Lifetime and rotational relaxation time of dansylgalactoside bound to the lac carrier protein

The results presented in this paper demonstrate that the excited state lifetime, anisotropy, and rotational relaxation time of 2'-(N-dansyl)aninoethyl 1-thio-beta-D-galactopyranoside (DG2) increase when the probe is bound specifically to the lac carrier protein in "energized" Escherichia coli membrane vesicles. Although the probe also binds nonspecifically to the vesicle membrane, such binding is independent of the lac carrier protein and is unaffected by "energization" of the vesicles. The experiments provide further evidence that the dansylgalactosides are useful probes for the beta-galactoside transport system and support the hypothesis that the changes in dansylgalactoside fluorescence observed on "energization" of membrane vesicles reflect changes in the binding of the probe.