STARK SPECTROSCOPY OF TRYPTAMINE IMMOBILIZED ON A GOLD ELECTRODE

Electroreflectance (ER), ellipsometry, and surface-enhanced Raman spec troscopy (SERS) measurements were performed for immobilized tryptamine on polycrystalline gold electrodes, SERS study indicates that the ind ole ring does not interact directly with the gold surface. Ellipsometr ic measurements yielded a thickness of the tryptamine layer oft 1.98 /- 0.05) nm and an inferred surface concentration of 3.3 x 10(-6) mol/ m(2), ER measurements were performed in the wavelength region 200-300 nm using the NIST synchrotron ultraviolet radiation facility (SURF), T he ER response was interpreted as a Stark shift in the tryptamine abso rption line at 218 nm, Assuming that the tryptamine layer consists of two stacked tryptamine molecules associated at the indole rings, a uni fied analysis of ER and ellipsometric data gave the difference between the static dipole moments of the solvated ground and excited states o f 0.65 x 10(-30) C m in low ionic strength buffer (0.01 IM PBS) and 0. 25 x 10(-30) C m in high ionic strength buffer (0.01 M PBS + 1.0 M NaC lO4), The results point to the importance of the composition of the so lvation shell. Semiempirical quantum chemistry calculations of tryptam ine molecules gave a qualitative explanation of the ER response in the low ionic buffer, The large ER response suggests that the method coul d be applied to the study of the tryptophan environment in adsorbed pr oteins. (C) 1998 Academic Press.