Second harmonic generation from tryptophan-rich short peptides: W(n)K(m) and gramicidin A.

We report the first hyperpolarizability of a series of tryptophan-rich short peptides with the respective sequence KWK, KWWK, KWWWK, KWWKWWK, where W and K stand for tryptophan and lysine. The measurements were performed with the technique of hyper-Rayleigh scattering in the bulk of an aqueous Tris buffer solution at a pH of 8.5 and a salt concentration of 150 mM at the non-resonant fundamental wavelength of 784 nm. The first hyperpolarizability of the different peptides follows a simple additive model scaling with the number of tryptophan residues contained in the peptide. However, it appears that the first hyperpolarizability response of a single tryptophan residue in the peptide strongly differs from that of an isolated tryptophan. Hence, it is therefore demonstrated that the local environment of the tryptophan residues within the peptide strongly influences its nonlinear optical response. A comparison with the first hyperpolarizability of the natural peptide gramicidin A measured in trifluoroethanol (TFE) further confirms the key role of the local environment on the first hyperpolarizability of tryptophan residues in peptides.

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.

Second harmonic generation from small gold metallic particles: from the dipolar to the quadrupolar response.

Hyper Raleigh scattering, a common technique to investigate the second harmonic light scattered from a liquid suspension of molecular compounds and to determine their quadratic hyperpolarizability, has been used for aqueous suspensions of gold nanoparticles, the diameter of which ranges from 20 up to 150 nm. The hyper Rayleigh signal intensity was recorded as a function of the angle of polarization of the incident fundamental wave. For the particles with a diameter smaller than 50 nm, the response is dominated by the dipolar contribution arising from the deviation of the particle shape from that of a perfect sphere. For larger diameter particles, retardation effects in the interaction of the electromagnetic fields with the particles cannot be neglected any longer and the response deviates from the pure dipolar response, exhibiting a strong quadrupolar contribution. It is then shown that in order to quantify the relative magnitude of these two dipolar and quadrupolar contributions, a weighting parameter zeta(V) which equals unity for a pure quadrupolar contribution and vanishes for a pure dipolar response, can be introduced.