Ultraviolet Photodissociation of Native Proteins Following Proton Transfer Reactions in the Gas Phase.

The growing use of mass spectrometry in the field of structural biology has catalyzed the development of many new strategies to examine intact proteins in the gas phase. Native mass spectrometry methods have further accelerated the need for methods that can manipulate proteins and protein complexes while minimizing disruption of noncovalent interactions critical for stabilizing conformations. Proton-transfer reactions (PTR) in the gas phase offer the ability to effectively modulate the charge states of proteins, allowing decongestion of mass spectra through separation of overlapping species. PTR was combined with ultraviolet photodissociation (UVPD) to probe the degree of structural changes that occur upon charge reduction reactions in the gas phase. For protein complexes myoglobin·heme (17.6 kDa) and dihydrofolate reductase·methotrexate (19.4 kDa), minor changes were found in the fragmentation patterns aside from some enhancement of fragmentation near the N- and C-terminal regions consistent with slight fraying. After finding little perturbation was caused by charge reduction using PTR, homodimeric superoxide dismutase/CuZn (31.4 kDa) was subjected to PTR in order to separate overlapping monomer and dimer species of the protein that were observed at identical m/z values.