Rapid Communications
lactate deshydrogenase, 6 staphylococcal nuclease, 7 T4 lysozyme, 8 subtilisin inhibitor, 9 phosphoglycerate kinase, 10 chey, 11 and barstar 12 were all found to undergo cold denaturation.
Very few synthetic polypeptides have been reported to undergo cold denaturation. Urry and co-workers reported that an elastic protein-based polypeptide experiences a folding process with increased temperature. Lacassie and co-workers showed that sequential polymer (Leu-Lys-Glu-Lys-Pro-Glu-Lys-Leu) n undergoes a reversible transition upon heating leading to a partly a- helical structure. 17 Anderson and co-workers reported that cold denaturations were observed in aqueous hexafluoroisopropanol solutions for both amphiphilic a-helices and alanine-rich a-helices. 18 Kitakuni and co-workers also observed the cold denaturation of an amphiphilic 30-residue peptide. The dissociation of collagen fibrils upon cooling has been reported. However, it is likely that the nonassociated triple helices in these supramolecular structures are not disrupted. 12 Therefore, there appears to be no report of the cold denaturation of a triple-helical collagen peptide that is made up of three polyproline II-like helices supercoiled around each other. We have established in prior work that a series of peptoid-containing collagen mimetics are capable of assembling into triple-helical structures. While we were studying the thermal stability of collagen mimetic Ac-(Gly-Pro-Nleu) 9 -NH 2 (Ac Γ
acetyl, Nleu Γ
N-isobutylglycine) in various ethylene glycol (EG)/H 2 O solutions, we came across, quite surprisingly, the phenomenon of Yangbo Feng