First Hyperpolarizability of the Natural Aromatic Amino Acids Tryptophan, Tyrosine, and Phenylalanine and the Tripeptide Lysine−Tryptophan−Lysine Determined by Hyper-Rayleigh Scattering

We report the first hyperpolarizability of tryptophan (Trp) and tyrosine (Tyr) and an upper limit for that of phenylalanine (Phe), three natural aromatic amino acids. The measurements were performed with hyper-Rayleigh scattering in an aqueous Tris buffer solution at a pH of 8.5 and 150 mM salt concentration with a fundamental wavelength of 780 nm. A value of (4.7 ± 0.7) × 10−30 esu is found for Trp and (4.1 ± 0.7) × 10−30 esu for Tyr whereas the upper limit of 1.4 × 10−30 esu is found for that of Phe due to its limited solubility. The influence of the presence of lysine (Lys) in close vicinity of Trp is investigated with a measurement of the first hyperpolarizabilty of Trp in an excess of Lys and compared to the first hyperpolarizability obtained for the tripeptide Lys-Trp-Lys. The clear decrease of the values measured in these two cases indicates that the first hyperpolarizabilty of Trp is very sensitive to its local environment.