Reversible Freeze-Induced Beta-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 β-sheet-rich ionic co-aggregate of poly-(L-Lysine), PLL, and poly-(L-glutamic acid), PLGA. Freezing aqueous suspensions of 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 re-arrangement 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 here described process is fully reversible: the subsequent thawing results in an instantaneous disorder-to-β-sheet ‘refolding’. However, in the absence of traces of soluble salts, β-sheet framework of PLL-PLGA aggregate remains resistant to freezing as no transition is observed. We note that the occurr...