Study of peptide bonding under ionizing radiation in solid condensed phase under an inert atmosphere: Gas emission and radiation-induced macromolecular defects in beta-sheet-rich polyglycine

Abstract

Proteins are components of the human body. In the treatment of cancers, these molecules can be degraded due to the effect of irradiation. The objective of this study is to understand, at the molecular scale, the evolution of these particular molecules under ionizing radiation. Homopolypeptides were used as models to investigate the effect of molecular structure on the degradation of proteins under ionizing radiation. Since very few studies have targeted this topic, this study was performed to understand the evolution of polyglycine, a homopolymer formed from glycine, the simplest amino acid. Polyglycine samples with the highest β-sheets ratio reported were prepared and irradiated with ionizing radiation at room temperature under an inert atmosphere. Macromolecular defects and gas emission were characterized by infrared spectroscopy and mass spectrometry respectively. By infrared spectroscopy, deconvolution of amide-related bands revealed a higher susceptibility of β-sheets to radiation-induced scissions compared to 3₁₀-helices. At very high doses (7.5 MGy), half of the initial structures were lost through structural reorganization due to the buildup of radiation-induced defects, such as secondary amides at chain ends or imine groups, resulting in a disorganized structure. The formation of new carbonyl groups, most probably ketones, was confirmed. Among radiolysis gases, H₂, CO, CO₂ and CH₄ were identified, with H₂ and CO being primary products. The formation of CO correlates with amide bond scission, while CO₂ yields exceeded expectations if its formation were solely based on carboxyl end-chain radiolysis; suggesting a contribution from residual solvent used during sample preparation. Notably, neither NH₃ nor primary amides were detected. These findings provide valuable insights into how the peptide bond responds to ionizing radiation and could be extended to the study of more structurally complex glycine-based peptides.