Br-labelled elastin-like polypeptide incorporating the unnatural amino acid diBrK (N-ε-3,5-dibromobenzyloxycarbonyl-L-lysine). The positions of the diBrK (*) in the protein sequence are: SSGGVGVPG*GVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGV GVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGV GVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGVGVPGV GV*GWPSGA