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DMSO-Induced Denaturation of Hen Egg White Lysozyme

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journal contribution
posted on 16.09.2010, 00:00 by Ilja K. Voets, Willemberg A. Cruz, Christian Moitzi, Peter Lindner, Elizabeth P. G. Arêas, Peter Schurtenberger
We report on the size, shape, structure, and interactions of lysozyme in the ternary system lysozyme/DMSO/water at low protein concentrations. Three structural regimes have been identified, which we term the “folded” (0 < φDMSO < 0.7), “unfolded” (0.7 ≤ φDMSO < 0.9), and “partially collapsed” (0.9 ≤ φDMSO < 1.0) regime. Lysozyme resides in a folded conformation with an average radius of gyration of 1.3 ± 0.1 nm for φDMSO < 0.7 and unfolds (average Rg of 2.4 ± 0.1 nm) above φDMSO > 0.7. This drastic change in the protein’s size coincides with a loss of the characteristic tertiary structure. It is preceded by a compaction of the local environment of the tryptophan residues and accompanied by a large increase in the protein’s overall flexibility. In terms of secondary structure, there is a gradual loss of α-helix and concomitant increase of β-sheet structural elements toward φDMSO = 0.7, while an increase in φDMSO at even higher DMSO volume fractions reduces the presence of both α-helix and β-sheet secondary structural elements. Protein−protein interactions remain overall repulsive for all values of φDMSO. An attempt is made to relate these structural changes to the three most important physical mechanisms that underlie them: the DMSO/water microstructure is strongly dependent on the DMSO volume fraction, DMSO acts as a strong H-bond acceptor, and DMSO is a bad solvent for the protein backbone and a number of relatively polar side groups, but a good solvent for relatively apolar side groups, such as tryptophan.