4. Conclusions
Ab initio MD simulations were carried out to determine the environmental and temperature effects on EPR parameters of L-α-Ala radicals in acidic and alkali solutions and compared with the experimental data. Geometrical parameters of all radicals in both gas and solution phases clearly demonstrated substantial changes in bond lengths and angles upon transferring from gas to solution phase. It is found that similar to the amino acid alanine, the zwitterionic form is the stable form of this radical in the gas phase just as the well-known result for the closed-shell amino acid. In addition, the magnetic moments of all radicals in both gas and solution phases suggest that the combinations of hydrogen bonds associated with the polar -NH2, -NH3 and -COO(H) groups are driving the arrangement of the solvent molecules around the radicals. Besides, it is found that all radicals had similar number of hydrogen bonds around the carboxyl group while the number of hydrogen bonds around the amino group forradical is more than those around and radicals. Furthermore, the solvation structure for and radicals is such that the hydrogen bonds involving O1 and O2 are of approximately the same strength whereas, in the case of the radical, O1 had stronger hydrogen bond than O2.
The g -tensor components have characteristic functional dependence on the magnetic moment of the oxygen atoms of the carboxylic groups for all radicals. Thus, it is found that the gxx(gyy ) components have a strong linear dependence on this magnetic moment for all radicals, while thegzz component exhibits not such correlation. The lack of dependence of the gzz component on the magnetic moment may be due to the smallness of the magnitude of the spin-orbital coupling of the oxygen atoms of the water molecules.
The HFCC of the methyl group is found to strongly depend on the magnetic moment for all radicals with a linear correlation coefficient of almost unity. The transfer from the gas to the solution phase is accompanied by a decrease in the HFCC of atoms in solution phase compared to the gas phase in the case of the radical since it exhibited a lower spin density in solution phase compared to the gas phase. The radical exhibit a spin density that remained quasi-constant on passing from the gas to the solution phase which is accompanied by a constancy of the HFCC of the atoms of radical.
Isotropic hyperfine coupling and g -tensor of all radicals were compared with their experimental counterparts in acidic and alkaline solutions. In the case of the alkaline, the calculated HFCC and giso for the nitrogen atom, the methyl group, and the amino group, are all in good agreement with experiment, which supports the high-quality claim of our calculations and results. Under acidic solution, the calculated HFCCs of the nitrogen atom and the methyl group for the have a better agreement with experiment compared to the corresponding results for the radical. However, the calculatedgiso and HFCC for the Hα atom of shows a better agreement with experimental results.