We have examined the adsorption of thymine on (111), (100), and (210) gold single-crystal surfaces. The adsorption behavior on these three surfaces has been investigated by classical electrochemical methods like cyclic voltammetry and capacitance−potential measurements. Additionally in situ scanning tunneling microscopy (STM) and ex situ photoelectron spectroscopy (XPS) measurements have been performed for the adsorption of thymine on the (111) surface. The capacitance measurements as well as cyclic voltammetry investigations show the three adsorption states of thymine on all Au electrodes. The first adsorption state refers to a random adsorption of thymine molecules at negative surface charges. The second state can be characterized as a condensed but weakly adsorbed adlayer on the (100) and (111) crystals, whereas a noncondensed state has been found on the (210) surface. The condensed thymine film is stabilized mainly by hydrogen bonding. High-resolution STM images for this film on the (111) electrode point to an ordered adlayer with a unit cell which is incommensurate with the underlying Au surface. The images indicate flat adsorbing thymine molecules in this state. The third adsorption state is characterized by charge transfer from deprotonated thymine molecules to the gold surface. XPS data show one chemically modifed nitrogen atom for the chemisorbed thymine film. This adsorption state shows a commensurate 2√3 × 2√3 overstructure in the STM image. The STM images are interpreted by stacks of adsorbed thymine molecules with the molecular plane perpendicular to the surface. The stacks are connected by coadsorbed water molecules. The molecules are bound by a deprotonated nitrogen to the surface.

Microorganisms accumulate molar concentrations of compatible solutes like ectoine to prevent proteins from denaturation. Direct structural or spectroscopic information on the mechanism and about the hydration shell around ectoine are scarce. We combined surface plasmon resonance (SPR), confocal Raman spectroscopy, molecular dynamics simulations, and density functional theory (DFT) calculations to study the local hydration shell around ectoine and its influence on the binding of a gene-5-protein (G5P) to a single-stranded DNA (dT25). Due to the very high hygroscopicity of ectoine, it was possible to analyze the highly stable hydration shell by confocal Raman spectroscopy. Corresponding molecular dynamics simulation results revealed a significant change of the water dielectric constant in the presence of a high molar ectoine concentration as compared to pure water. The SPR data showed that the amount of protein bound to DNA decreases in the presence of ectoine, and hence, the protein-DNA dissociation constant increases in a concentration-dependent manner. Concomitantly, the Raman spectra in terms of the amide I region revealed large changes in the protein secondary structure. Our results indicate that ectoine strongly affects the molecular recognition between the protein and the oligonucleotide, which has important consequences for osmotic regulation mechanisms.

Ectoine is an important osmolyte, which allows microorganisms to survive in extreme environmental salinity. The hygroscopic effects of ectoine in pure water can be explained by a strong water binding behavior whereas a study on the effects of ectoine in salty solution is yet missing. We provide Raman spectroscopic evidence that the influence of ectoine and NaCl are opposing and completely independent of each other. The effect can be explained by the formation of strongly hydrogen-bonded water molecules around ectoine which compensate the influence of the salt on the water dynamics. The mechanism is corroborated by first principles calculations and broadens our understanding of zwitterionic osmolytes in aqueous solution. Our findings allow us to provide a possible explanation for the relatively high osmolyte concentrations in halotolerant bacteria.