APPLICATION OF ISODESMIC REACTIONS TO THE CALCULATION OF THE ENTHALPIES OF H-CENTER-DOT AND OH-CENTER-DOT ADDITION TO DNA BASES - ESTIMATEDHEATS OF FORMATION OF DNA-BASE RADICALS AND HYDRATES
Ao. Colson et al., APPLICATION OF ISODESMIC REACTIONS TO THE CALCULATION OF THE ENTHALPIES OF H-CENTER-DOT AND OH-CENTER-DOT ADDITION TO DNA BASES - ESTIMATEDHEATS OF FORMATION OF DNA-BASE RADICALS AND HYDRATES, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(47), 1997, pp. 8935-8941
In this work, we employ semiempirical and low-level ab initio molecula
r orbital theories with isodesmic/isogyric reactions to estimate the e
nergetics of free radical reactions important to radiolytic damage to
DNA. Results are presented (1) for H . and OH . addition reactions to
the natural DNA bases, (2) for the addition of H . to the DNA base rad
ical adducts so obtained, and (3) for the dehydration of the DNA base
hydrates and regeneration of the undamaged bases. The technique employ
ed sums the calculated enthalpy of an isodesmic reaction with the expe
rimentally known reaction enthalpy of an associated reaction to estima
te the enthalpies of the DNA base reactions of interest. The calculati
onal techniques employed are the ROHF/6-31G ab initio method and the
ROHF/PM3 semiempirical method. For H . addition reactions, the ROHF ab
initio method employing the 6-31G basis set gives results that are i
n closest agreement with known experimental results. For the OH . addi
tion processes, the ROHF/PM3 semiempirical technique results in predic
tions more in keeping with the experimental model systems. The values
calculated are employed to estimate the heats of formation of various
DNA base radicals and DNA base hydrates. Economical PM3 calculations o
f isodesmic reactions when combined with well-known experimental entha
lpies of reaction are found to yield reliable thermodynamic quantities
.