Infrared Photodissociation Spectroscopy of Electrosprayed Ions in a Fourier Transform Mass Spectrometer

Abstract

Previous gas-phase methods for infrared photodissociation spectroscopy (IRPD) require sample volatility. Our method instead uses electrospray ionization to introduce even large nonvolatile molecules into a Fourier transform mass spectrometer, where extended (>15 s) ion storage makes possible high sensitivity spectral measurements with an OPO laser over a range of 3050-3800 cm(-1). The spectra of 22 gaseous proton-bound amino acid complexes are generally correlated with the H-stretching frequencies established for O-H and N-H functional groups in solution. For theoretical structure predictions of the Gly2H+ and N-acylated Asp2H+ dimers, IRPD spectra clearly differentiate between the predicted lowest energy conformers. In contrast to solution, in the gas phase the glycine zwitterion is approximately 20 kcal/mol less stable than the neutral; however, glycine is clearly zwitterionic in the gaseous GlyLysH+ dimer. The level of theory is inadequate for the larger Lys2H+ dimer, as all low energy predicted structures have free carboxyl O-H groups, in contrast to the IR spectrum. IRPD appears to be a promising new technique for providing unique information on a broad range of biomolecular and other gaseous ions, especially on noncovalent bonding involving O-H and N-H groups.

Stepwise evolution of protein native structure with electrospray into the gas phase, 10 −12 to 10 2 s

“…A variety of methods have provided important data for the characterization of gaseous conformations of (22,48). The initial sidechain collapse ( Fig.…”

probing the gas-phase folding kinetics of peptide ions by IR activated DR-ECD

“…The effects of aqueous solvation and the intrinsic intramolecular interactions may be better revealed by studying the protein conformation in the gas phase in the absence of solvents. Making use of the soft ionization method of electrospray ionization (ESI) [3], a number of mass spectrometry (MS) based methods have been applied to investigate the protein conformation in the gas phase, including the ESI charge state distribution to determine the availability of ionization basic sites [4,5], HID exchange (HDX) to identify the exposed region of the conformation [6][7][8][9][10], drift tube ion mobility spectrometry (IMS) to measure the conformational cross section [10][11][12][13], high-field asymmetric waveform ion mobility spectrometry (FAIMS) to separate different conformers [10,14,15], infrared photodissociation spectroscopy (IRPDS) to probe the hydrogen bonding [16][17][18][19][20], and electron capture dissociation (ECD) [21,22] to locate the noncovalent tertiary bonding [23][24][25][26]. Particularly, ECD based methods have been used to study the gas-phase unfolding and refolding kinetics of protein ions [24, 25J. ECD in Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS or FTMS) [27, 28J has quickly found wide application in both top-down and bottom-up proteomics [21, 29-33J, as well as in identifying and locating post-translational modifications (PTMs) [34-38J.…”

Clear evidence of fluorescence resonance energy transfer in gas-phase ions

“…The general idea behind this is to simultaneously obtain completely "orthogonal" information on gasphase molecular conformation from optical spectroscopy. In principle, optical spectroscopic investigations thereby can be combined with any of the mass spectrometric methods described previously [9,10] Fluorescence resonance energy transfer (FRET) [16,17] is a distance-sensitive method that correlates changes in fluorescence intensity with changes in distance and orientation of specific spectroscopically active (donor, acceptor, quencher) moieties. The distance between fluorophores is defined as the distance between…”

“…The general idea behind this is to simultaneously obtain completely "orthogonal" information on gasphase molecular conformation from optical spectroscopy. In principle, optical spectroscopic investigations thereby can be combined with any of the mass spectrometric methods described previously [9,10]. Marshall et al have measured laser-induced fluorescence (LIF) excitation and emission spectra as well as LIF lifetimes of ions trapped in a Fourier transform ion cyclotron resonance (FTICR) cell [11][12][13].…”