the pH values, the majority of lysozyme molecules assume different structural forms. The protein has a zero net charge at the isoelectric point (pI). Interactions of neutral protein molecules with water is less favorable than when they are in the charged state. At the pI, the solubility of proteins in water is low and the repulsion between protein molecules is also a minimum. These two factors help in adsorption of the molecules on metal surfaces for pH of the solution close to pI. For lysozyme the value of pI is 10.5. At pH 10.5 the protein is not soluble in water, which would cause problem in comparing the SERS spectrum and normal Raman spectrum of lysozyme molecule. At pH 11 the molecule undergoes rapid hydrolysis [22]. Moreover, the pH of untreated metal sol was between 7.0-7.5. For these reasons we have maintained the pH of the lysozyme solution in slightly acidic state, such that the effective pH in the final sol (lysozyme+Ag sol) is close to 7.0-7.5 in all our experiments. 4 mM solution of lysozyme was prepared in deionized water. For SERS measurement the volume ratio of Ag sol to protein solution was maintained at 1:1. SERS spectra were measured using a 488 nm Argon ion laser as an excitation source using a microRaman spectrometer with 100X objective lens. The spectrometer is equipped with 1200 g / mm holographic grating, a holographic super-notch filter, and a Peltier cooled CCD detector. The laser power on the samples was 5 mW. The unchanged circular dichro- ism spectra of laser irradiated and un-irradiated samples eliminate the possibility of dam- age/conformational changes in the protein structure by laser irradiation under the given experimental conditions. The data acquisition time for each Raman and SERS spectrum was 120 sec. UV-Visible spectra were measured by Spectrascan UV 2600 (Chemito). To study inter- action kinetics, measurements were carried out immediately after addition of lysozyme in Ag sol and subsequently after 15 mins, 30 mins, 1 hr, 2hrs and 3 hrs. For optical absorption measurements the samples were kept in a microcuvette with optical pathlength 1 cm. For SERS measurements a drop of solution was taken on a glass slide and allowed to dry. It took around 10-15 minutes to dry the samples. During kinetic measurements, to investigate the interaction between metal colloids and protein molecules in the solution, drops were deposited after the specific time intervals as given above. Thus, we carried out all measurements after the completion of metal-molecule interaction for the required time, in ambient conditions. All measurements (SERS and optical absorption) and analysis have been carried out in triplicate. 4