PyMol [23] was used for visualization of the protein conformation. The accessible surface area (ASA) of all the amino acid residues in uncomplexed lysozyme are estimated using NACCESS [24]. All biomolecules do not interact with metal colloids in the same way. Some of them result in agglomeration of the colloidal particles, others do not cause aggregation but modify the surface charge of the particle [7]. To understand the effect of added lysozyme molecules on Ag colloids, we study the plasmon band of Ag particles in the sol on adsorption of the protein molecules. Plasmon band of Ag colloids is shown by black dotted line in Fig. 1 (a). Ag sols absorb light at 398 nm, due to the dipolar surface plasmon of small spherical particles of Ag. The average particle size of Ag has been estimated to be ∼ 100 ˚ A. Time-dependance of the plasmon band of Ag colloids in the sols is shown in Fig. 1 (a). Spectra, recorded just after the addition of protein molecules in the metal sols, are shown by black solid line. Subsequent changes in the spectra with time (after 15 mins., 30 mins, 1 hr, 2 hrs and 3 hrs.) are shown by colored solid lines in the Fig. 1 (a). Addition of lysozyme to the sol results in (i) decrease in intensity and (ii) broadening with a (iii) red shift of the plasmon resonance band of Ag particles in the sol. The decrease in relative intensity of the plasmon absorption band in solutions (ratio of maximum intensity of the plasmon band of Ag colloids after addition of protein to that of the Ag colloids), is shown in Fig. 1(b). We observe that the intensity of the plasmon band decays exponentially. The decay time constant of intensity is estimated to be 60 min in the sol. The observed broadening and the red shift of the plasmon band indicate formation of bigger particles in the sol by added lysozyme molecules. Furthermore, the decrease in the intensity of the peak can be explained by precipitation of metal particles from the sol followed by immediate aggregation. Analysis of SERS Spectra IV. EFFECT OF METAL COLLOIDS ON ADDED LYSOZYME MOLECULES A. Analysis of Plasmon Band of Silver Colloids III. EFFECT OF ADDED LYSOZYME MOLECULES ON METAL COLLOIDS 5