Infrared photoactivation enables nano-DESI MS of protein complexes from tissue on a linear ion trap mass spectrometer.

Native mass spectrometry analysis of proteins directly from tissues can be performed 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. The prototype system demonstrates declustering of intact protein complex ions up to approx. 50 kDa in molecular weight that were sampled directly from brain and eye lens tissues by nano-DESI. For example, signals attributable to different metal binding states of hSOD1G93A homodimers (approx. 32 kDa) separated by only approx. 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 non-specific 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.