Nanoflow Size Exclusion Chromatography – Native Mass Spectrometry of Intact Proteoforms and Protein Complexes

Native size-exclusion chromatography (SEC) coupled with native mass spectrometry (nMS) enables the characterization of proteins and protein complexes by combining liquid-phase separation (SEC) and mass measurement of the complexes (nMS). This approach allows for an increase in the throughput of native MS experiments, reduces the bias that may be present due to the co-ionization of oligomers, and facilitates online sample buffer exchange. SEC-nMS uses volatile buffers and relatively wide-diameter columns (e.g., ≥ 1 mm), with flow rates in the tens of µL/minute. To ionize sample components under this flow regime, relatively harsh electrospray ionization (ESI) desolvation conditions are needed, which may result in protein dissociation/denaturation. Also, relatively large amounts of samples are required (several µgs). Herein, we describe the development of a nanoflow SEC-nMS method using 200 µm I.D. columns, operated at 500 nL•min-1. This approach allows buffer exchange, oligomer separation, and mild ionization conditions (e.g., without the assistance of heated gas flow or temperature). Compared to microflow (1 mm I.D. column), the nanoflow method achieved a 4-fold increase in MS peak intensity, despite using a sample 20 times less concentrated (0.05 mg mL-1 for nanoflow vs. 1 mg mL-1 for microflow). Furthermore, we evaluated the impact in terms of sensitivity and separation efficiency of three injection approaches: large-volume injection (1μL), nano-volume injection (50nL), and an online mix-bed ion-exchange capillary trap column. The final method, using nano-volume injection, was applied to several model protein and protein complexes to showcase performance and applicability to the study of sample-limited analysis.