Cysteine-specific blue fluorescence probe

A strong fluorescence probe has been designed to investigate cytochrome c folding kinetics. The cysteine specific modifier, 2-[(-Iodo-acetyl)-methyl- amino]-6-methylaminobenzoic acid methyl ester (1), has been prepared, and the reaction with cysteine can be performed in aqueous solution to ensure its application to protein modification. The model compound, 2-{[2-(2-Acetylamino-2- methyoxycarbonylethylsulfanyl)-acetyl]-methyl-amino}-6-methyl amino-benzoic acid methyl ester (2), resulting from the reaction of 1 with N-acetyl cysteine has emission quantum yields of 0.627 and 0.185 in nonaqueous and aqueous buffer solutions, respectively. The lifetime of 4.6 ns for compound 2 manifests a singlet excited-state. The Förster energy transfer distance of compound 2 and cytochrome c is calculated to be 45 Å.