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\begin{document}

\title{New Insight into the Substituent Effects on the Hydrolytic Deamination
of Saturated and Unsaturated Cytosine}



\author[1]{Kabir Uddin}%
\author[2]{Md.  Islam}%
\author[3]{Shahidul  Islam}%
\author[4]{Raymond Poirier}%
\affil[1]{North South University}%
\affil[2]{Bangladesh Council of Scientific and Industrial Research, BCSIR, Laboratories, Chattogram-4220, Bangladesh}%
\affil[3]{University of Illinois Chicago}%
\affil[4]{Memorial University of Newfoundland}%


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  \date{May 25, 2024}


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\begin{abstract}
Ab initio calculations were carried out to understand the effect of
electron donating groups (EDG) and electron withdrawing groups (EWG) at
the C5 position of cytosine (Cyt) and saturated cytosine (H2Cyt) of the
deamination reaction. Geometries of the reactants, transition states,
intermediates, and products were fully optimized at the B3LYP/6-31G(d,p)
level in the gas phase as this level of theory has been found to agree
very well with G3 theories. Activation energies, enthalpies, and Gibbs
energies of activation along with the thermodynamic properties (\selectlanguage{greek}Δ\selectlanguage{english}E, \selectlanguage{greek}Δ\selectlanguage{english}H,
and \selectlanguage{greek}Δ\selectlanguage{english}G) of each reaction were calculated. A plot of the Gibbs energies
of activation (\selectlanguage{greek}Δ\selectlanguage{english}G++) for C5 substituted Cyt and H2Cyt against the Hammett
\selectlanguage{greek}σ-\selectlanguage{english}constants reveal a good linear relationship. In general, both EDG and
EWG substituents at the C5 position in Cyt results in higher \selectlanguage{greek}Δ\selectlanguage{english}G++ and
lower \selectlanguage{greek}σ \selectlanguage{english}values compared to those of H2Cyt deamination reactions. C5
alkyl substituents (-H, -CH3, -CH2CH3, -CH2CH2CH3) increase \selectlanguage{greek}Δ\selectlanguage{english}G++ values
for Cyt, while the same substituents decrease \selectlanguage{greek}Δ\selectlanguage{english}G++ values for H2Cyt which
is likely due to steric effects. However, the Hammett \selectlanguage{greek}σ-\selectlanguage{english}constants were
found to decrease for both the Cyt and H2Cyt. Both \selectlanguage{greek}Δ\selectlanguage{english}G++ and \selectlanguage{greek}σ \selectlanguage{english}values
decrease for the substituents Cl and Br in the reaction Cyt, while \selectlanguage{greek}Δ\selectlanguage{english}G++
values increase and \selectlanguage{greek}σ \selectlanguage{english}decrease in the reaction H2Cyt. This may be due to
high polarizability of bromine which results in a greater stabilization
of the transition state in the case of bromine compared to chlorine.
Regardless of the substituent at C5, the positive charge on C4 is
greater in the TS compared to the reactant complex for both the Cyt and
H2Cyt. Moreover, as the charges on C4 in the TS increase compared to
reactant, \selectlanguage{greek}Δ\selectlanguage{english}G++ also increase for the C5 alkyl substituents (-H, -CH3,
-CH2CH3, -CH2CH2CH3) in Cyt, while \selectlanguage{greek}Δ\selectlanguage{english}G++ decrease in H2Cyt. In addition,
analysis of the frontier MO energies for the transition state structures
shows that there is a correlation between the energy of the HOMO-LUMO
gap and activation energies.%
\end{abstract}%



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\textbf{Hosted file}

\verb`New Insight into the Substituent Effects on the Hydrolytic Deamination of Saturated and Unsaturated Cytosine.doc`  available at \url{https://authorea.com/users/488699/articles/572739-new-insight-into-the-substituent-effects-on-the-hydrolytic-deamination-of-saturated-and-unsaturated-cytosine}

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