Measuring the hydrodynamic radii of peptides and proteins with an unmodified LC-ESI-MS instrument operating in a Taylor dispersion regime

Abstract

Native ion mobility – mass spectrometry has been well established in recent years as a structural biology method. However, it is still useful to combine gas-phase techniques with complementary solution-phase measurements, for example hydrodynamic radius (R_h) determination. A common method to measure protein R_h is the use of Taylor dispersion analysis (TDA), which relies on measuring the longitudinal dispersion of an analyte in a laminar flow within a capillary. This type of analysis most often relies on spectroscopic (e.g., fluorescence-based) detection, although a few studies have reported the use of custom-built or modified fluidics systems coupled with MS-based detection. Here, we have repurposed a standard, unmodified LC-MS system, operated without a column, to measure hydrodynamic radii of peptides and proteins, including noncovalent complexes, ranging from 300 Da to 133 kDa. The R_h values we measured showed excellent correlation with gas-phase collision cross-sections as well as R_h values measured with a conventional TDA instrument with fluorescence-based detection. Notably, due to the mass-selective readout of our method, which is based on analysis of extracted ion chromatograms, we were able to perform multiplexed R_h measurements of several proteins in a single experiment, and also measured apparent R_h values for different oligomeric states of BSA and concanavalin A. Consistent with prior literature, the latter data suggested that concanavalin A primarily exists as a dimer and tetramer in solution, with monomer signals being largely due to dissociation during electrospray ionisation. We have named our method Hydrodynamic Radii from an Unmodified Liquid Chromatography-Mass Spectrometer (HYDRAULIC-MS).