Alanine Water Complexes Vanesa Vaquero, # M. Eugenia Sanz, § Isabel Peñ a, Santiago Mata, Carlos Cabezas, Juan C. Ló pez, and Jose ́ L. Alonso* Grupo de Espectroscopia Molecular (GEM), Edificio Quifima, Laboratorios de Espectroscopia y Bioespectroscopia, Unidad Asociada CSIC, Universidad de Valladolid, 47005 Valladolid, Spain * S Supporting Information ABSTRACT: Two complexes of alanine with water, alanine- (H 2 O) n (n = 1,2), have been generated by laser ablation of the amino acid in a supersonic jet containing water vapor and characterized using Fourier transform microwave spectroscopy. In the observed complexes, water molecules bind to the carboxylic group of alanine acting as both proton donors and acceptors. In alanine-H 2 O, the water molecule establishes two intermolecular hydrogen bonds forming a six-membered cycle, while in alanine- (H 2 O) 2 the two water molecules establish three hydrogen bonds forming an eight-membered ring. In both complexes, the amino acid moiety is in its neutral form and shows the conformation observed to be the most stable for the bare molecule. The microsolvation study of alanine-(H 2 O) n (n = 1,2) can be taken as a first step toward understanding bulk properties at a microscopic level. ■ INTRODUCTION The role of water as solvent is crucial in many biological processes because solvation may influence the structure and properties of the biomolecules involved by changing their function and reactivity. 1-4 A relevant example is that of amino acids, which exist as neutral species (NH 2 -CH(R)-COOH) when isolated in the gas phase but as doubly charged zwitterions ( + NH 3 -CH(R)-COO - ) upon solvation. This change in the preferred form of amino acids is driven by the establishment of hydrogen bonds between the functional groups of the amino acid and water molecules. Knowledge of how this phenomenon proceeds at a molecular level is essential to develop a deeper understanding of the influence of water on biological processes such as protein-protein interactions and protein folding, where both water and amino acids participate. 3,4 The interactions of amino acids with water are difficult to study in condensed phases because of the interplay between inter- and intramolecular interactions and the dynamical nature of the hydrogen bonds. Isolation of amino acids in the gas phase provides an ideal environment to investigate and control solvent effects because water molecules can be sequentially added (microsolvation), eventually bridging the gap between the gas and solution phases. The number of water molecules necessary to stabilize the zwitterion rather than the neutral form of the amino acid can thus be investigated. In addition, because individual amino acids and their hydrated complexes are observed, inter- and intramolecular interactions (mainly hydrogen bonds) between the different moieties can be revealed. Rotational spectroscopy is the most incisive tool available for the structural characterization of gas-phase biomolecules. It allows unambiguous identification of different conformers of a given species and determination of molecular structures with extreme accuracy. In our research group, rotational spectros- copy has been coupled to laser ablation and molecular beams (LA-MB-FTMW spectroscopy) 5-7 to investigate the structures and conformations of biomolecules. The power of this approach is highlighted by recent studies on amino acids, 8-13 nucleic acid bases, 14-17 and drugs. 18 Generation of amino acid-water complexes is not favored in the hot plasma created by the laser pulse used to vaporize the amino acid. Until 2006, it was not possible to observe and characterize the 1:1 glycine- water complex. 19 Although gaining experimental insight into the effects of solvation of amino acids remains a challenge, recent improvements in our spectrometers made possible the observation of two water molecules complexed with glycine. 20 We further explore the process of microsolvation by investigating the complexes of the amino acid alanine with water. Questions to consider in the study include the determination of the preferred binding sites for water, the type of hydrogen bonds established between water and alanine, and the examination of possible changes in the conformational preferences of alanine upon solvation. These aspects have been taken into account in several theoretical investigations of mono- and dihydrated alanine reported in the literature, 21-26 where different arrangements for interaction between the monomers Received: January 24, 2014 Revised: March 7, 2014 Published: March 11, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 2584 dx.doi.org/10.1021/jp500862y | J. Phys. Chem. A 2014, 118, 2584-2590