AN ASSESSMENT OF THE USEFULNESS OF 5-HYDROXYTRYPTOPHAN AS AN OPTICAL PROBE

5-Hydroxytryptophan (5HT) has been the focus of much recent attention as a novel intrinsic fluorescence probe for proteins. Solvent-dependen ce studies reported here show that, compared to other fluorophores of related interest like tryptophan (Trp) and 7-azatryptophan (7AT), the fluorescence emission maximum (lambda(em)(max)) of 5HT is relatively i nsensitive to solvent polarity. Thus, upon change of solvent from acet onitrile to water, the lambda(em)(max) of 5HT fluorescence is red shif ted by only 3 nm, in contrast to corresponding red shifts of much larg er magnitudes observed for Trp and 7AT (14 and 23 nm, respectively). T his behaviour suggests the lack of significant solvent dipolar relaxat ion effects in 5HT, which is corroborated by the low temperature emiss ion measurements at 77 degrees K. This aspect is also revealed in the fluorescence characteristics of 5HT in a membrane-mimetic model system , namely in AOT reverse-micellar assemblies containing different amoun ts of water. With increase in water content, the lambda(em)(max) of 5H T remains almost unaffected, although fluorescence anisotropy and quen ching data clearly indicate significant changes in the microenvironmen ts of 5HT molecules. (C) 1998 Elsevier Science B.V.