| Abstract: | The CD of aqueous solutions of poly(S-carboxymethyl-L -cysteine) and poly(S-carboxyethyl-L -cysteine) has been measured at different pH, and the pH-induced β-coil transition is observed by changes in residue ellipticity of dichroic bands around 200 and 225 nm. The residue ellipticity at 200 nm of the former polypeptide is twice as large as that of the latter, when the β-conformation is formed in solution. However, the β-conformation of the latter polypeptide is more stable against electrostatic repulsion than that of the former. The transition curve of poly(S-carboxymethyl-L -cysteine) has also been determined for different molecular weights. The curves were found to be completely coincident with one another if the degree of polymerization were higher than about 100. Such a transition curve is generally divided into three steps: initiation, cooperative formation, and rearrangement of hydrogen bonds. The cooperative step is very sharp, occurring at a constant pH. These steps become agglomerated into two or one when the polypeptide concentration or added salt concentration is increased. |
