Polymerized peptide and gel having collagen-like structure

A gelling agent comprising at least one kind of a polymerized peptide that has, as a polymerization unit, a triple-chain peptide having a triple-helical structure, and is polymerized by oxidative cross-linking, wherein peptide chains composing the triple-chain peptide are identical to or different from one another, and each peptide chain has:

• one or more triple helix forming peptidyl groups each having a repeating structure in which -(Xaa-Yaa-Gly)- as a basic unit is repeated at least 5 times;
• and
• one or more cross-link forming peptidyl groups each comprising at least two cysteine (Cys) residues within 6 residues from each of an amino-terminus and a carboxy-terminus, and wherein
• each of Xaa and Yaa is independently selected from a proline (Pro or P) residue, a hydroxyproline (Hyp or O) residue, an arginine (Arg or R) residue, a lysine (Lys or K) residue, a valine (Val or V) residue, a leucine (Leu or L) residue, an isoleucine (Ile or I) residue, a serine (Ser or S) residue, a threonine (Thr or T) residue, an alanine (Ala or A) residue, a glycine (Gly or G) residue, an N-isobutyl glycine residue, a phenylalanine (Phe or F) residue, a methionine (Met or M) residue, a glutamate (Glu or E) residue, an aspartate (Asp or D) residue, an asparagine (Asn or N) residue, a glutamine (Gln or Q) residue, a histidine (His or H) residue, a tryptophan (Trp or W) residue or a tyrosine (Tyr or Y) residue, the proline residue may be modified by an amino group or fluorine atom.