The structure of alanine anionic-zwitterionic dimers on Pd(111); formation of salt bridges

Abstract

The structure of alanine zwitterion-anion dimers previously proposed to form on Pd(111) is investigated using low-energy electron diffraction (LEED). Because of the propensity of amino acids on Pd(111) to undergo electron-beam-induced decomposition, LEED intensity versus beam voltage (I-V) curves were measured using a delay line detector LEED (DLD-LEED) system, which enables the complete LEED I-V curves to be obtained for an electron exposure of less than one electron per adsorbate. Since no long-range order is found following alanine adsorption on Pd(111), the adsorbate structure is determined from the substrate diffraction spots. The I-V data were analyzed using the CLEED: Automated Surface Structure package for an initial input structure from previous DFT calculations of the alanine zwitterion-anion dimer, the best fit structure yielded a satisfactory Pendry R-factor of 0.249, thus confirming the correctness of the originally proposed structure. Such anionic-zwitterionic dimers are a class of hydrogen-bonding intermolecular interactions in proteins that are dominated by direct electrostatic interactions, in particular for residues such a lysine and arginine, known as salt bridges. While such salt bridges interactions are relatively weak in biological systems, the attractive interaction energy between the anion and the zwitterion in the dimer on Pd(111) in vacuo is found to be ∼95 kJ/mol. It is proposed that the weaker binding biological systems occurs because the electrostatic screening in aqueous media weakens the electrostatic interaction between the anion and zwitterion, causing their structures to relax.