Permanent electric dipole and conformation of unsolvated tryptophan.

We have coupled a matrix assisted laser desorption source to an electric beam deflection setup to measure the permanent electric dipole of tryptophan isolated in a molecular beam. The permanent electric dipole is sensitive to the geometry and can be used as a tool to probe the conformation. For tryptophan, the dominant conformation present in the molecular beam has a dipole moment that agrees with the lowest energy conformation found at the B3LYP-DFT/6-31G* and MP2/6-31G* levels of theory. This conformation is stabilized by (COOHNH2) hydrogen bonding and through a favorable interaction between the NH2 group and the indole ring. Other low-energy conformations found in the calculations have dipole moments that are either too small or too large to account for the experimental results. In recent years there has been considerable interest in obtaining structural information for unsolvated and partially solvated biomolecules.1 These studies can provide detailed information about intramolecular interactions and solvent interactions that determine the conformations in all environments. In particular, a variety of sophisticated spectroscopic methods have been employed to examine isolated amino acids such as glycine,2,3 alanine,4 arginine,5 phenylalanine,6 tryptophan7-9 and tyrosine.8,10 The first spectroscopic results for tryptophan isolated in a molecular beam were obtained in 1985 by Levy and collaborators.7 They identified six different conformations in the resonantly enhanced two-photon ionization spectrum of jet-cooled tryptophan. More recently, high-resolution vibronic spectra were recorded for tryptophan at 0.38 K in helium droplets.8 However, the equilibrium structure has still not been determined. In the work described here we have used electric deflection of a molecular beam to directly probe the permanent dipole of tryptophan. Until very recently,11 this approach has mainly been restricted to the study of alkali halide dimers.12 This is the first application of the electric deflection method to a biomolecule. The experimental apparatus consists of a matrix assisted laser desorption (MALD) source coupled to an electric beam deflection setup that incorporates a position-sensitive time-of-flight mass spectrometer. Tryptophan and nicotinic acid were mixed in a 1:10 molar ratio and pressed to form a rod. A pulsed molecular beam of tryptophan was produced by desorbing molecules from the rod with the third harmonic of a Nd:YAG laser into a helium carrier gas. At the exit of the source, the tryptophan molecules are thermalized to 85 K in a 50 mm long nozzle cooled by liquid nitrogen. The molecular beam is collimated by two slits, and then goes through an electric deflector with a “two-wire” electric field configuration.13 One meter after the deflector, the molecules are ionized with the fourth harmonic of a Nd:YAG laser (266 nm) in the extraction region of a position-sensitive time-of-flight mass spectrometer.13 The tryptophan is two-photon ionized. The wavelength used for ionization (266 nm) is close to the resonance band of the indole moeity in tryptophan. There is relatively little fragmentation, roughly 80% of the ion signal is observed at the parent mass channel. Measurements of the molecular beam profile were performed as a function of the electric field in the deflector. The beam velocity was determined with a chopper. Figure 1 shows examples of the molecular beam profiles obtained without and with an electric field of 6.7 × 106 V/m in the deflector. The profiles were averaged over 10 000 laser shots. A symmetrical broadening of the profile and a decrease in the intensity on the beam axis are observed when the field is turned on. Similar profiles with a regular increase in the broadening are observed as the electric field is increased from 0 to 1.5 × 107 V/m. The force in the deflector is due to the interaction between the electric field Fz and the dipole μ of the molecule. It can be written as:14