Oscillator strengths of the 1La and 1Lb absorption bands of tryptophan and several other indoles

The intensities of the indolyl ¹L_a and ¹L_b absorption bands were investigated by using 5-methoxyindole as a model compound. With 5-methoxyindole dissolved in weakly interacting solvents, almost the entire ¹L_b electronic transition occurs at longer wavelengths than the ¹L_a transition. The resolved spectrum of 5-methoxyindole permitted estimation of its oscillator strengths and also those of other indoles dissolved in cyclohexane: indole, 0.129 (¹L_a), 0.019 (¹L_b); 5-methylindole, 0.129 (¹L_a), 0.027 (¹L_b); 5-methoxyindole, 0.138 (¹L_a), 0.045 (¹L_b); 3-methylindole and N-stearyl-L -tryptophan n-hexyl ester, 0.127 (¹L_a), 0.027 (¹L_b). Hydrogen bonding to 1-methyl-2-pyrrolidinone does not measurably affect the total near-ultraviolet oscillator strength of indoles (less than 5% change). In water and ethanol, the oscillator strength of 3-methylindole and tryptophan is 15–20% less than that of 3-methylindole dissolved in cyclohexane. The spectra of the N-stearyl n-hexyl esters of tryptophan and 1-methyltryptophan dissolved in methylcyclohexane can be generated by using ¹L_a and ¹L_b bands having shapes similar to those observed for 5-methoxyindole, if the ¹L_a and ¹L_b bands are shifted so that their O-O bands overlap (289.5 nm for tryptophan and 299.5 nm for 1-methyltryptophan).