and photoelectron spectra of several fat-soluble vitamins including A (all-trans-retinol and its two derivatives, 13-cis-retinoic acid and all-trans-retinoic acid), D₂, D₃, E (consisting of α-tocopherol, β-tocopherol, γ-tocopherol and δ-tocopherol) and K were studied theoretically in this work. The vertical ionization energies of these compounds, including considerations of their electron correlations were calculated in the gas phase. The direct symmetry-adapted cluster/configuration interaction method, which employs single and double excitation operators (direct-SAC-CI SD-R), and the D95(df,pd) basis set were used for the calculations. The results indicate that more than one conformer contributes to the photoelectron spectrum of vitamin A, all-trans-retinoic acid, vitamin D₂ and D₃, suggesting that there is more than one biologically active form for each of these vitamins. The photoelectron spectrum of each vitamin was simulated and assigned, and the previously reported assignments from the literature were revisited. The ionization of vitamin D from highest occupied molecular orbit − 2 (HOMO − 2) and the lone electron pairs of oxygen was found to not take place below 10 eV. The first ionization band of vitamin E was assigned to the ionization from π_CC and π*_CC of its aromatic ring, unlike the previous assignment relating this ionization band to the lone electron pair of oxygen. In addition, the ionizations of vitamin A and its derivatives from the lone electron pairs of oxygen were found to not occur below 11 eV in the gas phase.