Glycine and lysine adsorption and reactivity on the surface of amorphous silica

Abstract

In order to better control the adsorption of proteins on mineral surfaces, it is useful to gain a thorough understanding of the behaviour of their constituting monomers, namely, amino acids. We have therefore investigated the adsorption of glycine, lysine, and other small biomolecules on well-characterised surfaces of silica. While similar systems have been studied before, these studies usually concentrated on evidencing the effect of surfaces on peptide bond formation in a phenomenological, mostly macroscopic approach. In contrast, we have considered the adsorption process from the view-point of the surface, using knowledge previously gained on the molecular identification of surface functional groups to better characterise their interaction with amino acids, both from the aqueous phase and from the vapour phase. We combined macroscopic level information such as adsorbed amounts, pH dependence, or TGA, with molecular characterisation by vibrational spectroscopies and 13 C NMR of the adsorbed molecules. In parallel, we have carried out molecular modelling of candidate clusters containing the amino acid and the adsorption site by DFT. The structures of lowest energy were also those that best reproduced the observed spectroscopic properties. Different adsorption mechanisms can be postulated, corresponding to different spectroscopic signatures of the amino acid/adsorption site complexes. On silica surfaces, this implies cooperative hydrogen bonding in a kind of molecular recognition. The existence of molecular recognition is also evidenced by experiments on the coadsorption of different amino acids, where strong selectivity effects may be observed. Such phenomena may have much practical significance in the fields of biofilms and prebiotic chemistry.