Abstract
Collagen is the most abundant protein in mammalian tissues with a unique right-handed triple helix structure which is composed of three left-handed PPⅡ peptide chains. Each peptide chain is rich in (Xaa-Yaa-Gly) sequence repeats，the triple helix is formed by hydrogen bonding between the chains. Among the nature amino acids, histidine is known for having high binding affinity with metal ions and is often found in the metal binding sites in protons. In this work, we try to find a simple way to induce the folding of a collagen triple helix. Thus, we have designed and synthesized a series of His-containing collagen mimics, including GHP(PPG)7, (PPG)7PHG, HG(PPG)7, HG(PPG)7GH and (PPG)7GH, for the metal binding study. Circular dichroism (CD) and NMR measurements showed that (PPG)7PHG could form slightly stable triple helices in the present of metal ions, while HG(PPG)7GH and (PPG)7GH could be induced to fold into the triple helical comformation by metal ions. The increase in melting temperatures (Tm) can be up to about 20 ˚C, HG(PPG)7 fail to fold into a triple helix upon adding metal ions, indicating that the C-terminal metal-histidine coordination is the driving force to assist the folding. The results are also consistent with the folding mechanism of collagen that initiates at C-terminus and propagates to the N-terminus. There is metal selectivity for the peptide mimics to form triple helices, due to the difference in the binding affinity between metal ions and histidine. The metal dependent measurements have further shown that Cu2+、Cu+ and Ni2+ could induce the folding of HG(PPG)7GH and (PPG)7GH while Fe3+、Co2+ and Zn2+ could not. Besides, the kinetic measurement also showed that the folding rate of triple helices induced by Cu2+ and Cu+ were faster than by Ni2+. In the present of the same metal ion, (PPG)7GH formed triple helices faster than HG(PPG)7GH because the additional metal-histidine coordivation on the N-terminus might perturb the inter-strand alignment and impede the folding process. In conclusion, the His-containing collagen mimetic peptides can fold into a stable triple helix with the assistance of metal ions, and their metal selectivity could provide a potential application in biosensor.