A Computational Study of Microhydrated N-Acetyl-Phenylalaninylamide (NAPA): Kinetics and Thermodynamics

The formations of [NAPA-A(H2O)n (n = 1, 2, 3, 4)] complexes have been studied employing DFT/wB97XD/cc-pVTZ computational level to understand the kinetics and thermodynamics for the hydration reactions of N-acetyl-phenylalaninylamide (NAPA). Thermodynamic parameters such as reaction energy (E), enthalpy (H), Gibb’s free energy (G), specific heat capacity (Cv), entropy (S), and change of these parameters (ΔEr, ΔHr, ΔGr, ΔCr, and ΔSr) were studied using the explicit solvent model. The predicted values of H, G, C, and S increase with the sequential addition of water in NAPA-A due to the increase in the total number of vibrational modes. On the other hand, the value of ΔEr, ΔHr, and ΔGr increases (more negative to less negative) gradually for n = 1, 2, 3, and 4 that indicates an increase of hydration in NAPA-A makes exothermic to endothermic reactions. The barrier heights for the transition states (TS) of [NAPA-A(H2O)n (n = 1, 2, 3, 4)] complexes are predicted to lie at 4.41, 4.05, 3.72 and 2.26 kcal/mol respectively below the reactants. According to the calculations, the formations of [NAPA-A(H2O)1] and [NAPA-A(H2O)2] complexes are barrierless reactions because both water molecules are strongly bonded via two hydrogen bonds in the backbone of NAPA-A. On the contrary, the reactions of [NAPA-A(H2O)3] and [NAPA-A(H2O)4] complexation are endothermic and the barrier heights are predicted to stay at 6.30 and 10.54 kcal/mol respectively above the reactants. The free energy of activation (Δ‡G0) for the reaction of [NAPA-A(H2O)1], [NAPA-A(H2O)2], [NAPA-A(H2O)3], and [NAPA-A(H2O)4] complexation are 4.43, 4.28, 3.83 and 5.11 kcal/mol respectively which are very low. As well as the rates of reactions are 3.490 × 109 s-1, 4.514 × 109 s-1, 9.688 × 109 s-1, and 1.108 × 109 s-1 respectively which are very fast and spontaneous.