Diffusion measurements by electrospray mass spectrometry for studying solution-phase noncovalent interactions

This study describes a novel approach for monitoring noncovalent interactions in solution by electrospray mass spectrometry (ESI-MS). The technique is based on measurements of analyte diffusion in solution. Diffusion coefficients of a target macromolecule and a potential low molecular weight binding partner are determined by measuring the spread of an initially sharp boundary between two solutions of different concentration in a laminar flow tube (Taylor dispersion), as described in Rapid Commun. Mass Spectrom. 2002, 16, 1454-1462. In the absence of noncovalent interactions, the measured ESI-MS dispersion profiles are expected to show a gradual transition for the macromolecule and a steep transition for the low molecular weight compound. However, if the two analytes form a noncovalent complex in solution the dispersion profiles of the two species will be very similar, since the translational diffusion of the small compound is determined by the slow Brownian motion of the macromolecule. In contrast to conventional ESI-MS-based techniques for studying noncovalent complexes, this approach does not rely on the preservation of solution-phase interactions in the gas phase. On the contrary, "harsh" conditions at the ion source are required to disrupt any potential gas- phase interactions between the two species, such that their dispersion profiles can be monitored separately. The viability of this technique is demonstrated in studies on noncovalent heme-protein interactions in myoglobin. Tight noncovalent binding is observed in solutions of pH 10, both in the absence and in the presence of 30% acetonitrile. In contrast, a significant disruption of the noncovalent interactions is seen at an acetonitrile content of 50%. Under these conditions, the diffusion coefficient of heme in the presence of myoglobin is only slightly lower than that of heme in a protein-free solution. A breakdown of the noncovalent interactions is also observed in aqueous solution of pH 2.4, where myoglobin is known to adopt an acid-unfolded conformation.