The thiol ester of N-methyl-(3-thioacetyl)-propionamide (4.6 mg, 3.1×10-5 mol, see Example 3) was hydrolyzed by the addition of 0.62 ml of 0.12M KOH in 80% methanol. After 10 min, 0.3 ml of 0.5M potassium phosphate, pH 7 was added to the thiol compound and the pH adjusted to 7.5 with 1N hydrochloric acid. The thiol concentration was 34 mM (determined by reaction with dithionitrobenzoic acid, DTNB). SMCC-caproate (8.7 mg, 2.5×10-5 mol) was dissolved in 0.4 ml of 0.5M potassium phosphate, pH 7.0, and the N-methyl-(3-mercapto)propionamide solution (0.742 ml, 2.5×10-5 mol) added. The solution was gassed with argon and the tube sealed. After 12 hr at room temperature, no thiol remained and the solvent was removed in vacuo. The residue was dissolved in 0.117 ml water to make a 213 mM solution of the desired compound (see FIG. 1D).
