Peptide hydrazide 1 (4.59 mg, 2.78 µmol, 1.00 equiv) was dissolved in 0.2 M phosphate buffer containing 6 M Gn•HCl pH 3.0 (560 µL). After adjusting the pH to 3.0, the mixture was cooled to -14 °C using a NaCl/ice bath. Then, a solution of 0.5 M NaNO2 (37.1 µL, 18.6 µmol, 6.67 equiv) was added to the reaction mixture which was agitated for 15 min. Then, p-hydroxythiophenol (23.4 mg, 186 µmol, 66.7 equiv) was suspended in 0.2 M phosphate buffer containing 6 M Gn•HCl pH 3.0 (600 µL) and the pH was adjusted to 6.7 using NaOH 6 M (24 µL). At this concentration and pH, p-hydroxythiophenol is only partly solubilized. This suspension was then added to the above reaction mixture. The NaCl/ice bath was removed and the pH of the reaction mixture was adjusted to 6.9 using NaOH 6 M (20 µL). The solution is cloudy at this stage.