Estimation The Intramolecular N-H...O=C Hydrogen Bonding Energies For The Peptides Via Substition Method

The hydrogen bond plays a very vital role in many life processes and is one of the most frequently used terms in chemistry and biology. Hydrogen bonds can be separated into two categories: intermolecular hydrogen bond and intramolecular hydrogen bond. Theoretically, intermolecular interactions are usually investigated through a supermolecular approach. However, such an approach cannot be used to evaluate the intramolecular interactions. At present, some theoretical procedures were proposed to investigate the energies of intramolecular hydrogen bonds for special systems, such as cis-trans method, ortho-para method, and isodesmic reaction method. None of them is suitable to predict the intramolecular N-H…O=C hydrogen bonding energies for peptides.In this paper, a method was proposed to evaluate the intramolecular N-H…O=C hydrogen bonding energies for peptides. The reasonability of the substitution method was explained. And then different kinds of intramolecular N-H…O=C hydrogen bonding energies for peptides were evaluated by the substitution method. The main content of this paper are as follows:(1) A new method was proposed for predicting the intramolecular N-H…O=C hydrogen bonding energies for peptides. In this method CH2 group was used to substitute the NH group involved in the intramolecular N-H…O=C hydrogen bond, so the new method proposed in this paper was named as substitution method. The intramolecular hydrogen bonding energies for some special systems were evaluated by means of the substitution method, cis-trans method, and supermolecular approach. The intramolecular hydrogen bonding energies predicted by the substitution method agree well with the values obtained by the cis-trans method and the supermolecular approach.(2) Evaluating the intramolecular N-H…O=C hydrogen bonding energies forα- andβ-peptides. The structural features of the intramolecular hydrogen bonds for the sixα-peptide conformers and the fourteenβ-peptide conformers were analyzed detailed. The hydrogen bonding energies for the twentyα- andβ-peptide conformers were evaluated by the substitution method. The results showed that the intramolecular hydrogen bonding energies obtained by the substitution method were in agreement with the structural features of the intramolecular hydrogen bonds. The values obtained in this paper are helpful for deeply understanding the structure and stability of theα- andβ-peptide conformers.(3) Predicting the intramolecular C-H…O=C hydrogen bonding energies. In order to perfect the substitution method proposed by us, the intramolecular C-H…O=C hydrogen bonding energies should be predicted. An acetamide and propaldehyde dimer was extracted from the each optimal structure containing the intramolecular C-H…O=C hydrogen bonding. The supermolecular approach was then used to predict the C-H…O=C intermolecular hydrogen bonding energies. At the same time, the proportions of the C-H…O=C hydrogen bonding energies in the N-H…O=C hydrogen bonding energies were also calculated.