Proteins may exhibit an unfolding or folding state in the presence of a surfactant. In the present study, the unfolding and folding pathway of hen egg white lysozyme (HEWL) induced by sodium dodecyl sulfate (SDS) is studied. The stoichiometry obtained from isothermal titration calorimetry (ITC) provides guidelines for other techniques. The fluorescence spectra and circular dichroism show that the fluorescence properties and secondary structure of proteins undergo a two-step change upon binding with SDS, in which the intensity decreases, the emission blue shifts and the helical conformation decreases at low ratios of SDS to HEWL, while all of them return to the native-like state upon the addition of SDS at higher ratios. At the end of the binding, HEWL presents a higher α-helical content but its tertiary structure is lost compared to its native state, which is namely a molten globule state. Small angle X-ray scattering (SAXS) analysis and the derived model reveal that the complexes possess a decorated core-shell structure, with the core composed of dodecyl chains and the shell consisting of SDS head groups with a protein in molten globule state. Five binding steps, including the individual details involved in the denaturation, were obtained to describe the unfolding and folding pathway of HEWL induced by SDS. The results of this study not only present details about the denaturation of protein induced by SDS and the structure of the complexes involved in each binding step, but also provide molecular insights into the mechanism of the higher helical conformation of proteins in the presence of surfactant micelles.
Two commercial exogenous pulmonary surfactants, Curosurf and Survanta,
are investigated. Their thermotropic behavior and associated structural
changes for the samples in bulk are characterized and described. For
Survanta, the obtained results of differential scanning calorimetry
showed a thermogram with three peaks on heating and only a single
peak on cooling. Curosurf on the other hand, presents calorimetric
thermograms with only one peak in both the heating and cooling scans.
This distinct thermotropic behavior between the two pulmonary surfactants,
a consequence of their particular compositions, is associated with
structural changes that were evaluated by simultaneous small- and
wide-angle X-ray scattering experiments with in situ temperature variation. Interestingly, for temperatures below ∼35
°C for Curosurf and ∼53 °C for Survanta, the scattering
data indicated the coexistence of two lamellar phases with different
carbon chain organizations. For temperatures above these limits, the
coexistence of phases disappears, giving rise to a fluid phase in
both pulmonary surfactants, with multilamelar vesicles for Curosurf
and unilamellar vesicles for Survanta. This process is quasi-reversible
under cooling, and advanced data analysis for the scattering data
indicated differences in the structural and elastic properties of
the pulmonary surfactants. The detailed and systematic investigation
shown in this work expands on the knowledge of the structure and thermodynamic
behavior of Curosurf and Survanta, being relevant from both physiological
and biophysical perspectives and also providing a basis for further
studies on other types of pulmonary surfactants.
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