Transition State of <scp>l</scp>-Alanine for Normal to Zwitterion Conversion and Its Interaction Study in Water Solvation

Alanine in the zwitterion form participated in many biophysiological activities/processes that occurred in the human body, which is abundant with water. When introducing the normal form of l-alanine to water, it will be converted to a zwitterion. The present paper deals with the study of the possibility and prediction of the transition state (TS) of l-alanine between its normal and zwitterion forms and its minimum energy path (MEP) by using the nudged elastic band (NEB) method in a density functional theory framework. Simultaneously, the paper unveils the perspective of the coexisting interactions (in terms of interaction energy) by implementing the tight binding (GNF2-XTB) method with a docking approach. The intratransportation of hydrogen by NEB is depicted via a skeletal central carbon atom, which is observed at TS for the final MEP. Few properties of TS, like electronic structure, thermochemistry, and nature of global indices of reactivity (quantum descriptors), were studied to understand changes. The study further conducted XTB-docking to understand interactions of TS by docking a single molecule of normal-form, TS-form, and zwitterion-form. Also, with the interaction preference of the first 10 water molecules by docking on normal, TS, and zwitterion forms, the lowest interaction energy per docked water on TS was reported. It covered noncovalent interactions.