There are several reagents available for the detection and determination of thiols by labeling with a fluorescent reagent.⁽¹⁾ MMBC (Methyl maleimidobenzochromenecarboxylate, also known as ThioGlo® was developed by Yang and Langmuir for the detection of thiols.⁽²⁾ The maleimide moiety is susceptible to conjugate addition by thiols, converting MMBC, which is essentially nonfluorescent, into highly fluorescent adducts. The advantages of MMBC include: (1) MMBC reacts quickly (ca. 1 min) with thiols under neutral conditions (pH 7.0-7.4). (2) The fluorescent thiol adducts of MMBC have emission maxima at relatively long wavelengths (513 nm), exhibiting high quantum yields. (3) MMBC itself exhibits very little fluorescence at this wavelength (ca. 40-60 times less fluorescent), thus obviating the separation of unreacted starting material from the mixture. (4) MMBC is relatively resistant to hydrolysis at neutral pH. Higher pH's should be avoided (e.g., = 8.0) due to maleimide hydrolysis and nonspecific reactions with amine residues.
ThioGlo is a registered trademark of Covalent Associates, Inc.

Ref:

1. Review: Tyagarajan, K.; Pretzer, E.; Wiktorowicz, J. E. Electrophoresis, 2003, 24, 2348-2358.
2. Yang, J.-R.; Langmuir, M. E., J. Heterocyclic Chem. 1991, 28, 1177-1180.