Water-Induced Structural Changes in the Membrane-Anchoring Monolayers Revealed by Isotope-Edited SERS

Water is required for biological function of tethered bilayer lipid membranes (tBLMs) consisting of a lipid bilayer anchored by a mixed self-assembled monolayer (SAM) to a solid support. In this work, water-induced structural changes in mixed SAMs and bonding with gold substrate were probed in situ by the isotope-edited SERS coupled with the first-principles calculations. The assignment of the bands was based on experimental analysis of deuterated 2-mercaptoethanol (ME-D-4) and quantum chemical modeling of adsorption complexes consisting of a Au cluster of 10 atoms. Evidence for interaction of the gauche conformer with the Au surface through both S and O atoms was obtained from the analysis of the Au S/Au-O vibrational mode near 303 cm(-1) and adsorption induced downshift of the C-O stretching band near 1100 cm(-1). First-principles calculations of adsorption complexes revealed shortening of the Au O bond upon hydrogen bonding of explicit water molecule to the OH group of ME-D-4. Intense bands in the SERS spectrum near 679 and 597 cm(-1) were assigned to C-S vibrational modes of adsorbed ME-D-4 in trans and gauche conformation, respectively. Analysis of relative intensities revealed a decrease in relative amount of trans conformers after 72 h incubation of the SAM in water. At the same time an increase in the population of a long carbon chain molecular anchor (WC14) in the all-trans hydrocarbon chain conformation was detected by SERS. Observed structural changes suggest water-induced clustering of long-chain anchors and conformational transition for short-chain thiols.