The first UV absorption band of l-tryptophan is not due to two simultaneous orthogonal electronic transitions differing in the dipole moment

Abstract

Based on UV/Vis spectroscopic evidence obtained in this work, the first band in the absorption spectrum of L-tryptophan is largely due to a single electronic transition from the ground state to the ¹L_b excited state. However, emission spectra of this compound recorded at a variable temperature in ethanol, n-butanol and diethyl ether are structureless and considerably red-shifted at room temperature; also, lowering the temperature causes the emission to become structured and to undergo such a strong blue shift that it appears to be due to the ¹L_b state of the compound. Based on these findings, the formation (from the excited ¹L_b state) of the excited state responsible for the structureless, markedly red-shifted emission in L-tryptophan is strongly dependent not only on the viscosity of the medium, but also on its dipolarity.