Inhibition of Ice Recrystallization by Nanotube-Forming Cyclic Peptides

While most native ice-binding proteins are rigid, artificial (macro)molecular ice-binders are usually flexible. Realizing a regular array with precisely positioned ice-binding motifs on synthetic proteins, (macro)molecular ice-binders are thus challenging. Here, we exploit the predictable assembly of cyclic peptides into nanotubes as a starting point to prepare large, rigid ice-binders bearing an ice-binding site that is found in hyperactive ice-binding proteins in insects. First, we designed, synthesized, and purified cyclic octapeptide Lys2CP8 bearing a TaT motif to promote ice binding and investigated their solution assembly and activity using circular dichroism (CD) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, light scattering (LS), cryogenic transmission electron microscopy (cryo-TEM), and ice recrystallization inhibition (IRI) assays. The cyclic peptide Lys2CP8 was synthesized in good yield using Fmoc chemistry and purified by reversed-phase HPLC. Upon dissolution in aqueous solutions, Lys2CP8 was observed to assemble in a pH- and concentration-dependent manner into objects with nanoscopic dimensions. LS revealed the presence of small and large aggregates at pH 3 and 11, held together through a network of intermolecular antiparallel β-sheets as determined by FTIR and CD spectroscopy. Cryo-TEM revealed the presence of one-dimensional objects at pH 3 and 11. These are mostly well-dispersed at pH 3 but appear to bundle at pH 11. Interestingly, the pH-dependent self-assembly behavior translates into a marked pH dependence of IRI activity. Lys2CP8 is IRI-active at pH 3 while inactive at pH 11 hypothetically because the ice-binding sites are inaccessible at pH 11 due to bundling.