D 2355 (OTTO) 5,5'-Dithio-bis-(2-nitrobenzoic Acid) [DTNB] Cas 69-78-3 - used to characterize reactive thiol groups. D 2355 (OTTO) 5,5'-Dithio-bis-(2-nitrobenzoic Acid) [DTNB] Cas 69-78-3 - used for measuring the free sulfhydryl content in proteins, peptides, and tissues. Also useful in photometric determination of thiols. A sulfhydryl reagent used to characterize reactive thiol groups.