Synthesis and characterization of a fragment of an ice nucleation protein

Synthetic peptides were used as models for studying the conformation of ice nucleation proteins. We chemically synthesized four peptides (16-, 24-, 32-, and 48-mer) that consisted of two to six repeats of the consensus repeating octapeptide unit of ice nucleation proteins and evaluated their conformation by circular dichroism spectroscopy. These model peptides exist predominantly as random coils in aqueous solution, but adopt alpha-helical structures in the presence of trifluoroethanol. The stability of their secondary structures was investigated by monitoring the pH and time dependence of their circular dichroism spectra. Our results indicated that the alpha-helical content of the 48-mer exhibited a significant pH dependence, while that of the 24- and 32-mer peptides did not. The 32-mer was the only peptide that transformed from the alpha-helical to a beta-sheet structure upon storage. We suggest that the overall conformation of the ice nucleation protein could be a beta-sheet.