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Molecular orbital theory of the hydrogen bond. XXX. Water‐cytosine complexes
Author(s) -
Del Bene Janet E.
Publication year - 1983
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540040213
Subject(s) - hydrogen bond , chemistry , dimer , cytosine , crystallography , intermolecular force , water dimer , molecule , ab initio , computational chemistry , molecular orbital , stereochemistry , dna , organic chemistry , biochemistry
Ab initio SCF and SCF ‐ CI calculations with the STO ‐3G basis set have been performed to investigate the structures and energies of water–cytosine complexes and the intermolecular water–cytosine surface in the cytosine molecular plane. Although there are six nominal hydrogen‐bonding sites in this plane, only three dimers are distinguishable in the ground state. The most stable has an energy of −10.7 kcal/mol, and is found in the N 1 H and O 2 region. An asymmetric cyclic structure in which the water molecule bridges adjacent N 1 H and O 2 sites is the preferred form of this dimer. The dimer in the region between O 2 and N 4 H′ of the amino group is slightly less stable at −10.4 kcal/mol, and also has an asymmetric cyclic structure as the preferred structure, with the water molecule bridging amino N 4 H′ and N 3 hydrogen‐bonding sites. The third dimer has the amino group as the proton donor to water through the hydrogen cis to C 5 , and a stabilization energy of −7.0 kcal/mol. The water‐cytosine surface is characterized by deeper minima and higher barriers than the water‐thymine surface and by a decreased mobility of the water molecule between adjacent hydrogen‐bonding sites. Absorption of energy by the C 2 O group leads to the first n → π* excited state in which interactions of water with O 2 are broken. The water‐cytosine dimers remain bound in this state, but may change structurally. In the second n → π* state interactions between water and N 3 are no longer stabilizing. As a result, the dimer in the O 2 and N 4 H′ region collapses to either a dimer with water the proton donor to O 2 , or one with N 4 H′ the proton donor to water. The other two dimers remain bound. All excited dimers are destabilized on vertical excitation relative to the ground state.