Raman study of the sulfoxide effect on the thermal denaturation of lysozime.

The addition of organic solvents with both polar and apolar moieties, like sulfoxides, to protein aqueous solutions is able to affect the protein structure by weakening the hydrophobic interaction between apolar residues as well as by perturbing the characteristic water structure around the protein molecule. In the present paper we present a Raman study on the effect ot two sulfoxides, dimethyl- and diethyl-sulphoxide (DMSO and DESO) on the thermal inactivation of lysozyme (Lyso). Raman spectra were recorded by using a Jasco NRS 5000 spectrometer (exciting line 488 nm; laser power at the sample 15 mW). The enzymatic activity of Lyso before and after heating in the presence of different sulfoxide amount was assayed by the colorimetric method based on the enzymatic breaking up of cells of M. lysodeikticus. The denaturing thermal effect on Lyso was noticeable reduced by the presence of sulfoxides. Indeed, after the heating cycle, the residual activity of Lyso resulted to be 93% and 87% in the presence of DESO (25% w/w) and DMSO (40% w/w) respectively, whereas it decreased up to 60% in the absence of sulfoxides. The presence of sulfoxides induced a small decrease in the enzymatic activity of Lyso also before heating. This effect was slightly more evident in the present of DESO than DMSO, according with their hydrophobic properties. The sulfoxide effect on the secondary structure of Lyso was evaluated before and after heating by the analysis of the Raman amide I band, appearing in the 1620-1700 cm-1 spectral region. The data confirmed the ability of both DMSO and DESO to act as effective protective agent against the thermal denaturation of the protein. In fact, in the absence of sulfoxides the heating induced a decrease in the -helix content of 9% instead of only 0-3% in the presence of DMSO (40%) and DESO (25%). Also the -sheet conformation showed a similar behaviour by increasing of 10% and 3% in absence and the presence of the two sulfoxides, respectively. Moreover, our data suggest that in the more concentrated DESO-containing samples (40%) the greater hydrophobic character prevails, leading to a destructurizing effect on the Lyso structure.