PROTEIN METAL INTERACTIONS IN PROTEIN-COLLOID CONJUGATES PROBED BY SURFACE-ENHANCED RAMAN-SPECTROSCOPY

Surface-enhanced Raman (SER) spectroscopy has been used to assess protein-metal interactions in colloidal immunoprobes. The absence of a detectable signal from conjugates of colloidal gold with anti-rabbit IgG, albumin-biotin, or protein A suggests that the proteins are not chemisorbed on the metal particles. Similarly, albumin, albumin-biotin, and protein A are not covalently bound to colloidal silver, but rather adsorb on silver initially with the protein hydration shell intact. Over a period of days, new SER bands appear in the albumin spectrum that are attributed to vibrations of aromatic amino acids. This implies that conformational changes are occurring in the protein near the interface. The time scale for these changes is consistent with what has been observed by circular dichroism and UV-visible spectroscopy. The effect of avidin on the binding of biotin to colloidal metal probes has also been characterized. The SERS results indicate that biotin binds to a cleft or pocketlike structure in avidin, restricting biotin-surface interactions. This study demonstrates the utility of SER spectroscopy for characterizing protein-metal interactions and the availability of adsorbed biomolecules for complexation.