Fig. 3. Monoconjugated derivatives of GSH obtained with various agents. (A) Structures of bioconjugated GSH products and relative conversions. All reactions were carried out in acetonitrile–buffer (4 : 1) solution, at 5.0 mM concentrations of the bioconjugation agent and GSH in pH 9.0 TRIZMA buffer (50.0 mM), at 25 °C for 2 hours. Relative conversions were calculated using ESI-MS peak intensities of the bioconjugated product and GSH (m/z = 307). (B) Representative MS spectra of products 14 and 15 (see ESI† for details). The undesired byproducts carrying two aryl rings were not detected. (C) Fluorescence emission spectra of increasing concentrations of monoconjugated product 12 (excitation wavelength was 335 nm). (D) The fluorescence intensity at 441 nm plotted against the GSH/1 ratio, see ESI† for details.