National Centre for Ultrafast Processes , University of Madras , Taramani Campus, Chennai 600113 , India.
J Phys Chem B. 2018 Jun 28;122(25):6462-6470. doi: 10.1021/acs.jpcb.8b01629. Epub 2018 Jun 14.
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 OH···NH and OH···OC bonds are -9.5 and -7.1 kcal/mol, respectively. This suggests that the higher clusters are formed through OH···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···NH mode of hydrogen bonding, unlike the dimer which is formed through the OH···OC bond. Feasibility of the amino acid cluster to function as a precursor for polypeptide formation is examined because the orientation of the OH···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.
采用 DLPNO2013-CCSD(T) 水平的精确量子化学计算研究了气相中 l-丙氨酸二聚体和四聚体簇的低能结构的稳定性。研究发现，与氢键（HB）能量（分别为-4.1 到-14.2 和-32.2 到-40.1 kcal/mol）相比，二聚体（-0.3 到-0.6 kcal/mol）和四聚体（-1.3 到-2.5 kcal/mol）中的色散相互作用能对簇的稳定性影响较小。丙氨酸簇中的 HB 能量是通过从 DLPNO2013-CCSD(T)//B2PLYP/def2-TZVP 的结合能（BE）中减去色散相互作用能获得的。从二聚体结构中推断出的局部 HB 能量被发现适合于估计类似环境中的总 HB 能量。OH···NH 和 OH···OC 键的 BE 分别为-9.5 和-7.1 kcal/mol。这表明更高的簇是通过 OH···NH 键形成的，因为它们赋予了更高的稳定性。对四聚体中键合的分析表明，低能四聚体和更高的簇是通过 OH···NH 模式的氢键形成的，而不是通过 OH···OC 键形成的二聚体。因为 OH···NH 模式的氢键的取向适合于化学缩合，所以研究了氨基酸簇作为多肽形成前体的可行性。根据丙氨酸四聚体的构象稳定性，研究了在更高的簇中形成卷曲结构的倾向，以及在多肽中的倾向。
