Modeling the Microhydration of Protonated Alanine

Entropic Effects on the Aqueous Microsolvation of Protonated Glycine and Protonated β-Alanine. Hybrid Density Functional Theory Born-Oppenheimer Molecular Dynamics Studies

Recent low-temperature infrared-based experimental studies provided information about the effects of aqueous microsolvation on the intramolecular hydrogen bond of protonated glycine and β-alanine [J. Phys. Chem. A 2019, 123, 3355]. Here we address the temperature-dependent entropic effects on the aqueous microsolvation patterns of these protonated amino acids using the AAH⁺(H₂O)_n (n = 1-8) cluster model at 50 K and room temperature with Born-Oppenheimer molecular dynamics using a calibrated hybrid density functional. The C_COOH-O_w, N-O_w, and center-of-mass-O_w radial distribution functions provide accurate structural data and temperature-dependent water coordination numbers vs. solvation degree. The solvation patterns for protonated glycine at 50 K show structural features in agreement with previous static optimizations. However, entropic effects at room temperature play a crucial role in the evolution of the intramolecular HB strength vs. solvation degree for both protonated amino acids. With increasing hydration entropic effects favor the making of solvent hydrogen bond networks over full solvation of protonated glycine. At room temperature four water molecules are needed to build the first solvation shell for protonated glycine while five are required for protonated β-alanine. A new statistical Cumulative Percentage of Structures (CPS) scheme is proposed; when the CPS data are analyzed in light of the empirical formula of Rozenberg et al. [Phys. Chem. Chem. Phys. 2000, 2, 2699] and the hydrogen bond relative strength (HBRS) criteria of Jeffrey [An Introduction to Hydrogen Bonding; Oxford University: 1997] we can provide a detailed molecular mechanism for the weakening of the intramolecular hydrogen bond based on the average dynamical structures, which clearly reveals the temperature dependence of this process. The new CPS-HBRS scheme proposed here can be utilized using any type of molecular dynamics trajectory (classical, BOMD, CPMD, etc.).

Theoretical Study of the Microhydration the Chemical Warfare Agent Sulfur Mustard

A microhydration study of sulfur mustard (SM) was carried out using M06-2X, B3LYP, B3LYP-D3, and MP2 levels of theory with the 6-311++G(2d,2p) basis set. The changes in energetics, structural parameters and vibrational wavenumbers following the addition of up to three discrete water molecules to SM were analyzed. We observed slight changes in the geometry of SM upon microhydration. The stability of hydrated clusters is due to weak C-H···O-H hydrogen bonds. The free energy change for the formation of the clusters is positive at room temperature and becomes exergonic when the temperature decreases. The infrared stretchings of C-Cl of SM and O-H of water are redshifted upon the addition of water molecules. The findings from this work add to the literature of hydrated SM and can be useful in its detection and subsequent destruction.

Microhydration of Protonated Nα-Acetylhistidine: A Theoretical Approach

Extensive exploration of the potential energy surfaces of protonated Nα-acetylhistidine hydrated by 0-3 molecules of water was performed. The methodology combined hierarchical and genealogical (Darwin family tree) approaches using polarizable AMOEBA force field and M06 functional. It is demonstrated that this mixed approach allows recovering a larger number of conformers than the number recovered by using any one of the two methods alone. Hydration enthalpies of protonated Nα-acetylhistidine and of model compounds have been computed using higher theoretical methods, up to the G4MP2 procedure. Excellent agreement with experiment is observed for successive hydration of methylamonium and imidazolium cations using MP2/6-311++G(2d,2p)//M06/6-311++G(d,p) and G4MP2 methods, thereby validating the theory levels used for hydrated protonated Nα-acetylhistidine. It is found that the first hydration enthalpy of protonated Nα-acetylhistidine is ca. 10 kJ mol(-1) lower than that of imidazolium, a result explained by the local environment of the positively charged imidazolium moiety.

Ab initio investigation of the hydration of deprotonated amino acids

The complexation of five deprotonated anionic amino acids (glycine, L-alanine, L-valine, L-Aspartic acid, and L-glutamine) with one water molecule, has been investigated using a MP2/63-11++G(d,p) approach fully accounting for the basis set superposition errors. For each amino acid, several energetic minima have been identified, and we provide spectroscopic information allowing to discriminate them. Our results strongly suggest that two complexes should coexist under the experimental conditions for [Ala - H](-), [Val - H](-), and [Asp - H](-). Comparisons with the experimental enthalpies, entropies, and Gibbs free energies recently obtained by Wincel [J. Am. Soc. Mass Spectrom. 2008, 19, 1091-1097] show that our simulation reproduces the most significant structure/energy experimental trends, though the entropic changes induced by hydration are slightly overestimated.