The fluorescence of a large set of tryptophan analogues, including several that are conformationally constrained, was studied. The constrained analogues include tetrahydrocarboline-3-carboxylic acid and 3-amino-3-carboxytetrahydrocarbazole. Steady state and time-resolved fluorescence measurements were made as a function of pH. The fluorescence quantum 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 increase in fluorescence of tryptophan with deprotonation of this group. These results are consistent with the existence of excited state proton transfer to carbon 4 of the indole ring as a quenching mechanism, which is sterically prohibited in the constrained analogues and 4-methyltryptophan. From quantum yield and lifetime data (most decays are nonexponential), the effective rate constant for nonradiative depopulation 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 induced change in the pK(alpha) of this group. Deprotonation of this cl-ammonium 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.