Adsorption and Thermal Condensation Mechanisms of Amino Acids on Oxide Supports. 1. Glycine on Silica

Abstract

Glycine was adsorbed on the surface of a well-defined silica from aqueous solutions of variable concentrations and pHs. The adsorbed molecules were characterized using middle-IR and UV-vis-NIR spectroscopies. Except at the lowest pH (2.0), they were predominantly present on the surface as zwitterions. Two successive deposition mechanisms were evidenced with increasing glycine concentration. At low concentrations, glycine is specifically adsorbed on silica surface sites, probably through its NH3+ moiety. The pH dependence suggests that these sites may be silanolate groups (approximately equal to Si-O-). At higher concentrations, specific adsorption sites are saturated and surface-induced precipitation of beta-glycine is observed. The thermal reactivity of adsorbed/deposited glycine was then investigated by thermogravimetric analysis, in situ diffuse reflectance IR spectroscopy, and thermoprogrammed desorption coupled with mass spectrometry. Adsorbed glycine molecules react to form peptide bonds at a temperature considerably lower than that for bulk crystalline alpha-glycine. The main reaction product is the cyclic dimer diketopiperazine, with no evidence of the linear dimer. The activation mechanism is not diffusionally limited; the formation of "surface acyls", previously proposed for related systems, has not been evidenced here. These findings are of relevance for the evaluation of prebiotic peptide synthesis scenarios.

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…It is worth mentioning that this situation was not observed by Costa et al 556 due to the limited number of water molecules considered. In the work of Rimola et al, 322 the vibrational frequencies of the structure shown in Figure 69e were also computed and compared with the experimental IR results of Meng et al 210 (data shown in Table 16). In this case, the  as (COO -) and (NH 3 + ) modes useful for comparison are hidden by the stretching and bending modes of solvent water, which cover the 3900-3500 cm -1 and 1700-1600 cm -1 regions.…”

Comparison of ordered mesoporous materials sorption properties towards amino acids

“…Amino acids, the components of protein, play an important role in many fields, including food industry and solid-phase peptide synthesis (Meng et al 2004). They are also applied as building blocks for the production of pharmaceutical and agrochemical compounds (O'Connor et al 2006).…”

Theoretical and Experimental Study of the Adsorption of Neutral Glycine on Silica from the Gas Phase

“…More recently, we have started to investigate the adsorption of small biological molecules such as amino acids on the same support. 8 Here, we have attempted as a first step to model the adsorption of glycine on silica in conditions of low water activity. For meaningful comparison, experimental results have been obtained by adsorbing glycine from the gas phase.…”