Stark Spectroscopy of Tryptamine Immobilized on a Gold Electrode

Electroreflectance (ER), ellipsometry, and surface-enhanced Raman spectroscopy (SERS) measurements were performed for immobilized tryptamine on polycrystalline gold electrodes. SERS study indicates that the indole ring does not interact directly with the gold surface. Ellipsometric measurements yielded a thickness of the tryptamine layer of (1.98 +/- 0.05) nm and an inferred surface concentration of 3.3 x 10(-6) mol/m2. ER measurements were performed in the wavelength region 200-300 nm using the NIST synchrotron ultraviolet radiation facility (SURF). The ER response was interpreted as a Stark shift in the tryptamine absorption line at 218 nm. Assuming that the tryptamine layer consists of two stacked tryptamine molecules associated at the indole rings, a unified analysis of ER and ellipsometric data gave the difference between the static dipole moments of the solvated ground and excited states of 0.65 x 10(-30) C m in low ionic strength buffer (0.01 M PBS) and 0.25 x 10(-30) C m in high ionic strength buffer (0.01 M PBS + 1.0 M NaClO4). The results point to the importance of the composition of the solvation shell. Semiempirical quantum chemistry calculations of tryptamine molecules gave a qualitative explanation of the ER response in the low ionic buffer. The large ER response suggests that the method could be applied to the study of the tryptophan environment in adsorbed proteins. Copyright 1998 Academic Press.