Phosphorylation status of peptide monolayers modulates hydrogen bonding and orientations of nematic liquid crystals

We report on the azimuthal orientations of nematic liquid crystals (LCs) on monolayers of dipeptides and tripeptides containing cysteine (C) and either tyrosine (Y), serine (S) or threonine (T). Measurements performed with 5CB and TL205 (with and without doping with trimethylamine), and dipeptide monolayers in which we removed the –OH groups of CY, CS or CT, led us to conclude that hydrogen bonding strongly influences LC orientations on CY but not CS or CT monolayers. Chemical force microscopy and cyclic voltammetry support the hypothesis that intra-monolayer hydrogen bonding within monolayers of CS and CT occurs preferentially to hydrogen bonding with 5CB. In contrast, the orientations of the LCs on tripeptide monolayers of CAY, CAS or CAT, where A is alanine, were all influenced by hydrogen bonding, consistent with a decrease in surface density of tripeptides (as compared to dipeptides; supported by x-ray photoelectron spectroscopy) and increase in hydrogen bonding with the LC overlayer. Phosphorylation of monolayers that oriented 5CB through hydrogen bonding led to easily measured changes in LC orientation. Overall, our results advance the rational design of LC materials with internal order that is coupled to biologically relevant chemical modifications of oligopeptides and proteins.