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

The intensities of the indolyl 1 L a and 1 L b absorption bands were investigated by using 5‐methoxyindole as a model compound. With 5‐methoxyindole dissolved in weakly interacting solvents, almost the entire 1 L b electronic transition occurs at longer wavelengths than the 1 L 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 ( 1 L a ), 0.019 ( 1 L b ); 5‐methylindole, 0.129 ( 1 L a ), 0.027 ( 1 L b ); 5‐methoxyindole, 0.138 ( 1 L a ), 0.045 ( 1 L b ); 3‐methylindole and N ‐stearyl‐ L ‐tryptophan n ‐hexyl ester, 0.127 ( 1 L a ), 0.027 ( 1 L 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 1 L a and 1 L b bands having shapes similar to those observed for 5‐methoxyindole, if the 1 L a and 1 L b bands are shifted so that their O‐O bands overlap (289.5 nm for tryptophan and 299.5 nm for 1‐methyltryptophan).