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 < DMSO < 0.7), "unfolded" (0.7 e DMSO < 0.9), and "partially collapsed" (0.9 e DMSO < 1.0) regime. Lysozyme resides in a folded conformation with an average radius of gyration of 1.3 ( 0.1 nm for DMSO < 0.7 and unfolds (average R g of 2.4 ( 0.1 nm) above DMSO > 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 R-helix and concomitant increase of -sheet structural elements toward DMSO ) 0.7, while an increase in DMSO at even higher DMSO volume fractions reduces the presence of both R-helix and -sheet secondary structural elements. Protein-protein interactions remain overall repulsive for all values of DMSO . 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.

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