Intramolecular disulfide loop formation in a peptide containing two cysteines.

The cyanogen bromide fragment comprising residues 115-181 of Kunitz soybean trypsin inhibitor is a soluble random-coil peptide at pH 7 containing two cysteines separated by eight other amino acids in the primary sequence. Four of the six rate constants have been determined for the three disulfide exchange reactions between this fragment and oxidized and reduced forms of N-acetylcysteine methyl ester. The rate constant for intramolecular loop formation in the fragment containing one thiolate anion and one sulfur connected by a disulfide bond to the small cysteine analogue is 0.36 +/- 0.15 s-1 at 23 degrees C in 3 M guanidine hydrochloride. This measurement provides a frame of reference corresponding to formation of a small but sterically unstrained loop, the fast limit for intramolecular disulfide exchange in a random-coil peptide.