Structure and Magnetic Properties of L-α-Alanine Radicals in Radiation
Dosimetry Applications: An Ab Initio Molecular Dynamics Simulation
Abstract
Alanine is a transfer standard dosimeter using in gamma-ray and electron
beam calibration. One of the important factor affecting its dosimetric
response is amount of humidity which can deviate the dosimetry expert
from the exact value of absorbed doses. Ab initio molecular dynamics
calculations were performed to determine the environmental effects on
the EPR parameters of L-α-Alanine radicals in acidic and alkaline
solutions. Similar to the closed-shell amino acid molecule alanine, the
zwitterionic form of alanine radical is the stable form in the gas phase
while the non-zwitterionic neutral alanine radical is not a stable
structure. Geometric and EPR parameters of radicals in both gas and
solution phases are found to be dependent on hydrogen bonding of water
molecules with the polar groups and by dynamic solvation. Calculations
on the optimized free radicals in the gas phase revealed that for
neutral radical, hydrogen bonding to water molecules drives a decrease
in the magnitudes of g-tensor components gxx and gyy without affecting
neither gzz component nor the HFCCs. For the transfer from the gas to
solution phase of the alanine radical anion is accompanied with an
increase in the spin density on the carboxylic group’s oxygen atoms.
However, for the neutral radical, this transfer from gas to solution
phase is accompanied with the decrease in the spin density on oxygen
atoms. Calculated isotropic HFCCs and g-tensor of all radicals were in
good agreement with their experimental counterparts in both acidic and
alkaline solutions, which enhances the confidence in our calculated
results.