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| - Cooperative Hydrogen-Bonding in Adenine−Thymine and Guanine−Cytosine Base Pairs.Density Functional Theory and Møller−Plesset Molecular Orbital Study
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| - The cooperative contributions to the H-bonding interaction energies of the adenine−thymine and guanine−cytosine base pairs have been evaluated using molecular orbital theory. The energies of the individual bondsin each base pair were ascertained by using models structures that keep one H-bond at a time intact byrotating one base with respect to the other about the axis of each H-bond to form structures with the basesperpendicular to each other. The energies of the individual H-bonds calculated in this way are compared withthose of the planar base pairs. Optimized geometries were obtained using ab initio molecular orbital theorywith electron correlation (MP2/D95**) and density functional theory (B3LYP/D95**). The cooperativecontributions are of similar magnitude for each base pair. However, since the A·T overall interaction is weaker,the cooperative interaction provides 31% of its stability versus only 12−16% for G·C. The relatively smallercooperative contribution to G·C is due to the difficulty of forming three optimal H-bonds between two rigidmolecules. Structural modifications that might strengthen one H-bond tend to weaken another. As a result thecentral H-bond of G·C appears to be compressed by the attractive interaction in the two outer H-bonds. Tothe extent that these observations can be generalized, they should be important to the design of materials thatutilize H-bonding motifs for self-assembly.
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