Carcinogenesis of Urethane:
Simulation versus Experiment
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Abstract
The
carcinogenesis of urethane (ethyl carbamate), a byproduct of
fermentation that is consistently found in various food products,
was investigated with a combination of kinetic experiments and quantum
chemical calculations. The main objective of the study was to find
Δ<i>G</i><sup>⧧</sup>, the activation free
energy for the rate-limiting step of the S<sub>N</sub>2 reaction among
the ultimate carcinogen of urethane, vinyl carbamate epoxide (VCE),
and different nucleobases of the DNA. In the experimental part, the
second-order reaction rate constants for the formation of the main
7-(2-oxoethyl)guanine adduct in aqueous solutions of deoxyguanosine
and in DNA were determined. A series of <i>ab initio</i>, density functional theory (DFT), and semiempirical molecular orbital
(MO) calculations was then performed to determine the activation barriers
for the reaction between VCE and nucleobases methylguanine, methyladenine,
and methylcytosine. Effects of hydration were incorporated with the
use of the solvent reaction field method of Tomasi and co-workers
and the Langevine dipoles model of Florian and Warshel. The computational
results for the main adduct were found to be in good agreement with
the experiment, thus presenting strong evidence for the validity of
the proposed S<sub>N</sub>2 mechanism. This allowed us to predict
the activation barriers of reactions leading to side products for
which kinetic experiments have not yet been performed. Our calculations
have shown that the main 7-(2-oxoethyl)deoxyguanosine adduct indeed
forms preferentially because the emergence of other adducts either
proceeds across a significantly higher activation barrier or the geometry
of the reaction requires the Watson–Crick pairs of the DNA
to be broken. The computational study also considered the questions
of stereoselectivity, the ease of the elimination of the leaving group,
and the relative contributions of the two possible reaction paths
for the formation of the 1,<i>N</i><sup>2</sup>-ethenodeoxyguanosine
adduct
