Characterization of a few Raman lines of tryptophan

The Raman spectrum of tryptophan has been observed in its neutral aqueous solution with 514.5, 488.0, 457.9, 363.8, and 257.3 nm excitation. The effects of N ‐deuteration and 15 N‐substitution of the indole ring on the Raman spectrum have been examined. A preliminary normal coordinate treatment of the indole ring has also been made. On the basis of these data, the vibrational mode and a possible origin of the Raman scattering intensity has been discussed for each of the 1623, 1555, 1436, 1344, 1016, 882, and 762 cm −1 Raman lines of tryptophan. The resonance enhanced line at 1623 cm −1 has been assigned to a vibration of the indole ring which is related to the b 3g ring stretching vibration of naphthalene at 1636 cm −1 . Its intensity is considered to come almost solely from the B̃ ← X̃ ( L a ) electronic transition at 273 nm. It is therefore probable that the geometrical structure of the molecule is distorted along this vibrational coordinate on going from the ground ( X̃ ) to the B̃ electronic excited state.