甘氨酸在SiO_2表面的吸附及热缩合反应

Adsorption and Thermal Condensation of Glycine on SiO_2

考察了甘氨酸水溶液平衡浓度和pH值对其在SiO2 表面吸附行为的影响,采用DTG ,XRD和FT IR等手段对甘氨酸在SiO2 表面的吸附量及晶相结构等进行了表征.结果表明,甘氨酸在SiO2 上的吸附不符合Langmuir吸附模型,当甘氨酸水溶液平衡浓度小于0 0 73mol/L时,主要为特定位置吸附,此时SiO2 对甘氨酸的吸附源于其表面≡Si-O-基团与甘氨酸离子的-NH+3 端的相互作用;当平衡浓度大于0 0 73mol/L时,呈多层吸附,形成α和β两种晶型的甘氨酸.用TPD MS对甘氨酸的热缩合行为进行了考察,结合FT IR结果表明,产物为环状二聚体哌嗪二酮(DKP) ,没有检测到线式二聚体以及表面硅酯类物种形成.SiO2 对表面吸附的甘氨酸分子产生了诱导活化,使其热缩合温度与体相α甘氨酸相比降低了5 0℃左右.

The study on the performance of amino acid adsorption, activation and thermal condensation on inorganic oxides is helpful to make clear the essential function of silica, alumina and clay during the prebiotic chemical evolution, such as the formation of peptide bonds. In this work, the adsorption and thermal condensation performance of glycine on silica were investigated. Glycine was adsorbed on the surface of silica from aqueous solutions with various concentrations and pHs, and the samples were characterized by DTG, XRD, FT-IR and TPD-MS. It is shown by DTG results that the adsorption performance does not meet the Langmuir model. XRD and FT-IR results indicate that when the equilibrium concentration is below 0.073 mol/L, the adsorption may take place only on specific sites; and it is probably resulted from the interaction between the NH3+ moiety of glycine and the silanolate group (&3bond; Si-O-) of silica. At higher equilibrium concentrations (> 0.073 mol/L), the precipitation of beta-glycine on silica surface was observed. The adsorbed or deposited glycine molecules react with each other to form peptide bonds at 166 similar to 220 degrees C, which is considerably lower than that of pure a-glycine (240 similar to 250 degrees C). The main product of the thermal condensation reaction is cyclodimer diketopiperazine (DKP), no linear dimer and surface acyl species were detected. The reconstruction of hydrogen bonds in the adsorbed glycine molecules makes the condensation reaction proceeded easier. Additionally, the product DKP on silica can be totally degradated below 600 degrees C in the oxidative atmosphere, while the DKP produced from bulk glycine is difficult to be degradated without silica under the same condition.