Thermal Stabilities of the Microhydrated Zwitterionic Glycine: A Kinetics and Dynamics Study

Aspartame and Its Microhydrated Aggregates Revealed by Laser Spectroscopy: Water-Sweetener Interactions in the Gas Phase

The popular sweetener, aspartame, is an agonist of the tongue's sweet taste receptor. How water molecules affect its conformation or which aspartame atoms are more prone to interact with solvent are helpful questions to understand its activity in different environments. Here, the combination of IR-UV spectroscopic techniques with computational simulations has been successfully applied to characterize aspartame·water0-2 clusters, showing that the addition of water molecules simplifies the conformational panorama of aspartame, favoring the formation of folded structures by interaction with the polar part of the molecule.

Amino Acid Polymerization on Silica Surfaces

The polymerization of unactivated amino acids (AAs) is an important topic because of its applications in various fields including industrial medicinal chemistry and prebiotic chemistry. Silica as a promoter for this reaction, is of great interest owing to its large abundance and low cost. The amide/peptide bond synthesis on silica has been largely demonstrated but suffers from a lack of knowledge regarding its reaction mechanism, the key parameters, and surface features that influence AA adsorption and reactivity, the selectivity of the reaction product, the role of water in the reaction, etc. The present review addresses these problems by summarizing experimental and modeling results from the literature and attempts to rationalize some apparent divergences in published results. After briefly presenting the main types of silica surface sites and other relevant macroscopic features, we discuss the different deposition procedures of AAs, whose importance is often neglected. We address the possible AA adsorption mechanisms including covalent grafting and H-bonding and show that they are highly dependent on silanol types and density. We then consider how the adsorption mechanisms determine the occurrence and outcome of AA condensation (formation of cyclic dimers or of long linear chains), and outline some recent results that suggest significant polymerization selectivity in systems containing several AAs, as well as the formation of specific elements of secondary structure in the growing polypeptide chains.

Unveiling Zwitterionization of Glycine in the Microhydration Limit

Charge separation under solvation stress conditions is a fundamental process that comes in many forms in doped water clusters. Yet, the mechanism of intramolecular charge separation, where constraints due to the molecular structure might be intricately tied to restricted solvation structures, remains largely unexplored. Microhydrated amino acids are such paradigmatic molecules. Ab initio simulations are carried out at 300 K in the frameworks of metadynamics sampling and thermodynamic integration to map the thermal mechanisms of zwitterionization using Gly(H₂O) _n with n = 4 and 10. In both cases, a similar water-mediated proton transfer chain mechanism is observed; yet, detailed analyses of thermodynamics and kinetics demonstrate that the charge-separated zwitterion is the preferred species only for n = 10 mainly due to kinetic stabilization. Structural analyses disclose that bifurcated H-bonded water bridges, connecting the cationic and anionic sites in the fluctuating microhydration network at room temperature, are enhanced in the transition-state ensemble exclusively for n = 10 and become overwhelmingly abundant in the stable zwitterion. The findings offer potential insights into charge separation under solvation stress conditions beyond the present example.

Can THF hydrogen bond to glycine as strong as water?

Hydrogen bond complexation between glycine and THF and between glycine and water involving four lowest-energy glycine conformers have been studied. The complexes have been investigated in the gas phase at the ab initio molecular orbital theory (MP2) with aug-cc-pVDZ basis set and density functional theory (B3LYP) with aug-cc-pVTZ basis set. Bader’s theory of atoms in molecules (AIM), natural bond orbital (NBO), and symmetry adapted perturbation theory (SAPT) analyses are employed to elucidate the interaction characteristics in the complexes. The premise that the hydrogen bond donor ability of the O–H group of the carboxyl group dominates the interaction between glycine and THF and between glycine and water is confirmed. It is found that in comparison with water, THF binds more strongly to glycine. The quantum studies indicate that contribution of N–H···O and C–H···O hydrogen bonds in the complexes, although lower in magnitude to O–H···O interactions, play an important role in the stability of complexes. The blue and red shifts in the stretching frequencies of the hydrogen bond donors X–H (X = O, C, N) have also been related to stabilization energies. Decomposition of the stabilization energy based on the SAPT method clearly indicates the dominant role of the electrostatic interactions in all the complexes under study; however, induction and dispersion interaction terms are relatively higher in glycine–THF complexes.

A proline-based neuraminidase inhibitor: DFT studies on the zwitterion conformation, stability and formation

The designs of potent neuraminidase (NA) inhibitors are an efficient way to deal with the recent "2009 H1N1" influenza epidemic. In this work, density functional calculations were employed to study the conformation, stability and formation of the zwitterions of 5-[(1R,2S)-1-(acetylamino)-2-methoxy-2-methylpentyl]-4-[(1Z)-1-propenyl]-(4S,5R)-d-proline (BL), a proline-based NA inhibitor. Compared to proline, the zwitterion stability of BL is enhanced by 1.76 kcal mol(-1) due to the introduction of functional groups. However, the zwitterion of BL will not represent a local minimum on the potential energy surface until the number of water molecules increases up to two (n = 2). With the addition of two and three water molecules, the energy differences between the zwitterions and corresponding canonical isomers were calculated at 3.13 and -1.54 kcal mol(-1), respectively. The zwitterions of BL are mainly stabilized by the H-bonds with the water molecules, especially in the case of three water molecules where the carboxyl-O atoms are largely coordination-saturated by three H-bonds of medium strengths, causing the zwitterion stability even superior to the canonical isomer. With the presence of two and three water molecules, the energy barriers for the conversion processes from the canonical isomers to the zwitterions are equal to 4.96 and 3.13 kcal mol(-1), respectively. It indicated that the zwitterion formation is facile to take place with addition of two molecules and further facilitated by more water molecules. Besides, the zwitterion formation of BL is finished in a single step, different from other NA inhibitors. Owing to the above advantages, BL is a good NA inhibitor candidate and more attention should be paid to explorations of BL-based drugs.