The Journal of
Physical Chemistry
0 Copyright, 1992, by the American Chemical Society
VOLUME 96, NUMBER 6 MARCH 19, 1992
LETTERS Activation Energies for Gas-Phase Dissociations of Multiply Charged Ions from Eiectrospray Ionization Mass Spectrometry Mark Busman, Alan L. Rockwood, and Richard D. Smith* Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352 (Received: October 3, 1991; In Final Form: January 29, 1992)
The reactions of multiply protonated melittin molecular ions of various charge states produced from an electrospray ionization source have been studied. The flow of ions entrained in gas through a heated metal capillary inlet serves as a reaction vessel for gas-phase measurements of molecular ion reaction rates using mass spectrometry. Activation energies for the unimolecular dissociation reactions are calculated from the temperature dependence of the reaction kinetics. The differences in activation energies for the reactions of the different charge states are attributed to the destabilizing effect of Coulombic repulsion for highly charged ions.
Introduction The demonstration of extensive multiple charging of large ions in electrospray ionization mass spectrometry (ESI-MS)’ has introduced a new type of chemical species to gas-phase chemistry. Ions as large as 2000000 Da with -1000 charges have been reported.’ Little is known about the chemical characteristics (e.g., structure, reactivity) of these ions, particularly the dependence of chemical characteristics upon the ion’s extent of charging.* The properties of such chemical species are, in themselves, fundamentally interesting. Furthermore, greater knowledge about these extensively charged ions would be of great use for the interpretation of ESI-MS result^,^ as well as for understanding the properties of these ions, such as Coulombic effects in their diss~iation.~ (1) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64. (2) LOO, J. A.; Edmonds, C. G.; Smith, R. D. Science 1990, 248, 201. (3) Smith, R. D.; Barinaga, C. J. Rapid Commun. Mass Spectrom. 1990, 4, 54. (4) Rockwood, A. L.; Busman, M.; Smith, R. D. Inr. J . Mass Specrrom. Ion Processes 1991, I l l , 103.
Our laboratory has been extensively involved in the study of the dissociation of large multiply charged ions by collisional acti~ation.~-’Preliminary results have also been presented using surface collisions for activation of such ions,* and studies of photoactivation processes may be anticipated. Unimolecular dissociation, or other reaction processes, yield reaction products that can be used to elucidate sequence or other structural information for biomolecules.2 Stabilities of the ions can (in principle) be evaluated in terms of activation energies required for fragmentation. The electrospray process is well suited for this type of evaluation, as it produces ions at atmospheric pressure, where they are rapidly thermalized to ambient temperature^.^ (5) Smith, R. D.; Loo, J. A,; Edmonds, C. G.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1990.62, 882. ( 6 ) Loo, J. A.; Edmonds, C. G.; Smith, R. D. Anal. Chem. 1991.63, 2488. (7) Smith, R. D.; Barinaga, C. J.; Udseth, H. R. J . Phys. Chem. 1989,93, 5019.
(8) McCormack, A. L.; Shebanowitz, J.; Hunt, D. L.;Wysocki, V. H. In Proceedings of the 39th Annual Conferenceon Mass Spectrometry and Allied Topics, June 1991, Nashville; American Society for Mass Spectrometry, 1991; p 823. (9) Mann, M . Org. Mass Spectrom. 1990, 25, 575
0022-365419212096-2397$03.00/0 0 1992 American Chemical Society
2398
Letters
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
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atmospheric pressure ; vacuum
heated metal caDillarv electrospray
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I
I 1 TAGA
Figure 1. Schematic diagram of the S I - h e a t e d capillary reactor apparatus.
Recently, we have begun to explore thermal activation of electrosprayed ions using a heated capillary reactor.I0 This device is related to a recently described electrospray interface" but is operated at higher temperatures and used as a thermal reactor rather than just as an aid to ion desolvation. In a heated capillary reactor ions are all heated to the same (well characterizable) temperature. This is in contrast to other commonly used activation methods in mass spectrometry, in which temperature is not very well l ~ n o w n . ~ This ~ J ~ feature of the heated capillary reactor has motivated us to use it to measure activation energies of gas-phase ion reactions. The design of the reactor makes it well suited for studies of gas-phase kinetics of ions produced at high pressures. The present measurements provide new insight into the role of Coulombic forces upon the stability of large multiply charged ions! Experimental Section The key part of the apparatus used for the present studies is a heated capillary used to transport ions between an atmospheric pressure electrospray ion source and the differentially pumped regions of the mass spectrometer. In the experiments reported here, this heated capillary effectively constitutes a "heated reaction vessel" for thermally induced dissociation. Figure 1 shows a schematic illustration of the apparatus. The electrospray is directed toward the end of a resistively heated stainless steel capillary (200 mm, 1.6 mm o.d., 0.5 mm i.d.). Power leads from a dc power supply (Model 6259B, Hewlett-Packard, Palo Alto, CA) are connected to the opposite ends of the capillary. Heating of the capillary is controlled by varying the current, and temperature is measured using a chromel-alumel (type K) thermocouple. The capillary is insulated with a loose fitting length of ceramic tubing and wrapped with a layer of glass wool. The thermocouple was placed at the midpoint of the capillary exterior. For given applied heating wattages, the temperatures indicated by the thermocouple were stable (A2 "C) after a few minutes' equilibration. The gas sampled through the capillary, together with the entrained ions, expands into a region that is pumped to about 0.2 Torr by a single-stage Roots blower. Ions are then sampled by a beam skimmer (1 mm diameter) and enter a TAGA 6000E triple quadrupole mass spectrometer (Sciex, Thornhill, Ontario, Canada). The cryopumped mass spectrometer chamber is maintained at pressures in the range of 104-10-5 Torr. The voltage bias between the capillary and the skimmer was kept at zero during the course of our studies, in order to eliminate possible complications from collision-induced dissociation due to acceleration of ions in the moderate pressure of the interface, an activation process that would be charge state dependent.I4 Similarly, the bias (10) Rockwood, A. L.; Busman, M.; Udseth, H. R.; Smith, R. D. Rapid Commun. Mass Spectrom. 1991, 5, 582. (1 1) Chowdhury, S.; Katta, V.; Chait, B. T. Rapid Commun. Mass Spectrom. 1990, 4, 81. (12) Huang, F.-S.; Dunbar, R. C. J. Am. Chem. SOC.1989, 1 1 1 , 6497. (13) Nourse, B. D.; Kenttamaa, H. I. J. Phys. Chem. 1990, 94, 5809.
between the skimmer and the fmt quadrupole lens was maintained at + Centre de Recherche Paul Pascal, UniversitP Bordeaux I, Avenue A . Schwietzer, 33600, Pessac, France, and Institute of Experimental and Theoretical Biophysics, Pushchino, Moscow Region, U.S.S.R. (Received: November 14, 1991; I n Final Form: January 17, 1992)
We show that the Turing structures recently discovered with the chlorite-iodide-malonic acid oscillating reaction in open gel-filled reactors depend neither on the chemical nature of the gel nor on the actual presence of gels. In contrast, the concentration of starch (the color indicator) can play a crucial role on the development of these structures. We conclude that the iodide-iodinestarch complex mediates the relative differences in the diffusivity of reacting species necessary to obtain Turing structures. We demonstrate that patterns developed in capillary tubes with a diameter comparable to the wavelength of the pattern can exhibit three-dimensional structures with no cylindrical symmetry.
Introduction Recently, experimental evidence of standing concentration patterns’-3 in an unstirred solution of reacting chemicals, ascribed to a spatial symmetry breaking in~tability,~ was obtained when the chlorite-iodide-malonic acid (CIMA) oscillating reaction5q6 was operated in a soft hydrogel containing a soluble starch. These chemical patterns were interpreted as genuine Turing patterns,’ a pattern mechanism often put forward in theoretical biology to explain some aspects of morphogenetic developments.* This interpretation implies that patterns solely result from the coupling of the local nonlinear single-phase kinetic processes and the linear diffusive transport of species. Furthermore, the natural or effective diffusion coefficients of species should b e positive scalars and exhibit appropriate differences. In aqueous solution, the natural difference between the diffusion coefficients of small species is usually modest. It is of basic importance to determine whether these small differences are sufficient for the onset of Turing structures in the CIMA reaction or if the appropriate differences
’Universitt Bordeaux I. I Institute
of Experimental and Theoretical Biophysics.
have to be mediated by interactions with the polymer matrix of the gel or with the immobilized starch molecule which forms complexes with iodine species. Turing patterns result from nonequilibrium self-organization phenomena43’x8 and can only be maintained by a permanent supply ( 1 ) Castets, V.; Dulos, E.; Boissonade, J.; De Kepper, P. Phys. Reu. Lett. 1990, 64, 2953. (2) De Kepper, P.; Castets, V.; Dulos, E.; Boissonade, J. Physica D 1991, 49, 161. ( 3 ) Ouyang, Q.;Swinney, H. L. Nature 1991, 352, 610. (4) Nicolis, G.; Prigogine, 1. Selforganization in Nonequilibrium Chemical Systems; Wiley: New York, 1977. Haken, H . Synergetics, an Introduction; Springer-Verlag: New York, 1977. ( 5 ) De Kepper, P.; Epstein, I. R.; Kustin, K.; OrbPn, M. J. Phys. Chem. 1982, 8 6 , 170. (6) Lengyel, 1.; Rabai, G.; Epstein, I. R. J . Am. Chem. Soc. 1990, 112. 4606.
(7) Turing, A. M. Philos. Trans. R . Soc. London, B 1952, 327, 37. (8) Murray, J. D. Mathematical Biology; Springer-Verlag: New York, 1989. Meinhardt, H. Models of Biological Patterns Formation; Academic Press: New York, 1982. Babloyantz, A . Molecules, Dynamics and Life; Wiley: New York, 1986. Harrison, L . G . J . Theor. Biol. 1987, 125, 365.
0 1992 American Chemical Society QO22-3654/92/2Q96-24QQ~Q3.QQ/Q
