The collagen-like peptide (GER)15GPCCG forms pH-dependent covalently linked triple helical trimers

A collagen-like peptide with the sequence (GER)(15) GPCCG was synthesized to study the formation of a triple helix in the absence of proline residues. This peptide can form a triple helix at acidic and basic pH, but is insoluble around neutral pH. The formation of a triple helix can be used to covalently oxidize the cysteine residues into a disulfide knot. Three disulfide bonds are formed between the three chains as has been found at the carboxyl-terminal end of the type III collagen triple helix. This is a new method to covalently link collagen-like peptides with a stereochemistry that occurs in nature. The peptide undergoes a reversible, cooperative triple helix coil transition with a transition midpoint (T(m)) of 17 to 20 degrees C at acidic pH and 32 to 37 degrees C at basic pH. At acidic pH there was little influence of the T(m) on the salt concentration of the buffer. At basic pH increasing the salt concentration reduced the T(m) to values comparable to the stability at acidic pH. These experiments show that the tripeptide unit GER which occurs frequently in collagen sequences can form a triple helical structure in the absence of more typical collagen-like tripeptide units and that charge-charge interactions play a role in the stabilization of the triple helix of this peptide.