Assembly and Thermal-Induced Polymerization of Histidine on Fumed Silica Surfaces

A thorough investigation into the thermal condensation reaction of histidine (His) on fumed silica nanoparticles (FSN) was performed. Thermogravimetric analysis (TGA) was used to track thermal events and characterize the optimal temperature for thermal condensation reactions for His adsorbed to FSN. Lower-temperature thermal cycling was used to investigate the possibility of linear polymerization. The products were characterized using one- (1D) and two-dimensional (2D) solution and solid-state nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The formation of His diketopiperazine (DKP) was the major product and is produced at two distinct thermal condensation temperatures. Under low-temperature thermal condensation conditions, a linear His-His dipeptide is also formed which is relevant to origin of life research and points to some potential conditions for promoting His linearization. Possible explanations for two discrete thermal events for His/FSN complexes are presented that indicate variable His binding modes influence His polymerization and that molecular reorientation may be required for complete conversion to DKP. DTG of other amino acids on FSN are included for context and illustrate that two thermal events are characteristic of amino acids with a functional basic side chain.