Structural Stability in Dimer and Tetramer Clusters of L-Alanine in the Gas Phase and the Feasibility of Peptide Bond Formation

Stability in low-energy structures of the dimer and tetramer clusters of L-alanine in the gas phase is studied by accurate quantum chemical computations at the DLPNO2013-CCSD(T) level. It is found that the dispersion interaction energies in the dimer (-0.3 to -0.6 kcal/mol) and in the tetramer (-1.3 to -2.5 kcal/mol) have a small role in the stability of the clusters as compared to the hydrogen bond (HB) energies -4.1 to -14.2 and -32.2 to -40.1 kcal/mol, respectively. The HB energy in the alanine cluster is obtained from the binding energy (BE) of DLPNO2013-CCSD(T)//B2PLYP/def2-TZVP by subtracting the dispersion interaction energy. Local HB energies deduced from the dimer structures are found to be suitable to estimate total HB energies in similar environments. The BEs of OFF-NH and OH center dot center dot center dot OC bonds are -9.5 and -7.1 kcal/mol, respectively. This suggests that the higher clusters are formed through 01-1-NH bonds as they confer more stability. Analysis of bonding in the tetramer shows that the low-energy tetramer and higher clusters are formed through the OH center dot center dot center dot NH mode of hydrogen bonding, unlike the dimer which is formed through the OH center dot center dot center dot OC bond. Feasibility of the amino acid cluster to function as a precursor for polypeptide formation is examined because the orientation of the OH center dot center dot center dot NH mode of hydrogen bonding is suitable for chemical condensation. The propensity of forming coiled structures in higher clusters and thus in the polypeptides is examined based on the conformational stability in the tetramer of alanine.