Spectroscopy and photophysics of indoline and indoline-2-carboxylic acid

This paper presents a detailed description of the fluorescence and photophysical behavior of indoline and indoline-2-carboxylic acid (I2CA). Indoline is an analogue of indole lacking the C-2=C-3 double bond, but unlike indole, indoline possesses well-separated L-1(a) and L-1(b) states. I2CA, which displays similar fluorescence properties, is an amino acid and can be regarded as a fluorescent analogue of proline. In view of the potential for indoline and I2CA as fluorescent probes in peptides and proteins, we have undertaken an in-depth study of the spectroscopic and photophysical properties of these molecules. A companion paper (Slaughter, B.D.; Allen, M. W.; Lushington, G. H.; Johnson, C. K. J. Phys. Chem. A 2003, 107, 5670) presents a theoretical treatment of the spectroscopic transitions of indoline and I2CA. The pH-dependent absorption and fluorescence characteristics are investigated for both indoline and I2CA. Ground- and excited-state dissociation constants are determined spectroscopically. Quantum yields and radiative lifetimes are reported, and relaxation mechanisms are explored for both indoline and I2CA. Comparisons are made to the model systems of aniline and indole. The fluorescence lifetime of indoline is 4 to 7 ns in nonaqueous solvents but is reduced to 0.18 ns in water by electron-transfer quenching. The fluorescence decays of I2CA indicate the presence of two conformations, one with a short (<1 ns) lifetime and the other with a lifetime of 4 to 5 ns in most solvents. The fluorescence properties of I2CA in water are unvaried from pH 6-10 with a fluorescence decay component of 5.0 ns that makes it useful as a potential fluorescence probe.