Next, we focus on the effect of metal particles on added lysozyme molecules in the sol. Raman spectra and SERS spectra of 4 mM lysozyme in water and in the Ag sol (just after addition) are shown by solid lines and dotted lines respectively in Fig. 2. Appearance of the characteristic feature of the Ag-N vibrational mode at 233 cm − 1 [8] in the SERS spectrum (shown in the inset of Fig. 2) is a clear signature of metal-protein interaction in the sol via N atom of the ad-molecule. Time variation of SERS spectra of lysozyme in Ag sol over the spectral range 400–900 cm − $^{1}$, 900–1100 cm − 1 and 1100–1700 cm − 1 are shown in Fig. 3(a)–(c) The assignments of the prominent bands (indicated by ⋆ marks in Fig. 2) are summarized in Table I. To study the interaction of the Ag-lysozyme complex, we have estimated the relative intensity of each feature in the range over 400–1100 cm − 1 [Fig. 3(a) and (b)] with respect to the intensity of the peak at 877 cm − $^{1}$(vibration of N 1 H site of Trp amino acid residues in lysozyme). The methylene δ (CH 2 ) vibration at 1442 cm − 1 has been used as reference for the spectral features in the range 1100–1700 cm − 1 [Fig. 3(c)]. Both these peaks (at 877 and 1442 cm − $^{1}$) maintained a constant frequency and were of quite strong intensities. Invariant Raman shift of a band indicates unchanged chemical bonding responsible for these particular vibrational modes over experimental duration. It was interesting to note that the relative intensity of all vibrational bands of the adsorbed lysozyme molecule do not follow the same trend. A possible explanation for the time- evolution of the spectra can be the conformational changes of the ad-molecules with time, which we discuss in detail below. Vibrations of peptide backbone in proteins, associated with amide I, mainly involve the C=O stretch with a small contribution of C-N stretching and N-H bending. The same for the amide III region arises predominantly by an in-plane N-H deformation coupled with a C α -N stretch. In lysozyme, amide bands consist of mainly three structural components — three stretches of α -helix, antiparallel pleated β sheet, in which the polypeptide is hydrogen- bonded between one region to another via a hairpin turn of the chain and random coils, which are folded in an irregular way. Lysozyme is a globular protein with distinctly demarcated α -helix and β -sheet regions. These secondary structural units are connected by random coils. The positions and intensities of the vibrational modes of amides can be used for an empirical estimation of these secondary structures in proteins. SERS spectrum of amide I band appears between 1610–1700 cm − 1 . The contributions of A. Vibrational modes of amide bands 6