PEPTIDE MODELS OF PROTEIN-FOLDING INITIATION SITES .3. THE G-H HELICAL HAIRPIN OF MYOGLOBIN

As part of an extensive dissection of the folding pathway of myoglobin, a series of peptides corresponding to fragments of sperm whale myoglobin have been synthesized, and their conformational preferences investigated using circular dichroism and nuclear magnetic resonance spectroscopy in aqueous solution and in solvent mixtures containing water and trifluoroethanol. The behavior of short fragments corresponding to the sequences of the G- and H-helices of myoglobin and to the turn region between these helices has been described in accompanying papers. At the next level of complexity, peptide model compounds have been synthesized to explore the longer-range interactions which may take place in protein folding after initial secondary structure formation has occurred. A series of disulfide-bridged dimeric peptides containing the complete sequences of the G- and H-helices of myoglobin were synthesized and their conformational preferences examined. CD spectra indicate that disulfide-bridged peptides consisting of two H-helix sequences (Mb-HssH) and of one G- and one H-helix (Mb-GssH) are highly helical in water solution, as a result of intermolecular association. A 51-residue peptide, Mb-GH51, encompassing the entire G-H helical hairpin of myoglobin, including the turn sequence between the two helices, has been successfully synthesized by standard methods. This peptide was designed to be monomeric in aqueous solution. Mb-GH51 does not appear from CD spectra to contain any additional helix in water solution above what would be expected from an equimolar mixture of the G- and H-helix peptides. NMR spectra indicate that the turn conformation observed in shorter peptide fragments is retained in Mb-GH51 in high population. The addition of TFE results in the formation of some helix, though not as much as might be expected even from a simple combination of the ellipticities of the component helical peptides in TFE. Current experimental and theoretical studies of myoglobin folding implicate the G-H helical hairpin in the earliest stages of folding: the present results imply that other parts of the polypeptide chain may be participating in these early events to a greater extent than heretofore imagined.