Glycine homopeptides: the effect of the chain length on the crystal structure and solid state reactivity

Abstract

A series of linear oligoglycines has been studied using X-ray diffraction, thermal analysis complemented with simultaneous FTIR spectroscopy, gas chromatography – mass spectrometry analysis, and ¹H/¹³C NMR. The new and previously reported data are rationalized to reveal the effect of the chain size on the crystal structure, molecular conformation and thermal stability of the oligopeptides in the solid state, as well as thermally induced transformations (pyrolysis) in the solid state and the gaseous phase. Tetraglycine (4G) and pentaglycine (5G) form triclinic crystals, P, where they adopt a zwitterionic form and fully extended chain conformation. The antiparallel β sheet arrangement is similar to that in β-triglycine (β3G) and α-diglycine (α2G) but differs from the Form I of polyglycine (polyG). The stability sequence α2G < β3G < 4G < 5G has been attributed to the stabilizing environment in the solid phase. For all studied glycines, the thermal degradation is a kinetic process complicated by multiple parallel pathways and sensitivity to the experimental conditions. These pathways include defragmentation reactions (mainly deamination and decarboxylation) and condensation reactions (formation of new peptide bonds), the latter yielding cyclo-diglycine (c2G, or 2,5-diketopiperazine or DKP) as a major product. Further pyrolysis results in polymerization (solid samples) or further fragmentation (gaseous phase). The overall cyclization reaction α2G(solid) = c2G(solid) + H₂O(gas) is endothermic with ΔH = 63.7(4) kJ mol⁻¹ at 492 K. The standard enthalpy of the reaction α2G(solid) = c2G(solid) + H₂O(liquid) was estimated to be ΔH²⁹⁸ ≈ 18 kJ mol⁻¹ meaning the reaction is thermodynamically unfavorable under ambient conditions. Finally, the cyclization reaction mechanisms are discussed for the whole series studied.