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.