FLUORESCENCE STUDIES WITH TRYPTOPHAN ANALOGS - EXCITED-STATE INTERACTIONS INVOLVING THE SIDE-CHAIN AMINO GROUP

The fluorescence of a large set of tryptophan analogues, including sev eral that are conformationally constrained, was studied. The constrain ed analogues include tetrahydrocarboline-3-carboxylic acid and 3-amino -3-carboxytetrahydrocarbazole. Steady state and time-resolved fluoresc ence measurements were made as a function of pH. The fluorescence quan tum yields of the constrained analogues are higher than those for the unconstrained counterparts. The emission intensity of the constrained analogues, as well as 4-methyltryptophan, decreases with deprotonation of the side chain alpha-ammonium group; this is in contrast to the in crease in fluorescence of tryptophan with deprotonation of this group. These results are consistent with the existence of excited state prot on transfer to carbon 4 of the indole ring as a quenching mechanism, w hich is sterically prohibited in the constrained analogues and 4-methy ltryptophan. From quantum yield and lifetime data (most decays are non exponential), the effective rate constant for nonradiative depopulatio n of the excited state was calculated. For tryptophan analogues having two side chain functional groups, there is a synergistic effect; the presence of two side chain groups causes more quenching than expected from the sum of the individual contributions. For analogues having ana -ammonium group, this synergism appears to be correlated with an induc ed change in the pK(alpha) of this group. Deprotonation of this cl-amm onium group also causes a red shift in the emission of these compounds ; this appears to be due to electrostatic repulsion between the alpha- NH3+ group and the excited indole dipole.