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

Abstract

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