Robert Hudgins - Academia.edu (original) (raw)
Papers by Robert Hudgins
Journal of the American Chemical Society, May 12, 2004
The factors affecting host-guest complexation between the molecular container compound cucurbit[6... more The factors affecting host-guest complexation between the molecular container compound cucurbit[6]uril (CB6) and various guests in aqueous solution are studied, and a detailed complexation mechanism in the presence of cations is derived. The formation of the supramolecular complex is studied in detail for cyclohexylmethylammonium ion as guest. The kinetics and thermodynamics of complexation is monitored by NMR as a function of temperature, salt concentration, and cation size. The binding constants and the ingression rate constants decrease with increasing salt concentration and cation-binding constant, in agreement with a competitive binding of the ammonium site of the guest and the metal cation with the ureido carbonyl portals of CB6. Studies as a function of guest size indicate that the effective container volume of the CB6 cavity is approximately 105 A(3). It is suggested that larger guests are excluded for two reasons: a high activation barrier for ingression imposed by the tight CB6 portals and a destabilization of the complex due to steric repulsion inside. For example, in the case of the nearly spherical azoalkane homologues 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, volume ca. 96 A(3)) and 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, volume ca. 110 A(3)), the former forms the CB6 complex promptly with a sizable binding constant (1300 M(-1)), while the latter does not form a complex even after several months at optimized complexation conditions. Molecular mechanics calculations are performed for several CB6/guest complexes. A qualitative agreement is found between experimental and calculated activation energies for ingression as a function of both guest size and state of protonation. The potential role of constrictive binding by CB6 is discussed.
Journal of the American Society For Mass Spectrometry, 2003
Chemical Engineering Education, 1989
Chemical Engineering Education, 1981
Physical Review Letters, Jun 1, 1997
... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whe... more ... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whetten, Phys. Rev. B 47, 7480 (1993). [22] DF Hagen, Anal. Chem. 51, 870 (1979); Z. Karpas, RM Stimac, and Z. Rappoport, Int. J. Mass Spectrom. Ion Proc. ...
Journal of the American Society For Mass Spectrometry, Apr 30, 2003
Desfuroylceftiofur (DFC) is a bioactive -lactam antibiotic metabolite that has a free thiol grou... more Desfuroylceftiofur (DFC) is a bioactive -lactam antibiotic metabolite that has a free thiol group. Previous experiments have shown release of DFC from plasma extracts after addition of a disulfide reducing agent, suggesting that DFC may be bound to plasma and tissue proteins through disulfide bonds. We have reacted DFC with [Arg 8 ]-vasopressin (which has one disulfide bond) and bovine insulin (which has three disulfide bonds) and analyzed the reaction products by use of electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD FT-ICR MS), which has previously shown preferential cleavage of disulfide bonds. We observe cleavage of DFC from vasopressin and insulin during ECD, suggesting that DFC is indeed bound to peptides and proteins through disulfide bonds. Specifically, we observed dissociative loss of one, as well as two, DFC species during ECD of [vasopressin ϩ 2(DFC-H) ϩ 2H] 2ϩ from a single electron capture event. Loss of two DFCs could arise from either consecutive or simultaneous loss, but in any case implies a gas phase disulfide exchange step. ECD of [insulin ϩ DFC ϩ 4H] 4ϩ shows preferential dissociative loss of DFC. Combined with HPLC, ECD FT-ICR-MS may be an efficient screening method for detection of drug-biomolecule binding.
Can J Chem Eng, 1971
A maximum likelihood ratio was used as a statistical criterion for discriminating among twenty-tw... more A maximum likelihood ratio was used as a statistical criterion for discriminating among twenty-two kinetic models posited to describe the hydrogenation of mixed pentenes (cis-and trans-2-pentenes) with a Raney nickel catalyst. Selectivity ratio, the ratio of reaction rates of two different species, was used and found to be a suitable extension of conventional techniques for identifying rate controlling steps owing in this case to its reproducibility. Selectivity ratio models were of the Langmuir-Hinshelwood form. Hydrogenation was performed isothermally at 20°@ in benzene at five constant pressures corresponding to partial pressures of hydrogen of 22.5, 42.3, 62.3, 112.3 and 122.1 psia. Discrimination was attempted at each pressure, but found acceptable only at the three intermediate pressures. At each of these pressures, a model of similar functional form was found to represent the selectivity data; however, this model appears to he empirical. The likelihood ratio approach appears to be a powerful one for discriminating among a large number of models even with only a moderate number (-20) of experimental observations. nformation on the rate-controlling step or steps is
Environmental Progress, 1993
A low-temperature process employing activated carbon-based catalysts and operating downstream of ... more A low-temperature process employing activated carbon-based catalysts and operating downstream of the electrostatic precipitator (ESP) is being developed jointly by Research Triangle Institute (RTI) and the University of Waterloo (Waterloo). The RTI-Waterloo process is capable of removing more than 95% SO[sub 2] and 75% NO[sub x] from coal combustion flue gas. The flue gas leaving the ESP is first cooled
International Journal of Mass Spectrometry and Ion Processes, 1997
Our high resolution ion mobility apparatus has been modified by attaching an electrospray source ... more Our high resolution ion mobility apparatus has been modified by attaching an electrospray source to perform measurements for biological molecules. While the greater resolving power permits the resolution of more conformations for BPTI and cytochrome c, the resolved features are generally much broader than expected for a single rigid conformation. A major advantage of the new experimental configuration is the much gentler introduction of ions into the drift tube, so that the observed gas phase conformations appear to more closely reflect those present in solution. For example, it is possible to distinguish between the native state of cytochrome c and the methanol-denatured form on the basis of the ion mobility measurements; the mass spectra alone are not sensitive enough to detect this change. Thus this approach may provide a quick and sensitive tool for probing the solution phase conformations of biological molecules.
The Journal of Physical Chemistry B, 2000
The conformations of unsolvated glycine-based peptides have been probed using high-resolution ion... more The conformations of unsolvated glycine-based peptides have been probed using high-resolution ion mobility measurements and molecular dynamics simulations. Alanine-based Ac-Ala n-LysH + peptides have previously been shown to form helices in the gas phase. In contrast, the polyglycine analogues (Ac-Gly n-LysH +) do not form helices at room temperature; they adopt random globular conformations. Thus, the stabilities of alanine and glycine helices in vacuo are consistent with the helix propensities in aqueous solution, where alanine has the highest helix propensity and glycine has one of the lowest.
The Journal of Physical Chemistry B, 2003
Peptides containing the chromophore, benzophenone (as the amino acid, 4-benzoyl-phenylalanine (Bp... more Peptides containing the chromophore, benzophenone (as the amino acid, 4-benzoyl-phenylalanine (Bpa)), have been synthesized to explore the feasibility of gas-phase photochemical cross-linking to investigate the conformations of unsolvated peptides. The main product of UV irradiation of a BP-containing peptide is CO 2 loss from the peptide C terminus. To test whether decarboxylation results from hydrogen abstraction from acidic residues, we synthesized peptides designed to limit the contact between the Bpa and the peptide C terminus. Such contact is necessary for hydrogen abstraction. The peptide, Ac-Bpa-Ala 10-Lys, forms an extended R-helix in the gas phase, thus preventing the N-terminal Bpa from approaching the C terminus. 1 In contrast, Ac-Bpa-Gly 10-Lys is a flexible peptide that can adopt multiple "random globule" conformations in the gas phase, some of which could allow contact between the termini and thus photoinduced reaction. On simultaneous UV irradiation of a mixture of both electrosprayed gas-phase peptides in a Fourier transform ion cyclotron resonance ion trap, the degree of decarboxylation is 50-fold less for the R-helical alanine-based peptide than for the flexible glycine-based peptide. The results suggest that intimate contact between the photoexcited Bpa and the C terminus is necessary for decarboxylation, implicating hydrogen atom abstraction as the photochemical mechanism. These results establish the feasibility of probing gas-phase peptide/protein ion conformations by photochemistry of photoaffinity-labeled ions.
The Journal of Physical Chemistry A, 1999
The laser desorption of C k (k) 60 and 70) fullerene is known to produce a broad distribution of ... more The laser desorption of C k (k) 60 and 70) fullerene is known to produce a broad distribution of cluster sizes strongly peaking around the integer multiples of original fullerene mass. The "exact dimers" (C n clusters with n) 2k) and species with slightly fewer atoms (n even) have been characterized previously as fully coalesced large single-shell fullerenes and [2 + 2] cycloadducts. Presently, we investigate the species encountered on the high-mass sides of exact dimers, that is, the clusters with n > 2k (n even), using highresolution ion mobility measurements. Specifically, the drift time distributions for C n + and C nwith n) 122-128, 132-136, and 142-146 have been obtained and compared with the results of trajectory calculations for various trial geometries optimized using the density functional tight binding and semiempirical (AM1) calculations. We find that, besides the normal near-spherical fullerenes and [2 + 2] cycloadducts, these species assume the "ball-and-chain" structures consisting of two fullerene cages (not necessarily those of the original material) connected by chains up to eight atoms long. C 122 and C 132 cations and anions also reveal a substantial abundance of isomers where the CC unit is sandwiched between the two fullerenes. Taken in conjunction with earlier findings for smaller fullerene dimers, presently reported results have allowed us to develop a comprehensive model for the chemical reactions occurring in the hot fullerene plasma.
![Research paper thumbnail of Structures of the Clusters Produced by Laser Desorption of Fullerenes: [2+2] Cycloadducts of Preshrunk Cages](https://attachments.academia-assets.com/81387569/thumbnails/1.jpg)
](The Journal of Physical Chemistry A, 1998
The laser desorption of C 60 or C 70 fullerenes yields a distribution of ions with masses corresp... more The laser desorption of C 60 or C 70 fullerenes yields a distribution of ions with masses corresponding to an even number of carbon atoms around integer multiples of 60 or 70. Clusters with 120 or 140 atoms have been characterized as large coalesced fullerenes and fullerene dimers joined by a [2+2] cycloaddition. Several structures had been proposed for the species with less than an integer multiple of 60 or 70 atoms, but none of them has been validated experimentally. We have examined C n (n) 112, 114, 116, 118, 136, and 138) cations and anions using high resolution ion mobility measurements and compared the results with trajectory calculations for a number of candidate geometries. We find that these species exist as near-spherical cages and [2+2] cycloadducts of smaller fullerenes derived from C 60 or C 70. Quadruply bound C 116 , C 118 , C 136 , and C 138 isomers that would result from the stepwise elimination of C 2 from the preformed [2+2] C 60 or C 70 dimers were not observed. Hence the fullerene dimers lacking a few atoms are formed by coalescence of dissociation products rather than by dissociation of the products of coalescence. Plausible reasons why the [2+2] cycloadducts are the only dimers produced by the laser desorption of fullerenes are discussed.
The Journal of Physical Chemistry A, 1997
C 120 , C 130 , and C 140 cluster anions and cations produced by laser desorption of fullerene fi... more C 120 , C 130 , and C 140 cluster anions and cations produced by laser desorption of fullerene films have been examined by high-resolution ion mobility measurements and trajectory calculation simulations. The results for anions and cations are similar. Two peaks are present in the measured drift time distributions. They are attributed to fullerene cage isomers and dimers. The fullerene peak is broad, indicating the presence of a range of fullerene cages with different shapes. The measured mobilities of the dimers are in good agreement with mobilities determined by trajectory calculations for the [2 + 2] cycloadduct geometries of (C 60) 2 , C 60 ‚C 70 , and (C 70) 2. Comparison of the mobilities calculated for the dimer geometries by the commonly used hardsphere projection approximation with those determined by trajectory calculations with a realistic potential shows that the projection approximation significantly underestimates the inverse mobilities. The deviations, which are large enough to assign the observed features to the wrong geometries, result mainly from the complete neglect of the scattering process in the hard-sphere projection approximation.
Physical Review Letters, 2000
Physical Review Letters, 1998
High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster an... more High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster anions in the drift-time region previously assigned to monocyclic rings. Tight-binding calculations have been performed to generate feasible geometries, which were compared to the experimental data by calculating their mobilities. Structures composed of a short chain of carbon atoms attached to a carbon ring are found to be the most plausible geometries for these new isomers. A number of different bicyclic ring isomers have also been resolved for the first time. [S0031-9007(98)06131-6]
Physical Review Letters, 1997
... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whe... more ... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whetten, Phys. Rev. B 47, 7480 (1993). [22] DF Hagen, Anal. Chem. 51, 870 (1979); Z. Karpas, RM Stimac, and Z. Rappoport, Int. J. Mass Spectrom. Ion Proc. ...
Journal of the American Society for Mass Spectrometry, 2002
We have used electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass... more We have used electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry to characterize amino acid side chain losses observed during electron capture dissociation (ECD) of ten 7-to 14-mer peptides. Side-chain cleavages were observed for arginine, histidine, asparagine or glutamine, methionine, and lysine residues. All peptides containing an arginine, histidine, asparagine or glutamine showed the losses associated with that residue. Methionine side-chain loss was observed for doubly-protonated bombesin. Lysine side-chain loss was observed for triply-protonated dynorphin A fragment 1-13 but not for the doubly-protonated ion. The proximity of arginine to a methoxy C-terminal group significantly enhances the extent of side-chain fragmentation. Fragment ions associated with side-chain losses were comparable in abundance to those resulting from backbone cleavage in all cases. In the ECD spectrum of one peptide, the major product was due to fragmentation within an arginine side chain. Our results suggest that cleavages within side chains should be taken into account in analysis of ECD mass spectral data. Losses from arginine, histidine, and asparigine/glutamine can be used to ascertain their presence, as in the analysis of unknown peptides, particularly those with non-linear structures.
Journal of the American Chemical Society, May 12, 2004
The factors affecting host-guest complexation between the molecular container compound cucurbit[6... more The factors affecting host-guest complexation between the molecular container compound cucurbit[6]uril (CB6) and various guests in aqueous solution are studied, and a detailed complexation mechanism in the presence of cations is derived. The formation of the supramolecular complex is studied in detail for cyclohexylmethylammonium ion as guest. The kinetics and thermodynamics of complexation is monitored by NMR as a function of temperature, salt concentration, and cation size. The binding constants and the ingression rate constants decrease with increasing salt concentration and cation-binding constant, in agreement with a competitive binding of the ammonium site of the guest and the metal cation with the ureido carbonyl portals of CB6. Studies as a function of guest size indicate that the effective container volume of the CB6 cavity is approximately 105 A(3). It is suggested that larger guests are excluded for two reasons: a high activation barrier for ingression imposed by the tight CB6 portals and a destabilization of the complex due to steric repulsion inside. For example, in the case of the nearly spherical azoalkane homologues 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, volume ca. 96 A(3)) and 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, volume ca. 110 A(3)), the former forms the CB6 complex promptly with a sizable binding constant (1300 M(-1)), while the latter does not form a complex even after several months at optimized complexation conditions. Molecular mechanics calculations are performed for several CB6/guest complexes. A qualitative agreement is found between experimental and calculated activation energies for ingression as a function of both guest size and state of protonation. The potential role of constrictive binding by CB6 is discussed.
Journal of the American Society For Mass Spectrometry, 2003
Chemical Engineering Education, 1989
Chemical Engineering Education, 1981
Physical Review Letters, Jun 1, 1997
... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whe... more ... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whetten, Phys. Rev. B 47, 7480 (1993). [22] DF Hagen, Anal. Chem. 51, 870 (1979); Z. Karpas, RM Stimac, and Z. Rappoport, Int. J. Mass Spectrom. Ion Proc. ...
Journal of the American Society For Mass Spectrometry, Apr 30, 2003
Desfuroylceftiofur (DFC) is a bioactive -lactam antibiotic metabolite that has a free thiol grou... more Desfuroylceftiofur (DFC) is a bioactive -lactam antibiotic metabolite that has a free thiol group. Previous experiments have shown release of DFC from plasma extracts after addition of a disulfide reducing agent, suggesting that DFC may be bound to plasma and tissue proteins through disulfide bonds. We have reacted DFC with [Arg 8 ]-vasopressin (which has one disulfide bond) and bovine insulin (which has three disulfide bonds) and analyzed the reaction products by use of electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD FT-ICR MS), which has previously shown preferential cleavage of disulfide bonds. We observe cleavage of DFC from vasopressin and insulin during ECD, suggesting that DFC is indeed bound to peptides and proteins through disulfide bonds. Specifically, we observed dissociative loss of one, as well as two, DFC species during ECD of [vasopressin ϩ 2(DFC-H) ϩ 2H] 2ϩ from a single electron capture event. Loss of two DFCs could arise from either consecutive or simultaneous loss, but in any case implies a gas phase disulfide exchange step. ECD of [insulin ϩ DFC ϩ 4H] 4ϩ shows preferential dissociative loss of DFC. Combined with HPLC, ECD FT-ICR-MS may be an efficient screening method for detection of drug-biomolecule binding.
Can J Chem Eng, 1971
A maximum likelihood ratio was used as a statistical criterion for discriminating among twenty-tw... more A maximum likelihood ratio was used as a statistical criterion for discriminating among twenty-two kinetic models posited to describe the hydrogenation of mixed pentenes (cis-and trans-2-pentenes) with a Raney nickel catalyst. Selectivity ratio, the ratio of reaction rates of two different species, was used and found to be a suitable extension of conventional techniques for identifying rate controlling steps owing in this case to its reproducibility. Selectivity ratio models were of the Langmuir-Hinshelwood form. Hydrogenation was performed isothermally at 20°@ in benzene at five constant pressures corresponding to partial pressures of hydrogen of 22.5, 42.3, 62.3, 112.3 and 122.1 psia. Discrimination was attempted at each pressure, but found acceptable only at the three intermediate pressures. At each of these pressures, a model of similar functional form was found to represent the selectivity data; however, this model appears to he empirical. The likelihood ratio approach appears to be a powerful one for discriminating among a large number of models even with only a moderate number (-20) of experimental observations. nformation on the rate-controlling step or steps is
Environmental Progress, 1993
A low-temperature process employing activated carbon-based catalysts and operating downstream of ... more A low-temperature process employing activated carbon-based catalysts and operating downstream of the electrostatic precipitator (ESP) is being developed jointly by Research Triangle Institute (RTI) and the University of Waterloo (Waterloo). The RTI-Waterloo process is capable of removing more than 95% SO[sub 2] and 75% NO[sub x] from coal combustion flue gas. The flue gas leaving the ESP is first cooled
International Journal of Mass Spectrometry and Ion Processes, 1997
Our high resolution ion mobility apparatus has been modified by attaching an electrospray source ... more Our high resolution ion mobility apparatus has been modified by attaching an electrospray source to perform measurements for biological molecules. While the greater resolving power permits the resolution of more conformations for BPTI and cytochrome c, the resolved features are generally much broader than expected for a single rigid conformation. A major advantage of the new experimental configuration is the much gentler introduction of ions into the drift tube, so that the observed gas phase conformations appear to more closely reflect those present in solution. For example, it is possible to distinguish between the native state of cytochrome c and the methanol-denatured form on the basis of the ion mobility measurements; the mass spectra alone are not sensitive enough to detect this change. Thus this approach may provide a quick and sensitive tool for probing the solution phase conformations of biological molecules.
The Journal of Physical Chemistry B, 2000
The conformations of unsolvated glycine-based peptides have been probed using high-resolution ion... more The conformations of unsolvated glycine-based peptides have been probed using high-resolution ion mobility measurements and molecular dynamics simulations. Alanine-based Ac-Ala n-LysH + peptides have previously been shown to form helices in the gas phase. In contrast, the polyglycine analogues (Ac-Gly n-LysH +) do not form helices at room temperature; they adopt random globular conformations. Thus, the stabilities of alanine and glycine helices in vacuo are consistent with the helix propensities in aqueous solution, where alanine has the highest helix propensity and glycine has one of the lowest.
The Journal of Physical Chemistry B, 2003
Peptides containing the chromophore, benzophenone (as the amino acid, 4-benzoyl-phenylalanine (Bp... more Peptides containing the chromophore, benzophenone (as the amino acid, 4-benzoyl-phenylalanine (Bpa)), have been synthesized to explore the feasibility of gas-phase photochemical cross-linking to investigate the conformations of unsolvated peptides. The main product of UV irradiation of a BP-containing peptide is CO 2 loss from the peptide C terminus. To test whether decarboxylation results from hydrogen abstraction from acidic residues, we synthesized peptides designed to limit the contact between the Bpa and the peptide C terminus. Such contact is necessary for hydrogen abstraction. The peptide, Ac-Bpa-Ala 10-Lys, forms an extended R-helix in the gas phase, thus preventing the N-terminal Bpa from approaching the C terminus. 1 In contrast, Ac-Bpa-Gly 10-Lys is a flexible peptide that can adopt multiple "random globule" conformations in the gas phase, some of which could allow contact between the termini and thus photoinduced reaction. On simultaneous UV irradiation of a mixture of both electrosprayed gas-phase peptides in a Fourier transform ion cyclotron resonance ion trap, the degree of decarboxylation is 50-fold less for the R-helical alanine-based peptide than for the flexible glycine-based peptide. The results suggest that intimate contact between the photoexcited Bpa and the C terminus is necessary for decarboxylation, implicating hydrogen atom abstraction as the photochemical mechanism. These results establish the feasibility of probing gas-phase peptide/protein ion conformations by photochemistry of photoaffinity-labeled ions.
The Journal of Physical Chemistry A, 1999
The laser desorption of C k (k) 60 and 70) fullerene is known to produce a broad distribution of ... more The laser desorption of C k (k) 60 and 70) fullerene is known to produce a broad distribution of cluster sizes strongly peaking around the integer multiples of original fullerene mass. The "exact dimers" (C n clusters with n) 2k) and species with slightly fewer atoms (n even) have been characterized previously as fully coalesced large single-shell fullerenes and [2 + 2] cycloadducts. Presently, we investigate the species encountered on the high-mass sides of exact dimers, that is, the clusters with n > 2k (n even), using highresolution ion mobility measurements. Specifically, the drift time distributions for C n + and C nwith n) 122-128, 132-136, and 142-146 have been obtained and compared with the results of trajectory calculations for various trial geometries optimized using the density functional tight binding and semiempirical (AM1) calculations. We find that, besides the normal near-spherical fullerenes and [2 + 2] cycloadducts, these species assume the "ball-and-chain" structures consisting of two fullerene cages (not necessarily those of the original material) connected by chains up to eight atoms long. C 122 and C 132 cations and anions also reveal a substantial abundance of isomers where the CC unit is sandwiched between the two fullerenes. Taken in conjunction with earlier findings for smaller fullerene dimers, presently reported results have allowed us to develop a comprehensive model for the chemical reactions occurring in the hot fullerene plasma.
The Journal of Physical Chemistry A, 1998
The laser desorption of C 60 or C 70 fullerenes yields a distribution of ions with masses corresp... more The laser desorption of C 60 or C 70 fullerenes yields a distribution of ions with masses corresponding to an even number of carbon atoms around integer multiples of 60 or 70. Clusters with 120 or 140 atoms have been characterized as large coalesced fullerenes and fullerene dimers joined by a [2+2] cycloaddition. Several structures had been proposed for the species with less than an integer multiple of 60 or 70 atoms, but none of them has been validated experimentally. We have examined C n (n) 112, 114, 116, 118, 136, and 138) cations and anions using high resolution ion mobility measurements and compared the results with trajectory calculations for a number of candidate geometries. We find that these species exist as near-spherical cages and [2+2] cycloadducts of smaller fullerenes derived from C 60 or C 70. Quadruply bound C 116 , C 118 , C 136 , and C 138 isomers that would result from the stepwise elimination of C 2 from the preformed [2+2] C 60 or C 70 dimers were not observed. Hence the fullerene dimers lacking a few atoms are formed by coalescence of dissociation products rather than by dissociation of the products of coalescence. Plausible reasons why the [2+2] cycloadducts are the only dimers produced by the laser desorption of fullerenes are discussed.
The Journal of Physical Chemistry A, 1997
C 120 , C 130 , and C 140 cluster anions and cations produced by laser desorption of fullerene fi... more C 120 , C 130 , and C 140 cluster anions and cations produced by laser desorption of fullerene films have been examined by high-resolution ion mobility measurements and trajectory calculation simulations. The results for anions and cations are similar. Two peaks are present in the measured drift time distributions. They are attributed to fullerene cage isomers and dimers. The fullerene peak is broad, indicating the presence of a range of fullerene cages with different shapes. The measured mobilities of the dimers are in good agreement with mobilities determined by trajectory calculations for the [2 + 2] cycloadduct geometries of (C 60) 2 , C 60 ‚C 70 , and (C 70) 2. Comparison of the mobilities calculated for the dimer geometries by the commonly used hardsphere projection approximation with those determined by trajectory calculations with a realistic potential shows that the projection approximation significantly underestimates the inverse mobilities. The deviations, which are large enough to assign the observed features to the wrong geometries, result mainly from the complete neglect of the scattering process in the hard-sphere projection approximation.
Physical Review Letters, 2000
Physical Review Letters, 1998
High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster an... more High-resolution ion-mobility measurements have resolved a number of isomers for carbon cluster anions in the drift-time region previously assigned to monocyclic rings. Tight-binding calculations have been performed to generate feasible geometries, which were compared to the experimental data by calculating their mobilities. Structures composed of a short chain of carbon atoms attached to a carbon ring are found to be the most plausible geometries for these new isomers. A number of different bicyclic ring isomers have also been resolved for the first time. [S0031-9007(98)06131-6]
Physical Review Letters, 1997
... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whe... more ... Poncharal, and M. Sence, J. Chem. Phys. 103, 6362 (1995). [21] EC Honea, ML Homer, and RL Whetten, Phys. Rev. B 47, 7480 (1993). [22] DF Hagen, Anal. Chem. 51, 870 (1979); Z. Karpas, RM Stimac, and Z. Rappoport, Int. J. Mass Spectrom. Ion Proc. ...
Journal of the American Society for Mass Spectrometry, 2002
We have used electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass... more We have used electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry to characterize amino acid side chain losses observed during electron capture dissociation (ECD) of ten 7-to 14-mer peptides. Side-chain cleavages were observed for arginine, histidine, asparagine or glutamine, methionine, and lysine residues. All peptides containing an arginine, histidine, asparagine or glutamine showed the losses associated with that residue. Methionine side-chain loss was observed for doubly-protonated bombesin. Lysine side-chain loss was observed for triply-protonated dynorphin A fragment 1-13 but not for the doubly-protonated ion. The proximity of arginine to a methoxy C-terminal group significantly enhances the extent of side-chain fragmentation. Fragment ions associated with side-chain losses were comparable in abundance to those resulting from backbone cleavage in all cases. In the ECD spectrum of one peptide, the major product was due to fragmentation within an arginine side chain. Our results suggest that cleavages within side chains should be taken into account in analysis of ECD mass spectral data. Losses from arginine, histidine, and asparigine/glutamine can be used to ascertain their presence, as in the analysis of unknown peptides, particularly those with non-linear structures.