Collision cross sections for protein ions

A dynamic method is applied to measure the mobility of gas-phase ions in the dual ion funnel interface of the electrospray source of a quadrupole orthogonal time-of-flight mass spectrometer. In a new operational mode, a potential barrier was formed in the second ion funnel of the mass spectrometer and then progressively increased. In this region, a flow of gas drags the ions into the mass spectrometer while the electric force applied by the potential barrier decelerates them. Ions with lower mobility can be carried by the gas flow more easily than those with high mobility. Thus, electrical forces can block the more mobile ions more easily. Hence, the electric barrier formed in the ion funnel permits only ions below a certain mobility threshold to enter the mass spectrometer. When the barrier voltage is increased, this threshold moves from high to low mobilities. Ions with mobilities above the threshold cannot enter the mass spectrometer, and their signal decreases to zero. Thus, in a barrier voltage scan, mass spectrometric signals of ions sequentially disappear. Differentiation of these decreasing ion signal curves produces peaks from which an ion mobility spectrum can be reconstructed. Blocking voltages, i.e., the positions of the peaks on the barrier voltage scale are directly related to the mobility of these ions. An internal calibration using ions with known mobility values helps determine the unknown ion mobilities and allows calculation of ionic cross sections. (J Am Soc Mass

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 91%

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Applying a dynamic method to the measurement of ion mobility

Baykut

Halem

Raether

2009

“…Collision cross sections of ions were measured with axial kinetic energy loss experiments with a triple quadrupole mass spectrometer [13,39,40]. Protonated protein ions, generated by ESI, pass through an orifice in a curtain plate (880 V), an orifice (220 V), a skimmer (120 V), (orifice-skimmer voltage difference ⌬V OS ϭ 100 V), and enter an rf-only quadrupole ion guide (Q 0 ) with a DC offset of 117.5 V. The pressure in the ion guide is ϳ4 mTorr, measured with a precision capacitance manometer (model 120AA; MKS Instruments, Andover, MA).…”

Section: Collision Cross Sectionsmentioning

confidence: 99%

Gas-phase H/D exchange and collision cross sections of hemoglobin monomers, dimers, and tetramers

Wright

Douglas

2009

Self Cite

The conformations of gas-phase ions of hemoglobin, and its dimer and monomer subunits have been studied with H/D exchange and cross section measurements. During the H/D exchange measurements, tetramers undergo slow dissociation to dimers, and dimers to monomers, but this did not prevent drawing conclusions about the relative exchange levels of monomers, dimers, and tetramers. Assembly of the monomers into tetramers, hexamers, and octamers causes the monomers to exchange a greater fraction of their hydrogens. Dimer ions, however, exchange a lower fraction of their hydrogens than monomers or tetramers. Solvation of tetramers affects the exchange kinetics. Solvation molecules do not appear to exchange, and solvation lowers the overall exchange level of the tetramers. Cross section measurements show that monomer ions in low charge states, and tetramer ions have compact structures, comparable in size to the native conformations in solution. Dimers have remarkably compact structures, considerably smaller than the native conformation in solution and smaller than might be expected from the monomer or tetramer cross sections. This is consistent with the relatively low level of exchange of the dimers. . Electrospray ionization has allowed the study of protein ions in the gas phase, free of water. In the absence of water, monomeric proteins can adopt compact conformations similar to the solution conformations, but also extended conformations far removed from the native state [2,3]. Much of the current understanding of the "size" of protein ions in the gas phase comes from physical methods of measuring collision cross sections using ion mobility spectrometry at pressures of a few Torr [4 -7], at atmospheric pressure [8], or at pressures of ca. 1 ϫ 10 Ϫ3 Torr [7,9,10]. Measurements of ion kinetic energy loss in triple quadrupole systems have also been used to determine collision cross sections of proteins [11][12][13] and protein-ligand complexes [11,14,15]. Chemical probes of protein conformation have been used as well, with much of the work using gas-phase hydrogen deuterium exchange (H/Dx) of trapped ions, either in linear [16 -19] or 3D [20] quadrupole ion traps, or ion cyclotron resonance (ICR) mass spectrometers [21][22][23][24][25].The study of the structure of monomeric proteins in the gas phase has been extended to assemblies containing more than one protein. Ion mobility measurements of complex macromolecular protein complexes at atmospheric pressure [8] or at a pressures of ca. 1 ϫ 10 Ϫ3 Torr have shown some success [9,10]. The resolution of these experiments, ca. 5-20, is lower than more conventional mobility measurements at pressures of a few Torr.Protein-protein and protein-ligand complexes can be studied by gas-phase H/Dx of trapped ions. The pressures of deuterating reagent in linear trap experiments can be ϳ5 ϫ 10 Ϫ3 Torr, ca. 10 3 times greater than used in 3D traps [20], 10 1 to 10 2 times greater than used in mobility experiments [26,27], and 10 3 to 10 4 times greater than in ICR experiments [21][22][2...

“…The number of collisions is then proportional to the relative velocity of the ion and neutral where the magnitude of the relative velocity is defined as [24] V rel 2 ϭ V 2 ϩ U 2 Ϫ 2VU cos (5) where V is the thermal velocity of the collision gas, U is the velocity of the ion and is the centre of mass scattering angle. The average centre of mass scattering angle can be taken as 90° [25]. At low ion kinetic energy the thermal motion of the gas becomes significant whereas at high kinetic energies (Ͼ1-2 eV in the case of the ion reserpine and nitrogen as the collision gas) the gas thermal velocity becomes insignificant.…”

Section: Computer Simulations Of Resonant Excitationmentioning

confidence: 99%

Resonant excitation in a low-pressure linear ion trap

Collings¹,

Stott²,

Londry³

2003

It has been shown that through the process of resonant excitation the fragmentation of ions confined in a low-pressure (Ͻ0.05 mTorr) linear ion trap (LIT) can be accomplished while maintaining both high fragmentation efficiency and high resolution of excitation. The ion reserpine, 609.23 Da, has been fragmented with efficiencies greater than 90% while a higher mass ion, a homogeneously substituted triazatriphosphorine of mass 2721.89 Da, has been fragmented with 48% efficiency. This was accomplished by extended resonant excitation by low-amplitude auxiliary RF signals. Computer modelling of ion trajectories and analysis of the trapping potentials have demonstrated that a reduction in neutralization of ions on the rods (or losses on the rods) and increased fragmentation is a consequence of higher order terms in the potential introduced by the round-rod geometry of the LIT. (LIT), leading to neutralization on the rods and/or fragmentation, has been demonstrated previously using both quadrupolar and dipolar auxiliary RF signals [1][2][3][4]. Typically, these experiments were carried out using nitrogen collision gas, at pressures ranging from 1.5 to 7 mTorr, with exposures to resonant excitation of a few milliseconds. At these pressures and exposures it was possible to obtain increased resolutions of excitation from 75 to 250 as the pressure in the LIT was decreased [3]. In those experiments, resolution decreased as the amplitude of the auxiliary signal was increased. However, the auxiliary amplitude required for either neutralization on the rods or fragmentation was dependent upon the pressure of the buffer gas. When the auxiliary amplitude was too low, thermalizing collisions reduced ion energies, both translational and internal, at a rate greater than energy was added by the auxiliary signal. In consequence, there existed a threshold for the auxiliary amplitude below which neutralization/fragmentation did not occur [5,6]. When the auxiliary amplitude was only slightly above this threshold, resolution of excitation was high, but the degree of neutralization/fragmentation was low [3]. In order to achieve a high degree of neutralization/fragmentation, it was necessary to increase the auxiliary amplitude and suffer the decreased resolution.The range of frequencies, about the secular frequency, to which ions respond (frequency response) increases with pressure. This behavior is predicted by the theory of a forced damped harmonic oscillator (FDHO). Models based on the FDHO theory have been used to describe the behavior of ions confined in a Paul trap at pressures in the mTorr regime [7,8]. However, when excited on-resonance at pressures below one mTorr, it has been assumed generally that ions would be lost on the rods before significant fragmentation occurred. In contrast, it is well known that effective fragmentation at low pressures (1 ϫ 10 Ϫ4 to 1 ϫ 10 Ϫ6 torr) can be achieved through the use of off-resonance excitation, as demonstrated in sustained off-resonance irradiation (SORI) experiments in FT-ICR [9 -11].In t...