Infrared Photoactivation Enables nano-DESI MS of Protein Complexes in Tissue on a Linear Ion Trap Mass Spectrometer

Native mass spectrometry analysis of proteins directly from tissues can be performed by using nanospray-desorption electrospray ionization (nano-DESI). Typically, supplementary collisional activation is essential to decluster protein complex ions from solvent, salt, detergent, and lipid clusters that comprise the ion beam. As an alternative, we have implemented declustering by infrared (IR) photoactivation on a linear ion trap mass spectrometer equipped with a CO₂ laser (λ = 10.6 μm). The prototype system demonstrates declustering of intact protein complex ions up to approximately 50 kDa in molecular weight that were sampled directly from brain and eye lens tissues by nano-DESI. For example, signals for different metal binding states of hSOD1^G93A homodimers (approximately 32 kDa) separated by only approximately 6 Th (10+ ions) were resolved with IR declustering, but not with collisional activation. We found IR declustering to outperform collisional activation in its ability to reduce chemical background attributable to nonspecific clusters in the nano-DESI ion beam. The prototype system also demonstrates in situ native MS on a low-cost mass spectrometer and the potential of linear ion trap mass spectrometers for this type of analysis.