ABSTRACT
A technique that can fulfill many of the above points is a relatively novel biophysical method named plasmon-waveguide resonance (PWR) spectroscopy. In this chapter, the principles underlying this technique will be presented. Plasmon-waveguide resonance combined with techniques for forming solid-supported proteolipid membranes allows kinetic, thermodynamic, and structural characterization of anisotropic thin films such as the case of lipid model systems and molecules therein embedded or interacting with such films. The anisotropic properties of such oriented systems can be directly probed, as can thermodynamic and kinetic properties that result from these interactions, without the need for any chemical modification.In this chapter, the uses and power of this method in studies involving the activation and signaling of GPCRs as example of membrane proteins will be discussed. Additionally, applications regarding the interaction of membrane active peptides with lipid membranes will be presented. Finally, an overview of the current and future developments of this technique will be briefly highlighted. 4.2 Plasmon Spectroscopy
The phenomenon of anomalous dispersion on diffraction gratings due to the excitation of surface plasma waves was first described in the beginning of the twentieth century. In the late 1960s, optical excitation of surface plasmons by the method of attenuated
total reflection (ATR) was demonstrated by Kretschmann [1] and Otto [2]. The potential of surface plasmon resonance (SPR) for characterization of thin films and monitoring processes at metal interfaces was recognized in the late seventies [3]. In 1982, the use of SPR for biosensing was demonstrated by Nylander and Liedeberg [4]. Since then, SPR has been intensively studied [5, 6] and found to be in good agreement with theoretical concepts based on Maxwell’s theory of electromagnetism (EM). In Maxwell’s theory of electromagnetism, plasmons are treated as charge density oscillations of the free electrons of a metal. These electron density fluctuations generate a surface localized electromagnetic (EM) wave, which nonradiatively propagates along a metal/dielectric interface. The electric field is normal to this interface and vanishes exponentially with distance. The EM characteristics are the same as those describing optically generated evanescent waves in total internal reflection techniques. The EM fields do not stop at the boundary, but penetrate a distance into the emerging medium in the form of a surface wave. Surface plasmon excitation is a resonance phenomenon that occurs when energy and momentum conditions between incident light photons and surface plasmons are matched according to the following equation: kSP = kph = (w/c ) e01/2 sin a0,(4.1)where kSP = (w/c) (e1 e2/e1 +e2)1/2 (4.2) kSP is the longitudinal component of the surface plasmon wave vector, kph is the component of the exciting light wave vector parallel to the active (metal) medium surface, w is the frequency of the surface plasmon excitation wavelength (l), c is the velocity of light in vacuo, e0, e1, and e2 are the complex dielectric constants for the incident, surface active and dielectric (or emerging) media, respectively, and a0 is the incident coupling (resonance) angle.
