Models, electrostatic systems, hydrogen

At the same time, the CNDO/2 method describes fairly accurately those reactions in which electrostatic interactions rather than covalent bonding is the moving force. The explanation is simple the interactions of this type are governed by the character of the electron distribution. Thus, this method reproduces quite satisfactorily the systems with hydrogen bonding, and the protonation reactions of heteroatomic compounds where the reaction course is determined by the form of the electrostatic potential. The example of the model peptide given in Fig. 2.5 shows how well the CNDO/2 method can reproduce the picture of the electrostatic potential. 

Classical electrostatic modeling based on the Coulomb equation demonstrated that the model system chosen could account for as much as 85% of the effect of the protein electric field on the reactants. Several preliminary computations were, moreover, required to establish the correct H-bond pattern of the catalytic water molecule (WAT in Fig. 2.6). Actually, in the crystal structure of Cdc42-GAP complex [60] the resolution of 2.10 A did not enable determination of the positions of the hydrogen atoms. Thus, in principle, the catalytic water molecule could establish several different H-bond patterns with the amino acids of the protein-active site. Both classical and quantum mechanical calculations showed that WAT, in its minimum-energy conformation,... 

E. R. Lippincott The proposed model is certainly empirical. However, the internuclear potential function used for the terms V1 and F2 may be derived from a quantum mechanical model which lends support to their use in such a treat-ment of hydrogen bond systems. Professor Pauling is quite right in suggesting that the terms Vx and F2 may include some electrostatic contribution, since it is known that the internuclear potential function used correlates properties fairly well for partial polar bonds. Nevertheless the fact that additional terms of the electrostatic type are not needed to describe a number of the important properties of hydrogen bond systems, suggests that the covalent, repulsion and dispersions energy contributions are more important than the electrostatic contribution. 

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