Figure 1.

HX of the amide group in unstructured polypeptides. At the chemical level, amide HX rates vary linearly with hydronium and hydroxide ion concentration (eq 1). They depend also on temperature, neighboring side chains, and isotope effects [5, 6] (see URL HX2.Med.UPenn.edu/download). The HX halftime for unprotected amides is over 1 h at the pH minimum, between pH 2 and 3 at 0 °C, which makes possible the peptide separation/mass spectrometry technology.