| Abstract
| - This paper presents a detailed description of the fluorescence and photophysical behavior of indoline andindoline-2-carboxylic acid (I2CA). Indoline is an analogue of indole lacking the C2C3 double bond, butunlike indole, indoline possesses well-separated 1La and 1Lb states. I2CA, which displays similar fluorescenceproperties, is an amino acid and can be regarded as a fluorescent analogue of proline. In view of the potentialfor indoline and I2CA as fluorescent probes in peptides and proteins, we have undertaken an in-depth studyof 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. A2003, 107, 5670) presents a theoreticaltreatment of the spectroscopic transitions of indoline and I2CA. The pH-dependent absorption and fluorescencecharacteristics are investigated for both indoline and I2CA. Ground- and excited-state dissociation constantsare determined spectroscopically. Quantum yields and radiative lifetimes are reported, and relaxationmechanisms are explored for both indoline and I2CA. Comparisons are made to the model systems of anilineand indole. The fluorescence lifetime of indoline is 4 to 7 ns in nonaqueous solvents but is reduced to 0.18ns in water by electron-transfer quenching. The fluorescence decays of I2CA indicate the presence of twoconformations, 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 componentof 5.0 ns that makes it useful as a potential fluorescence probe.
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