Ionization of vitamin C in gas phase: Theoretical study

Electronic structure analysis of riboflavin: OVGF and EOM-CCSD study

The computational simulation of the photoelectron spectrum of active form of vitamin B2 is reported in the gas phase. In this work, we determine relative stability of eight riboflavin conformers by conformational search first with molecular mechanics AMMP potential in VEGA software at 553 K. Relative abundance of conformers was deduced from Boltzmann population weighting method (BPW). The three most stable conformers were then selected for computing valence, vertical ionization energies. We used high-level Equation-of-Motion Coupled-Cluster (EOM-IP-CCSD) method to obtain valence ionization energies (IP). In order to characterize the nature of ionization processes pertaining to different spectral bands, natural bonding orbital (NBO) method and molecular electrostatic potentials (MEP) were used to obtain orbital electron densities. The influence of the electronic structure of riboflavin on its biological activity is manifested via reduction of ionization energies of outermost orbitals which makes electron densities of these orbitals more readily available to participate in ligand-receptor bonding.

A TD-DFT study of the excited dissociative electronic states of the DNA nucleobases bound to Li. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2021;244:118862. [PMID: 32927359 DOI: 10.1016/j.saa.2020.118862]

In this work, the first and second dissociative potential curves of adenineLi⁺ (ADLi⁺), guanineLi⁺ (GUALi⁺), cytosineLi⁺(CYTLi⁺), and thymineLi⁺ (THYLi⁺) complexes, related to the dissociation of their LiO and LiN bonds, have been calculated in the gas phase and water, separately. For this purpose, the fifteen excited potential curves, in order of increasing energy, were calculated for each complex and the dissociative potential curves were distinguished from them considering the conical intersection points. The time-dependent density functional theory (TD-DFT) method employing the M06-2X functional was used for the calculations. It was observed that the electron transfer from the DNA base to the Li⁺ took place during the dissociation of complexes in the gas phase. The electrostatic field of water blocked this charge transfer and led to the excited DNA base and Li⁺ in its ground state. The vertical excitation energy for the desorption of the Li fragment as a neutral and cation species from the DNA bases was determined. The effect of the interaction site of Li⁺ on the dissociative potential curves was also investigated.