Surface Plasmon Resonance Applied to DNA-Protein Complexes
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Surface plasmon resonance (SPR) measures refractive index changes (Δn) at or near a surface and relates these to changes in mass at the surface Fig 1 ). This relationship is given by the Clausius Mossotti form (Eq. 2 ) of the Debye equation (Eq. 1 ):
(1)
Fig. 1. Schema showing the principle of surface plasmon resonance. Light from a laser source arriving through a prism at a gold surface at the angle of total internal reflection (θ) induces a nonpropagative evanescent wave that penetrates into the flow cell opposite the prism. The intensity of the reflected light is continuously monitored. At a given angle (λ) dependent on the refractive index of the solution in the flow cell, resonance between the evanescent wave and free electrons in the gold layer results in a reduction in the intensity of reflected light. The change in angle of reduced intensity (Δλ) reflects changes in the refractive index (n ) of the solution in the flow cell immediately adjacent to the gold layer. A dextran surface coupled to the gold layer allows immobilization of ligands (e.g., DNA) within the evanescent field.
where ɛ is the real part of the dielectric constant or permittivity constant related to the refractive index by ɛ = n 2 , N is the number density given by N A ρ/Ma (N A is Avogadro’s number, ρ is the density and M a is the molecular mass). It is assumed that Δn/ΔC is a constant.
