Surface-enhanced Raman spectroscopy in the structural studies of biomolecules: the state of the art

Abstract

It has been recently demonstrated'3 that very large (in some cases up to iOn) enhancement of Raman crosssection for molecules in the close vicinity of a metal surface results from superposition of two main mechanisms: electromagnetic and so-called molecular (or "chemical"). Enhancement of the local electromagnetic field near a "rough" metal surface induces the electromagnetic mechanism, while the "molecular" mechanism is connected with the appearance of new excited states for the molecule/metal complexes in the process of chemisorption. The techniques of surface-enhanced Raman (SER) and surface-enhanced resonance Raman (SERR) spectroscopy are based on these mechanisms and widely used in investigations ofbiological molecules.27 The three main questions of applicability of SERS and SERRS for resolving sophisticated problems in molecular biology, bioorganic and physical chemistry are the following: (i) What are the molecular mechanisms of interaction of the biomolecules with a metal surface in the experimental conditions typical of SERS appearance and is it possible to take measurements while preserving the native conformation of the molecule? (ii) What is the exact relationship between the Raman cross-section enhancement and the distance between the metal and the molecule? Is the mechanism for enhancement short-range or long-range and is it be possible to detect all normal vibrations of macromolecules or only vibrations of groups which directly contact the surface? (iii) What are the lowest concentrations for the detection of SER spectra of biomolecules in different experimental systems (electrodes, hydrosols, surfaces with regular roughness)? Is it possible to detect high-quality SER spectra of subpicogram amounts of different classes of biomolecules for successful competition with the traditional techniques in biotechnology and genetic engineering? This paper deals with applications of SERS to the study of membrane proteins and nucleic acids.