the three secondary structures— α -helix, β -sheet and random coil— are expected to occur over the spectral windows 1665-1680 cm − $^{1}$, 1610–1632 cm − 1 and 1636–1644 cm − $^{1}$, respec- tively [28]. The amide III band appears over the spectral range between 1230 and 1310 cm − $^{1}$— the signatures of α -helix, β -sheet and random coils are characterized by spectral peaks over the range 1280–1320 cm − $^{1}$, 1235–1242 cm − 1 and 1250–1260 cm − $^{1}$, respectively [29]. The region 1254–1260 cm − 1 has been assigned to β -turn [30]. In the measured SERS spectra of lysozyme in Ag sol, the vibrational modes of amide III appear at 1260 cm − $^{1}$, as a shoulder of the strong peak at 1331 cm − 1 of Trp (Fig. 3). Thus, it is difficult to extract the subtle spectral features of amide III band from spectral analysis. To study the correlation between three secondary structural components of the adsorbed protein molecule, the broad envelope of the amide I band of each spectrum needs to be deconvoluted into the contributions from each of them. Such analysis may also provide information on conformational changes in lysozyme with time upon adsorption on colloidal surface. The time-variation of the vibrational spectra of the amide I band are shown in Fig. 4 by + signs. The deconvoluted and net fitted spectra are shown in Fig. 4 by dotted and solid lines. From the analysis of the spectral profile of amide I band we obtain the following: (i) the spectral feature over the spectral range between 1610 and 1632 cm − 1 for β -sheet is missing in all spectra and (ii) the relative vibrational intensities of random coils (I R ) to α -helix (I α ) increases with time (in the inset of Fig. 4). As mentioned before, the sharp SERS spectral line at 877 cm − 1 corresponds to the vibra- tion of N 1 H site of Trp residues [31]. The frequency of this band depends on the strength of the H-bond between Trp and other side chain molecule. The same feature is expected to shift by 8 cm − 1 for non-H-bonding Trp derivatives and lowered by 7 cm − 1 for H-bonded Trp derivatives. The single sharp peak at 877 cm − 1 in the Raman and SERS spectra indicates that in the metal-protein complex the Trp-residues do not form any new chemical bond directly with the Ag surface or with other residues. The variation in relative intensities of other vibrational modes of Trp are shown in Fig. 5(a). The intensity of ring breathing mode of Phe-residue at 1004 cm − 1 first increases and then decreases with time (Fig. 5(b)). The change in δ (CH 2 ) mode of glutamic acid at 1449 cm − 1 (just next to δ (CH 2 ) methylene vibrational mode at 1442 cm − $^{1}$) is shown in Fig. 5(c). In each plot, the solid line is the guide to the eye. 7 B. Vibrational modes of amino acid residues