The theoretical equations for unfolding of globular proteins are applied to the unfolding of monomeric bacteriorhodopsin by the action of urea. The optical properties of the monomeric bacteriorhodopsin detergent (MBD) complex demonstrate an equilibrium between two conformations - unique native and unfolded. The stability of the MBD complex is significantly changed in urea buffer solution and the destabilizing energy of the protein is estimated to be about 20 kcal/mol. The model of unfolding used proves the existence of two steps - 'molecular swelling' - a process of low cooperativity and a lowerstability of about 36 kcal/mol and the 'true unfolding' - a highly cooperative characterized with stability of about 55 kcal/mol. The mechanism of unfolding demonstrates a value of 6 for the ratio of the theoretical to the observed sharpness of the transition which is too large to beassumed as an existence of intermediate forms. © 1989 Elsevier Science Publishers B.V. (Biomedical Division).
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