Extensive ab initio molecular calculations have been first performed to thoroughly characterize the gas-phase potential energy surfaces (PES) of the 21th amino acid selenocysteine and related ions (neutral, protonated and deprotonated). A wide range of trial structures generated by considering the combinations of all internal single-bond rotamers was surveyed at the BHandHLYP/6-31G(d) level, and then refined at the BHandHLYP/6-311++G(d,p) level. A total of 76, 23, 38, and 3 unique stable conformers respectively for neutral, protonated, deprotonated, and doubly deprotonated selenocysteine is identified, and neutral zwitterionic forms are found to be as local minima on the gas-phase PES. The properties of the low energy conformers, such as relative energies, dipole moments, rotational constants, and intramolecular hydrogen bonds, were determined and analyzed. The thermochemical properties of proton affinity (PA), gas-phase basicity (GB), proton dissociation energy (PDE), gas-phase acidity (GA), and the vertical ionization energies (VIEs) were computed by the theoretical approaches of BHandHLYP, B3LYP, MP2, and CCSD(T). Moreover, the conformational equilibrium effect (CEE) on thermochemical properties was analyzed. The statistical simulation predicts that the CEE generally yields a physical correction on about a 1 k_BT scale in GA/GB calculations for multi-conformer systems.
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