Characterization of the conformational properties of denatured proteins is essential to our understanding of the molecular basis of protein folding and stability. Here we combine small angle neutron and X-ray scattering to study the interaction of urea with the protein ubiquitin. Comparing coherent intensities scattered at zero angle, and exploiting the scattering densities of H(2)O, D(2)O, ubiquitin, and urea for X-rays and neutrons, we quantitatively determine the number of urea molecules preferentially bound during unfolding of ubiquitin. We find that a pH change from 6.5 to 2.5 triggers recruitment of approximately 20 urea molecules from bulk solution per ubiquitin molecule during the unfolding process.
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