Ion mobility–mass spectrometry for structural proteomics

References

• Alberts B. The cell as a collection of protein machines: preparing the next generation of molecular biologists. Cell92(3), 291–294 (1998).
   PubMed Web of Science ®Google Scholar
• Robinson CV, Sali A, Baumeister W. The molecular sociology of the cell. Nature450(7172), 973–982 (2007).
   PubMed Web of Science ®Google Scholar
• Alber F, Kim MF, Sali A. Structural characterization of assemblies from overall shape and subcomplex compositions. Structure13(3), 435–445 (2005).
   Google Scholar
• Sali A, Glaeser R, Earnest T, Baumeister W. From words to literature in structural proteomics. Nature422(6928), 216–225 (2003).
   PubMed Web of Science ®Google Scholar
• Von Heijne G. Membrane-protein topology. Nat. Rev. Mol. Cell Biol.7(12), 909–918 (2006).
   PubMed Web of Science ®Google Scholar
• Pierce KL, Premont RT, Lefkowitz RJ. Seven-transmembrane receptors. Nat. Rev. Mol. Cell Biol.3(9), 639–650 (2002).
   PubMed Web of Science ®Google Scholar
• Steven AC, Baumeister W. The future is hybrid. J. Struct. Biol.163(3), 186–195 (2008).
   PubMed Web of Science ®Google Scholar
• Wyttenbach T, Bowers MT. Intermolecular interactions in biomolecular systems examined by mass spectrometry. Annu. Rev. Phys. Chem.58511–58533 (2007).
   Google Scholar
• Barran PE, Jurneczko E. How useful is ion mobility mass spectrometry for structural biology? The relationship between protein crystal structures and their collision cross sections in the gas phase. Analyst136(1), 20–28 (2011).
   Google Scholar
• Scrivens JH, Scarff CA, Thalassinos K, Hilton GR. Travelling wave ion mobility mass spectrometry studies of protein structure: biological significance and comparison with X-ray crystallography and nuclear magnetic resonance spectroscopy measurements. Rapid Commun. Mass Spectrom.22(20), 3297–3304 (2008).
   Google Scholar
• Ruotolo BT, Giles K, Campuzano I et al. Evidence for macromolecular protein rings in the absence of bulk water. Science310(5754), 1658–1661 (2005).
   PubMed Web of Science ®Google Scholar
• Benesch JLP, Ruotolo BT, Simmons DA, Robinson CV. Protein complexes in the gas phase: technology for structural genomics and proteomics. Chem. Rev.107(8), 3544–3567 (2007).
   PubMed Web of Science ®Google Scholar
• Heck AJR. Native mass spectrometry: a bridge between interactomics and structural biology. Nat. Methods5(11), 927–933 (2008).
   PubMed Web of Science ®Google Scholar
• Ruotolo BT, Robinson CV. Aspects of native proteins are retained in vacuum. Curr. Opin. Chem. Biol.10(5), 402–408 (2006).
   PubMed Web of Science ®Google Scholar
• Politis A, Park AY, Hyung SJ et al. Integrating ion mobility mass spectrometry with molecular modelling to determine the architecture of multiprotein complexes. PLoS One5(8), e12080 (2010).
   Google Scholar
• Taverner T, Hernandez H, Sharon M et al. Subunit architecture of intact protein complexes from mass spectrometry and homology modeling. Acc. Chem. Res.41(5), 617–627 (2008).
   PubMed Web of Science ®Google Scholar
• Hernandez H, Robinson CV. Determining the stoichiometry and interactions of macromolecular assemblies from mass spectrometry. Nat. Protoc.2(3), 715–726 (2007).
   PubMed Web of Science ®Google Scholar
• Sharon M, Robinson CV. The role of mass spectrometry in structure elucidation of dynamic protein complexes. Annu. Rev. Biochem.76167–76193 (2007).
   Google Scholar
• Zhou M, Robinson CV. When proteomics meets structural biology. Trends Biochem. Sci.35(9), 522–529 (2010).
   PubMed Web of Science ®Google Scholar
• Hernandez H, Dziembowski A, Taverner T, Seraphin B, Robinson CV. Subunit architecture of multimeric complexes isolated directly from cells. EMBO Rep.7(6), 605–610 (2006).
   PubMedGoogle Scholar
• Leary JA, Schenauer MR, Stefanescu R et al. Methodology for measuring conformation of solvent-disrupted protein subunits using t-wave ion mobility MS: an investigation into eukaryotic initiation factors. J. Am. Soc. Mass Spectrom.20(9), 1699–1706 (2009).
   Google Scholar
• Ruotolo BT, Hyung SJ, Robinson PM et al. Ion mobility–mass spectrometry reveals long-lived, unfolded intermediates in the dissociation of protein complexes. Angew. Chem. Int. Ed.46(42), 8001–8004 (2007).
   PubMed Web of Science ®Google Scholar
• Benesch JLP. Collisional activation of protein complexes: picking up the pieces. J. Am. Soc. Mass Spectrom.20(3), 341–348 (2009).
   PubMedGoogle Scholar
• Pukala TL, Ruotolo BT, Zhou M et al. Subunit architecture of multiprotein assemblies determined using restraints from gas-phase measurements. Structure17(9), 1235–1243 (2009).
   PubMedGoogle Scholar
• Mason EA, Mcdaniel EW. Transport Properties of Ions in Gases. John Wiley & Sons, NY, USA (1988).
   Google Scholar
• Harvey SR, Macphee CE, Barran PE. Ion mobility mass spectrometry for peptide analysis. Methods54(4), 454–461 (2011).
   Google Scholar
• Kanu AB, Dwivedi P, Tam M, Matz L, Hill HH. Ion mobility–mass spectrometry. J. Mass Spectrom.43(1), 1–22 (2008).
   PubMed Web of Science ®Google Scholar
• Eiceman GA, Karpas Z. Ion Mobility Spectrometry. CRC Press, FL, USA (2005).
   Google Scholar
• Shvartsburg AA. Differential Ion Mobility Spectrometry: Nonlinear Ion Transport and Fundamentals of FAIMS. CRC Press, FL, USA (2008).
   Google Scholar
• Kolakowski BM, Mester Z. Review of applications of high-field asymmetric waveform ion mobility spectrometry (FAIMS) and differential mobility spectrometry (DMS). Analyst132(9), 842–864 (2007).
   PubMed Web of Science ®Google Scholar
• Kurulugama R, Nachtigall F, Lee S, Valentine S, Clemmer D. Overtone mobility spectrometry: part 1. Experimental observations. J. Am. Soc. Mass Spectrom.20(5), 729–737 (2009).
   Google Scholar
• Valentine S, Stokes S, Kurulugama R, Nachtigall F, Clemmer D. Overtone mobility spectrometry: part 2. Theoretical considerations of resolving power. J. Am. Soc. Mass Spectrom.20(5), 738–750 (2009).
   Google Scholar
• Merenbloom SI, Glaskin RS, Henson ZB, Clemmer DE. High-resolution ion cyclotron mobility spectrometry. Anal.Chem.81(4), 1482–1487 (2009).
   PubMed Web of Science ®Google Scholar
• Clemmer DE, Jarrold MF. Ion mobility measurements and their applications to clusters and biomolecules. J. Mass Spectrom.32(6), 577–592 (1997).
   Web of Science ®Google Scholar
• Gillig KJ, Ruotolo B, Stone EG et al. Coupling high-pressure MALDI with ion mobility/orthogonal time-of flight mass spectrometry. Anal. Chem.72(17), 3965–3971 (2000).
   PubMed Web of Science ®Google Scholar
• Gillig KJ, Ruotolo BT, Stone EG, Russell DH. An electrostatic focusing ion guide for ion mobility–mass spectrometry. Int. J. Mass Spectrom.239(1), 43–49 (2004).
   Google Scholar
• Valentine SJ, Koeniger SL, Clemmer DE. A split-field drift tube for separation and efficient fragmentation of biomolecular ions. Anal. Chem.75(22), 6202–6208 (2003).
   Google Scholar
• Tang K, Shvartsburg AA, Lee HN et al. High-sensitivity ion mobility spectrometry/mass spectrometry using electrodynamic ion funnel interfaces. Anal. Chem.77(10), 3330–3339 (2005).
   Google Scholar
• Koeniger SL, Merenbloom SI, Valentine SJ et al. An IMS–IMS analogue of MS–MS. Anal. Chem.78(12), 4161–4174 (2006).
   Google Scholar
• Koeniger SL, Merenbloom SI, Clemmer DE. Evidence for many resolvable structures within conformation types of electrosprayed ubiquitin ions. J. Phys. Chem. B.110(13), 7017–7021 (2006).
   Google Scholar
• Laschober C, Kaddis CS, Reischl GP et al. Comparison of various nano-differential mobility analysers (nDMAs) applying globular proteins. J. Exp. Nanosci.2(4), 291–301 (2007).
   Web of Science ®Google Scholar
• Hogan CJ Jr, Ruotolo BT, Robinson CV, Fernández De La Mora J. Tandem differential mobility analysis–mass spectrometry reveals partial gas-phase collapse of the GroEL complex. J. Phys. Chem. B.115(13), 3614–3621 (2011).
   PubMed Web of Science ®Google Scholar
• De La Mora JF, De Juan L, Eichler T, Rosell J. Differential mobility analysis of molecular ions and nanometer particles. Trends Anal. Chem.17(6), 328–339 (1998).
   Web of Science ®Google Scholar
• Hogan CJ, Jr., De La Mora JF. Ion mobility measurements of nondenatured 12–150 kDa proteins and protein multimers by tandem differential mobility analysis-mass spectrometry (DMA-MS). J. Am. Soc. Mass Spectrom.22(1), 158–172 (2011).
   PubMed Web of Science ®Google Scholar
• Kaufman SL, Kuchumov AR, Kazakevich M, Vinogradov SN. Analysis of a 3.6 MDa hexagonal bilayer hemoglobin from lumbricus terrestris using a gas-phase electrophoretic mobility molecular analyzer. Anal. Biochem.259(2), 195–202 (1998).
   Google Scholar
• John JT, Brian B, Joe T, Benner WH, Gary S. Electrospray ion mobility spectrometry of intact viruses. Spectroscopy18(1), 31–36 (2004).
   Google Scholar
• Loo J, Berhane B, Kaddis C et al. Electrospray ionization mass spectrometry and ion mobility analysis of the 20S proteasome complex. J. Am. Soc. Mass Spectrom.16(7), 998–1008 (2005).
   PubMed Web of Science ®Google Scholar
• Giles K, Pringle SD, Worthington KR et al. Applications of a travelling wave-based radio-frequency only stacked ring ion guide. Rapid Commun. Mass Spectrom.18(20), 2401–2414 (2004).
   PubMed Web of Science ®Google Scholar
• Pringle SD, Giles K, Wildgoose JL et al. An investigation of the mobility separation of some peptide and protein ions using a new hybrid quadrupole/travelling wave IMS/oa-ToF instrument. Int. J. Mass Spectrom.261(1), 1–12 (2007).
   Google Scholar
• Shvartsburg AA, Smith RD. Fundamentals of traveling wave ion mobility spectrometry. Anal. Chem.80(24), 9689–9699 (2008).
   PubMed Web of Science ®Google Scholar
• Ruotolo BT, Benesch JLP, Sandercock AM, Hyung SJ, Robinson CV. Ion mobility–mass spectrometry analysis of large protein complexes. Nat. Protoc.3(7), 1139–1152 (2008).
   PubMed Web of Science ®Google Scholar
• Giles K, Wildgoose JL, Langridge DJ, Campuzano I. A method for direct measurement of ion mobilities using a travelling wave ion guide. Int. J. Mass Spectrom.298(1–3), 10–16 (2010).
   Google Scholar
• Zhong Y, Hyung SJ, Ruotolo BT. Characterizing the resolution and accuracy of a second-generation traveling-wave ion mobility separator for biomolecular ions. Analyst136(17), 3534–3541 (2011).
   PubMed Web of Science ®Google Scholar
• Giles K, Williams JP, Campuzano I. Enhancements in travelling wave ion mobility resolution. Rapid Commun. Mass Spectrom.25(11), 1559–1566 (2011).
   Google Scholar
• Dugourd P, Hudgins RR, Clemmer DE, Jarrold MF. High-resolution ion mobility measurements. Rev. Sci. Instrum.68(2), 1122–1129 (1997).
   Web of Science ®Google Scholar
• Kemper PR, Dupuis NF, Bowers MT. A new, higher resolution, ion mobility mass spectrometer. Int. J. Mass Spectrom.287(1–3), 46–57 (2009).
   Google Scholar
• Roscioli KM, Davis E, Siems WF et al. Modular ion mobility spectrometer for explosives detection using corona ionization. Anal. Chem.83(15), 5965–5971 (2011).
   Google Scholar
• Loo JA, Kaddis CS, Lomeli SH et al. Sizing large proteins and protein complexes by electrospray ionization mass spectrometry and ion mobility. J. Am. Soc. Mass Spectrom.18(7), 1206–1216 (2007).
   Google Scholar
• Mckay AR, Ruotolo BT, Ilag LL, Robinson CV. Mass measurements of increased accuracy resolve heterogeneous populations of intact ribosomes. J. Am. Chem. Soc.128(35), 11433–11442 (2006).
   Google Scholar
• Han L, Hyung SJ, Mayers JJ, Ruotolo BT. Bound anions differentially stabilize multiprotein complexes in the absence of bulk solvent. J. Am. Chem. Soc.133(29), 11358–11367 (2011).
   Google Scholar
• Freeke J, Robinson CV, Ruotolo BT. Residual counter ions can stabilise a large protein complex in the gas phase. Int. J. Mass Spectrom.298(1–3), 91–98 (2010).
   Google Scholar
• Liepold L, Oltrogge L, Suci P, Young M, Douglas T. Correct charge state assignment of native electrospray spectra of protein complexes. J. Am. Soc. Mass Spectrom.20(3), 435–442 (2009).
   Google Scholar
• Tseng YH, Uetrecht C, Heck AJR, Peng WP. Interpreting the charge state assignment in electrospray mass spectra of bioparticles. Anal. Chem.83(6), 1960–1968 (2011).
   Google Scholar
• Van Breukelen B, Barendregt A, Heck AJR, Van Den Heuvel RHH. Resolving stoichiometries and oligomeric states of glutamate synthase protein complexes with curve fitting and simulation of electrospray mass spectra. Rapid Commun. Mass Spectrom.20(16), 2490–2496 (2006).
   Google Scholar
• Hernández H, Makarova OV, Makarov EM et al. Isoforms of U1–70k control subunit dynamics in the human spliceosomal U1 snRNP. PLoS One4(9), e7202 (2009).
   PubMed Web of Science ®Google Scholar
• Bush MF, Hall Z, Giles K et al. Collision cross sections of proteins and their complexes: a calibration framework and database for gas-phase structural biology. Anal. Chem.82(22), 9557–9565 (2010).
   PubMed Web of Science ®Google Scholar
• Sharon M, Michaelevski I, Eisenstein M. Gas-phase compaction and unfolding of protein structures. Anal. Chem.82(22), 9484–9491 (2010).
   Google Scholar
• Morsa D, Gabelica V, De Pauw E. Effective temperature of ions in traveling wave ion mobility spectrometry. Anal. Chem.83(14), 5775–5782 (2011).
   Google Scholar
• Benesch JLP, Ruotolo BT. Mass spectrometry: come of age for structural biology. Curr. Opin. Struct. Biol.21(5), 641–649 (2011).
   PubMed Web of Science ®Google Scholar
• Ruotolo BT, Mclean JA, Gillig KJ, Russell DH. The influence and utility of varying field strength for the separation of tryptic peptides by ion mobility–mass spectrometry. J. Am. Soc. Mass Spectrom.16(2), 158–165 (2005).
   Google Scholar
• Bleiholder C, Wyttenbach T, Bowers MT. A novel projection approximation algorithm for the fast and accurate computation of molecular collision cross sections (I). Method. Int. J. Mass Spectrom.308(1), 1–10 (2011).
   Web of Science ®Google Scholar
• Felitsyn N, Kitova EN, Klassen JS. Thermal decomposition of a gaseous multiprotein complex studied by blackbody infrared radiative dissociation. Investigating the origin of the asymmetric dissociation behavior. Anal. Chem.73(19), 4647–4661 (2001).
   Google Scholar
• Jurchen JC, Williams ER. Origin of asymmetric charge partitioning in the dissociation of gas-phase protein homodimers. J. Am. Chem. Soc.125(9), 2817–2826 (2003).
   PubMed Web of Science ®Google Scholar
• Benesch JLP, Aquilina JA, Ruotolo BT, Sobott F, Robinson CV. Tandem mass spectrometry reveals the quaternary organization of macromolecular assemblies. Chem. Biol.13(6), 597–605 (2006).
   Google Scholar
• Pagel K, Hyung SJ, Ruotolo BT, Robinson CV. Alternate dissociation pathways identified in charge-reduced protein complex ions. Anal. Chem.82(12), 5363–5372 (2010).
   PubMedGoogle Scholar
• Boeri Erba E, Ruotolo BT, Barsky D, Robinson CV. Ion mobility–mass spectrometry reveals the influence of subunit packing and charge on the dissociation of multiprotein complexes. Anal. Chem.82(23), 9702–9710 (2010).
   PubMedGoogle Scholar
• Zhang H, Cui WD, Wen JZ, Blankenship RE, Gross ML. Native electrospray and electron-capture dissociation FTICR mass spectrometry for top-down studies of protein assemblies. Anal. Chem.83(14), 5598–5606 (2011).
   PubMed Web of Science ®Google Scholar
• Wysocki VH, Jones CM, Galhena AS, Blackwell AE. Surface-induced dissociation shows potential to be more informative than collision-induced dissociation for structural studies of large systems. J. Am. Soc. Mass Spectrom.19(7), 903–913 (2008).
   Google Scholar
• Zhou M, Sandercock AM, Fraser CS et al. Mass spectrometry reveals modularity and a complete subunit interaction map of the eukaryotic translation factor eIF3. Proc. Natl Acad. Sci. USA105(47), 18139–18144 (2008).
   PubMedGoogle Scholar
• Sharon M, Taverner T, Ambroggio XI, Deshaies RJ, Robinson CV. Structural organization of the 19S proteasome lid: insights from MS of intact complexes. PLoS Biol.4(8), 1314–1323 (2006).
   Google Scholar
• Levy ED, Boeri Erba E, Robinson CV, Teichmann SA. Assembly reflects evolution of protein complexes. Nature453(7199), 1262–1265 (2008).
   PubMed Web of Science ®Google Scholar
• Nooren IM, Thornton JM. Diversity of protein–protein interactions. EMBO J.22(14), 3486–3492 (2003).
   PubMed Web of Science ®Google Scholar
• Uetrecht C, Barbu IM, Shoemaker GK, Van Duijn E, Heck AJR. Interrogating viral capsid assembly with ion mobility–mass spectrometry. Nat. Chem.3(2), 126–132 (2011).
   Google Scholar
• Knapman TW, Morton VL, Stonehouse NJ, Stockley PG, Ashcroft AE. Determining the topology of virus assembly intermediates using ion mobility spectrometry–mass spectrometry. Rapid Commun. Mass Spectrom.24(20), 3033–3042 (2010).
   PubMed Web of Science ®Google Scholar
• Bornschein RE, Hyung SJ, Ruotolo BT. Ion mobility–mass spectrometry reveals conformational changes in charge reduced multiprotein complexes. J. Am. Soc. Mass Spectrom.22(10), 1690–1698 (2011).
   Google Scholar
• Bagal D, Kitova EN, Liu L et al. Gas phase stabilization of noncovalent protein complexes formed by electrospray ionization. Anal. Chem.81(18), 7801–7806 (2009).
   Google Scholar
• Flick TG, Merenbloom SI, Williams ER. A simple and robust method for determining the number of basic sites in peptides and proteins using electrospray ionization mass spectrometry. Anal. Chem.83(6), 2210–2214 (2011).
   Google Scholar
• Bohrer BC, Mererbloom SI, Koeniger SL, Hilderbrand AE, Clemmer DE. Biomolecule analysis by ion mobility spectrometry. Annu. Rev. Anal. Chem.293–327 (2008).
   Google Scholar
• Valentine SJ, Plasencia MD, Liu XY et al. Toward plasma proteome profiling with ion mobility–mass spectrometry. J. Proteome Res.5(11), 2977–2984 (2006).
   PubMed Web of Science ®Google Scholar
• Ruotolo BT, Gillig KJ, Stone EG, Russell DH. Peak capacity of ion mobility mass spectrometry: separation of peptides in helium buffer gas. J. Chromatogr. B.782(1–2), 385–392 (2002).
   PubMed Web of Science ®Google Scholar
• Ruotolo BT, Mclean JA, Gillig KJ, Russell DH. Peak capacity of ion mobility mass spectrometry: the utility of varying drift gas polarizability for the separation of tryptic peptides. J. Mass Spectrom.39(4), 361–367 (2004).
   Google Scholar
• Valentine SJ, Kulchania M, Barnes CAS, Clemmer DE. Multidimensional separations of complex peptide mixtures: a combined high-performance liquid chromatography/ion mobility/time-of-flight mass spectrometry approach. Int. J. Mass Spectrom.212(1–3), 97–109 (2001).
   Web of Science ®Google Scholar
• Hoadlund-Hyzer CS, Li JW, Clemmer DE. Mobility labeling for parallel CID of ion mixtures. Anal. Chem.72(13), 2737–2740 (2000).
   Google Scholar
• Stone E, Gillig KJ, Ruotolo B et al. Surface-induced dissociation on a MALDI–ion mobility–orthogonal time-of-flight mass spectrometer: sequencing peptides from an ‘in-solution’ protein digest. Anal. Chem.73(10), 2233–2238 (2001).
   Google Scholar
• Mclean JA, Fenn LS. Structural resolution of carbohydrate positional and structural isomers based on gas-phase ion mobility–mass spectrometry. Phys. Chem. Chem. Phys.13(6), 2196–2205 (2011).
   Google Scholar
• Woods AS, Ugarov M, Egan T et al. Lipid/peptide/nucleotide separation with MALDI-ion mobility-TOF MS. Anal. Chem.76(8), 2187–2195 (2004).
   Google Scholar
• Bernstein SL, Dupuis NF, Lazo ND et al. Amyloid-beta protein oligomerization and the importance of tetramers and dodecamers in the aetiology of Alzheimer’s disease. Nat. Chem.1(4), 326–331 (2009).
   PubMed Web of Science ®Google Scholar
• Ashcroft AE. Mass spectrometry and the amyloid problem – how far can we go in the gas phase? J. Am. Soc. Mass. Spectrom.21(7), 1087–1096 (2010).
   Google Scholar
• Radford SE, Smith DP, Ashcroft AE. Elongated oligomers in beta(2)-microglobulin amyloid assembly revealed by ion mobility spectrometry–mass spectrometry. Proc. Natl Acad. Sci. USA107(15), 6794–6798 (2010).
   Google Scholar
• Van Duijn E, Barendregt A, Synowsky S, Versluis C, Heck AJR. Chaperonin complexes monitored by ion mobility mass spectrometry. J. Am. Chem. Soc.131(4), 1452–1459 (2009).
   PubMed Web of Science ®Google Scholar
• Heck AJR, Uetrecht C, Rose RJ, Van Duijn E, Lorenzen K. Ion mobility mass spectrometry of proteins and protein assemblies. Chem. Soc. Rev.39(5), 1633–1655 (2010).
   Google Scholar
• Barrera NP, Wang SC, Politis A et al. Ion mobility mass spectrometry of two tetrameric membrane protein complexes reveals compact structures and differences in stability and packing. J. Am. Chem. Soc.132(44), 15468–15470 (2010).
   Google Scholar
• Ashcroft AE, Morton VL, Stockley PG, Stonehouse NJ. Insights into virus capsid assembly from non-covalent mass spectrometry. Mass Spectrom. Rev.27(6), 575–595 (2008).
   Google Scholar
• Heck AJR, Uetrecht C, Versluis C et al. Stability and shape of hepatitis B virus capsids in vacuo. Angew. Chem. Int. Ed.47(33), 6247–6251 (2008).
   Google Scholar
• Benesch JLP, Ruotolo BT, Sobott F et al. Quadrupole-time-of-flight mass spectrometer modified for higher-energy dissociation reduces protein assemblies to peptide fragments. Anal. Chem.81(3), 1270–1274 (2009).
   Google Scholar
• Dodds ED, Blackwell AE, Jones CM et al. Determinants of gas-phase disassembly behavior in homodimeric protein complexes with related yet divergent structures. Anal. Chem.83(10), 3881–3889 (2011).
   Google Scholar
• Blackwell AE, Dodds ED, Bandarian V, Wysocki VH. Revealing the quaternary structure of a heterogeneous noncovalent protein complex through surface-induced dissociation. Anal. Chem.83(8), 2862–2865 (2011).
   PubMed Web of Science ®Google Scholar