We studied by small angle x-ray scattering (SAXS) the tetramethylurea (TMU) effects on the tertiary structure of lysozyme in water. Samples containing TMU with mass fractions, w, ranging from 0 to 0.7 were analyzed. The SAXS results clearly demonstrate a structural transition from a spheroidal molecule (0⩽w⩽0.6) to a highly anisometric molecule for w=0.7. Additional SAXS results for samples with w=0.7, from which the TMU was removed, showed that this transition is reversible. The variation of the maximum dimension of the molecule with w fits an equation corresponding to a “two-state” model. At w=0.6 (critical fraction) a mixture of folded (spheroidal) and unfolded (anisometric) molecules coexists in the solution.
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Castelletto et al. (1998) studied this question.
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