Near UV optical second harmonic generation studies of surface-adsorbed tryptophan residues

Abstract

A new optical method intended for interfacial protein studies has surfaced from the observation of near UV second harmonic generation (SHG) from the aromatic amino acids. Here, the first hyperpolarizability, β=4.72±0. 52X10-30 esu, of t-butyloxycarbonyl-L-tryptophan is measured for SHG at 266 nm, and its polar orientation at the air-water interface is deduced. The indole ring nitrogen atom is found to point upwards at the interface. From a monolayer surface density of 1.74±0.15 adsorbate molecules/nm 2, it is estimated that a minimum of 0.49 Trp residues/nm2 of surface area can be detected above the water background for this wavelength. SHG studies of tryptophan-containing peptides and proteins based on these results have the potential to complement existing linear optical techniques by providing new information on interfacial molecular ordering. © 1995 American Institute of Physics.