Adsorption and thermal condensation mechanisms of amino acids on oxide supports. 1. Glycine on silica.
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.
Meng
Ming
M
Laboratoire de Réactivité de Surface, UMR CNRS 7609, Université Pierre et Marie Curie, case courrier 178, 4 place Jussieu, 75252 Paris Cedex 05, France.
Stievano
Lorenzo
L
Lambert
Jean-François
JF
D000327:Adsorption; D000596:Amino Acids / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D010087:Oxides / Q000737:chemistry*; D010455:Peptides / Q000138:chemical synthesis; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D012821:Silanes / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013057:Spectrum Analysis; D013696:Temperature
D000596:Amino Acids; D010087:Oxides; D010455:Peptides; D000644:Quaternary Ammonium Compounds; D012821:Silanes; C082343:silanol; D012822:Silicon Dioxide; D005998:Glycine