or in a loop with an α -helical element [33]. The increase in the intensity ratio (I R : I α ) in the inset of Fig. 4 indicates unfolding of the α -helix or the α -helical turn element to a random coil of adsorbed protein molecule with time. It is interesting to note that the φ , ψ angles of Trp-123 in lysozyme are -124.9,-2.5 [red arrow in Fig. 6], that is close to the α -helical region of the Ramachandran plot. Furthermore, the disulfide bond is expected to be affected by the change in protein backbone conformation [34]. Of all four S-S bonds, two of them, (S6-S127) and (S30-S115) are very close to Trp-123. As the turn element, at the arrow in Fig. 6, gets affected due to the interaction with the Ag surface, S6-S127/S30-S115 bond/bonds may break down and this explains the decrease in the intensity of S-S vibrational mode with time (Fig. 5(d)). Furthermore, Glu-7 of lysozyme lies very close to the S6-S127 bond with an ASA corresponding to 82.4 ˚ A $^{2}$. If the S-S bond breaks down (Fig. 5(d)), the vibrational mode of Glu-7 is expected to be affected in a similar fashion (Fig. 5(c)). Moreover, it is to be noted that this particular region of the molecule corresponds to the C-terminal end of the protein. Hence, this region of the molecule is more flexible and susceptible to changes in the environment. Addition of lysozyme in Ag sol results in an immediate agglomeration of the Ag particles. The initial increase in the relative intensity of vibrational bands of amino-acid residues of the molecule with time in Fig. 5(a) and Fig. 5(b) suggest re-orientation of the residues to obtain a low energy configuration from an initial high energy state due to initial favorable electrostatic interactions during change in conformation. The time constant for the unfold- ing of the α -helix is also estimated to be 45 min (inset of Fig. 4). If we assume only the local electrostatic interaction between the adsorbates and metal colloids, all our results suggest that lysozyme molecules interact with Ag colloids instantaneously; however, it takes longer time for the protein molecule to be stabilized on the metal surface. In general, irreversible laws of thermodynamics are involved in adsorption of proteins on metal surfaces [35]. Var- ious time scales are involved in metal-molecule interactions. The fast steps are sometimes reversible (can be probed only by time-resolved experimental measurements) and the rela- tively slow steps (of time-scale varies from seconds to minutes) result in rearrangements of protein structure by the surface environment and in many cases, can be irreversible [35]. We believe that the initial metal-protein interaction via agglomeration of the particles involves the fast process mentioned above and the conformational changes of the lysozyme molecule 9