Development of Photodissociation Methods for Biomolecule Analysis in a Quadrupole Ion Trap Mass Spectrometer

Photodissociation methods have been implemented and compared to collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer for the structural analysis of peptides, proteins, oligosaccharides, DNA and DNA/drug complexes. Infrared multiphoton dissociation (IRMPD) was applied to N-terminally sulfonated peptides which offers efficient photo-fragmentation and detection of important low m/z fragments in comparison to CID. Upon IRMPD of these modified peptides a simplified MS/MS spectrum comprised of only characteristic y ions allows for better identification through de novo software analysis. Oligonucleotides can undergo highly efficient IRMPD due to the phosphate moiety located on along their backbone structure which yields excellent photon absorption at lambda = 10.6 mum. IRMPD fragmentation pathways of DNA and DNA/drug complexes were shown to be comparable to CID, yielding cleavage at the [w / (a - B)] bond, except IRMPD allows for significantly improved MS/MS sensitivity through the secondary dissociation of uninformative duplex base losses which can further dissociate into useful fragment ions for sequencing. Ultraviolet photodissociation (UVPD) has been applied to chromophore-derivatized peptides and oligosaccharides which retains the advantages associated with IRMPD, but also has additional benefits due to the greater energy per photon at 355 nm (3.5 eV/photon) in comparison to 10.6 mum (0.12 eV/photon). Primarily, UVPD provides highly efficient secondary dissociation of chromophore-containing fragments allowing for simplified MS/MS spectra of chromophore-derivatized peptides. This concept was also implemented for the characterization of branched fluorescently-labeled oligosaccharides which produces different fragment ions complementary to CID experiments. Secondly, UVPD provides an ion activation method which is independent of the bath gas helium pressure in the ion trap in contrast to CID or IRMPD permits for optimal trap performance without compromise. Coordination of a chromogenic 18-crown-6 molecule to the lysine side chain of a peptide facilitates UVPD at both 266 nm and 355 nm. Energy absorbed by the crown ether is transferred intermolecularly to the peptide via the strong hydrogen bonds which hold the complex together, resulting in activation and fragmentation of the peptide. CID or IRMPD of these crown ether/peptide complexes results only in their disassembly without peptide fragmentation.