Structure of Muramidase (Lysozyme):VIII. Effect of Dimethyl Sulfoxide on the Stability of Muramidase

Previously, it has been concluded that the muramidase molecule consists of α-helical portion and internal fold and that the internal fold assumes intramolecular cross-β configura-tion and is stabilized by hydrophobic inter-actions. In order to substantiate the structure of the internal fold of the muramidase mole-cule, the hydrodynamic, optical rotatory, and spectral properties of muramidase in DMSO-water mixtures were studied.<br> Muramidase molecule has a configuration of greater flexibility in DMSO, while the molecular weight remains unchanged. In-frared absorption spectral data show the presence of β-structure as well as α-helix and disordered structure in the muramidase mole-cule in DO solution. In DMSO solution, on the other hand, the presence of β-structure was not detected. Thus, DMSO destroys mainly the β-structure in the muramidase molecule. The change in the optical rotatory properties of muramidase with the solvent composition in DMSO-water mixtures is also interpreted as the result of the disruption of β-structure. As inferred from the ultraviolet absorption spectra, the disruption of β-struc-ture accompanies the exposure of tryptophan residues which have been buried in the inter-nal fold. These facts suggest that the internal fold assumes β-structure. Previously, it was found that the presence of some organic solvents such as methanol, ethanol, or 2-chlo-roethanol increased only the value of -a₀, leaving the value of -b₀ unchanged. This fact can be also interpreted as the result of the disruption of cross-β structure.<br> It is also suggested that the refractive index at 546mμ of the internal fold is not far from 1.440.<br> The author wishes to thank Prof. T. Isemura for his encouragement. The author is greatly indebted to Dr. K. Kakiuchi for his helpful suggestion in deter-mining the molecular weight of muramidase by the Archibald method, to Drs. T. Miyazawa and K. Fukushima for their discussions on the infrared absor-ption spectral data, to Dr. S. Sakakibara for a sample of L-leucyl-L-tryptophan, and to Drs. A. Imanishi and T. Takagi for their valuable discussions. Miss A. Kotake kindly performed the ultracentrifuge experi-ments.