Protein Conformation and Supercharging with DMSO from Aqueous Solution |
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Authors: | Harry J Sterling James S Prell Catherine A Cassou Evan R Williams |
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Institution: | (1) Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA; |
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Abstract: | The efficacy of dimethyl sulfoxide (DMSO) as a supercharging reagent for protein ions formed by electrospray ionization from
aqueous solution and the mechanism for supercharging were investigated. Addition of small amounts of DMSO to aqueous solutions
containing hen egg white lysozyme or equine myoglobin results in a lowering of charge, whereas a significant increase in charge
occurs at higher concentrations. Results from both near-UV circular dichroism spectroscopy and solution-phase hydrogen/deuterium
exchange mass spectrometry indicate that DMSO causes a compaction of the native structure of these proteins at low concentration,
but significant unfolding occurs at ~63% and ~43% DMSO for lysozyme and myoglobin, respectively. The DMSO concentrations required
to denature these two proteins in bulk solution are ~3–5 times higher than the concentrations required for the onset of supercharging,
consistent with a significantly increased concentration of this high boiling point supercharging reagent in the ESI droplet
as preferential evaporation of water occurs. DMSO is slightly more basic than m-nitrobenzyl alcohol and sulfolane, two other supercharging reagents, based on calculated proton affinity and gas-phase basicity
values both at the B3LYP and MP2 levels of theory, and all three of these supercharging reagents are significantly more basic
than water. These results provide additional evidence that the origin of supercharging from aqueous solution is the result
of chemical and/or thermal denaturation that occurs in the ESI droplet as the concentration of these supercharging reagents
increases, and that proton transfer reactivity does not play a significant role in the charge enhancement observed. |
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