Development of ultraviolet photodissociation based tandem mass spectrometry methods for the characterization of protein macromolecular structures and glycolipids

Abstract

Photon-based tandem mass spectrometry provides a versatile ion activation strategy for the analysis of polypeptides, proteins, and lipids. 351-nm ultraviolet photodissociation mass spectrometry (UVPD-MS) is a facile and selective tandem dissociation technique used to elucidate chromophore-modified peptides within large mixtures. A bis-aryl chromogenic chemical probe was utilized to target solvent exposed primary amine residues within native protein states. Collision-induced dissociation (CID) was employed to indiscriminatly characterize the complete proteolytic digest while chromophore containing peptides were selectively dissociated with 351-nm UVPD; thus streamlining the identification of targeted peptides with structurally informative residues. Protein amine residue reactivities were then compared with predicted solvent exposures to elucidate protein tertiary structures, their mechanistic properties, and ligand-binding interactions. High-energy 193-nm UVPD is a fast, high-energy tandem mass spectrometry method and frequently generates fragment ions typically inaccessible to CID-based methods. Native mass spectrometry was coupled to top-down 193-nm UVPD for the gas phase characterization of non-covalent protein-ligand and protein-protein complexes. This method yielded a unique array of fragment ions for a comprehensive analysis of protein structures. UVPD of non-covalent complexes generated many polypeptide backbone fragments to characterize the primary sequence of proteins. Furthermore, top-down UVPD engendered cleavages with intact electrostatic interactions; this provided insight into the binding interfaces within protein-ligand complexes and the higher order structural architectures of oligomeric complexes. High-resolution 193-nm UVPD was paired with high performance liquid chromatography (LC) for the streamlined structural analysis of amphiphilic glycolipids within complex mixtures. For all glycolipids, UVPD provided the most comprehensive structural analysis tool by affording a diverse array of fragment ions to characterize both hydrophobic and hydrophilic moieties. UVPD based LC-MS separations of gangliosides shed light on the ceramide lipid bases, glycan moieties, and their isobaric structural variants. UVPD activation of lipid A and lipooligosaccharides (LOS) compounds generated a mixture of C-C, C-O, and C-N fragment ions to illustrate the hydrophobic acyl structures, while cleavages within the glycosidic, and cross-ring cleavages allowed the determination of acylation patterns. Novel LC-MS separation strategies were developed to elucidate and structurally characterize complex mixtures of lipopolysaccharide containing compounds.