Reversible Freeze-Induced β‑Sheet-to-Disorder Transition in Aggregated Homopolypeptide System

Conformational transitions involving aggregated proteins or peptides are of paramount biomedical and biotechnological importance. Here, we report an unusual freeze-induced structural reorganization within a β-sheet-rich ionic coaggregate of poly­(l-lysine), PLL, and poly­(l-glutamic acid), PLGA. Freezing aqueous suspensions of the PLL–PLGA β-aggregate in the presence of low concentrations of salt (NaBr) induces an instantaneous β-sheet-to-disorder transition, as probed by infrared spectroscopy in the amide I′ band region. The conformational rearrangement of polypeptide chains appears to be fully synchronized with the global liquid-to-ice phase transition. In contrast to the known freeze-induced transitions, the process described here is fully reversible: the subsequent thawing results in an instantaneous disorder-to-β-sheet “refolding”. However, in the absence of traces of soluble salts, the β-sheet framework of the PLL–PLGA aggregate remains resistant to freezing as no transition is observed. We note that the occurrence of the transition depends on the type of salt present in the sample. Our results highlight a hidden dimension of the structural dynamics within β-sheet-rich aggregates. Possible scenarios of freeze-induced salt-bridge rupture and removal of water from nanocanals are discussed.