Cysteine Modification 31
can alter reaction rates by several orders of magnitude (Bali and Akabas 2012 ).
Comparison of MTS reagent reaction rates is further complicated if charged MTS
reagents are used. The positively charge MTSEA and MTSET will experience
an electrostatic attraction to the negatively charged thiolate anion, whereas the
negatively charged MTSES will experience an electrostatic repulsion. Uncharged
MTS reagents, such as methylmethanethiosulfonate (MMTS), will not experi-
ence an electrostatic interaction with the thiolate. The relative reactivity of MTS
reagents with 2-mercaptothanol (2-ME) is MTSET > MTSEA > MMTS > MTSES
Fig. 1 Sulfhydryl reagent structures and reactions with simple thiols. a Methanethiosulfonates. b
p-Chloromercuribenzenesulfonate (pCMBS). c N-ethylmaleimide