The Strong Fluorescence-Quenching Effect of Ergothioneine.

Ergothioneine (Erg) which possesses an imidazole-2-thione (IT) moiety had a strong quenching effect on indole fluorescence. The Stern-Volmer quenching constants (KSV), which quantified its fluorescence-quenching ability were 1508 (Erg) and 517 (IT) with respect to indole at pH 5.0. Compared with the structure of IT, the KSV values of ethylenethiourea (ET), which lacks a double bond between the 4 and 5 position, imidazole (IM) which lacks a sulfur atom, and mercaptoethylamine (MEA) which exhibits no thione-thiol tautomerizm were 71, 14, and 18, respectively under the same instrumental conditions. According to these results, the fluorescence-quenching effect of Erg was about 20-100 times stronger than related compounds such as ET, IM, and MEA. This physico-chemical property may help clarify the biological role of Erg.