Gas phase structural stability of neutral and zwitterionic forms of alanine in presence of (H2O)n=1–7 clusters: A density functional theory study

Abstract

In the present contribution we have examined the gas phase structural stability of zwitterionic alanine (ZAla) relative to the stability of neutral alanine (Ala) in presence of water clusters of size n=1–7 using Density Functional Theory (DFT) calculations. The gas phase structural energy, and thermodynamical parameters of Ala–(H2O)n=1–7 and ZAla–(H2O)n=2–7 complexes are calculated at B3LYP/6-311++G(d,p) level of theory. We do not get stable structure of ZAla and ZAla+(H2O) in gas phase. This implies that the hydrogen bonding with one water molecule does not produce stable structure of ZAla. However, we found the existence of stable structure of ZAla–(H2O)2 complex in gas phase. This essentially means that at least two water molecules are required to produce stable structure of ZAla in gas phase. Further, we also observed that the Ala–(H2O)2 is relatively more stable than that of ZAla–(H2O)2. The optimized energy of Ala–(H2O)3 and ZAla–(H2O)3 complexes is found to be almost same and thus these two complexes are said to be isoenergetic. The ZAla–(H2O)n complex is found to be structurally as well as thermodynamically more stable than that of Ala–(H2O)n for n⩾4. It indicates that the possibility of finding the ZAla–(H2O)n complex is larger than that of Ala–(H2O)n for n⩾4 in gas phase. The above observations are also well supported by the thermodynamical parameters such as: Gibbs energy, enthalpy and entropy of the complexes.