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Dielectric Behavior of Lysozyme and Ferricytochrome-c in Water/Ethylene-Glycol Solutions

A. Bonincontro ^*, S. Cinelli , G. Onori  and A. Stravato ^*

^* INFM-Dipartimento di Fisica, Università "La Sapienza," I-00185 Rome, Italy; and INFM-Dipartimento di Fisica, Università di Perugia, I-06123 Perugia, Italy

Correspondence: Address reprint requests to A. Bonincontro, E-mail: adalberto.bonincontro{at}uniroma1.it.

This work deals with a dielectric study at radio frequencies of the influence at room temperature of two organic molecules, known as cryo-protectants, ethylene-glycol and glycerol, on conformational and dynamic properties of two model proteins, lysozyme (lys) from chicken egg-white and ferricytochrome-c (cyt-c) from horse heart. Cyt-c is a compact globular protein whereas lys is composed of two structural domains, separated by the active site cleft. Measurements were carried out at the fixed temperature of 20°C varying the concentration of the cosolvent up to 90% w/w. From the analysis of the dielectric relaxation of the protein solution, the effective hydrodynamic radius and the electric dipole moment of the protein were calculated as a function of the cosolvent concentration. The data show that glycerol does not modify significantly the conformation of both proteins and cyt-c is also stable in the presence of ethylene-glycol. On the contrary ethylene-glycol strongly affects the dielectric response of lysozyme denoting a specific effect on its conformation and dynamics. The data are coherently interpreted hypothesizing that glycol molecule wedges between and separates the two domains of lys making them rotationally independent.