This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
European journal of mass spectrometry, Feb 1, 2019
Rob Dunbar, who passed away on 31 October 2017 at the age of 74, was one of the most respected an... more Rob Dunbar, who passed away on 31 October 2017 at the age of 74, was one of the most respected and beloved ion chemists. The fundamental understanding gained from his work on ion spectroscopy, dissociation rates, and infrared emission processes arc his lasting legacy, which will continue to influence our field in the coming years. Those lucky enough to have known Rob personally remember his devotion to the pursuit of ever-deepening understanding of ion chemistry, his modesty, generosity, and his ability to balance his scientific work with his love for the arts and literature.
Journal of the American Society for Mass Spectrometry, Jun 12, 2012
The Co(II) complexes of twelve meso-tetraaryl-porphyrins,-chlorins, and chlorin analogues contain... more The Co(II) complexes of twelve meso-tetraaryl-porphyrins,-chlorins, and chlorin analogues containing non-pyrrolic heterocycles were synthesized and converted in situ to the corresponding Co(III) complexes coordinated to one or two imidazoles. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) in conjunction with the energy-variable collision-induced dissociation (CID) technique was used to compare the relative gas-phase binding strength of the axially coordinated imidazoles to the octahedral and square planar Co(III) porphyrinoid complex ions. The observed binding energies of these ligands were rationalized in terms of the effects of porphyrinoid core structure and meso-substitution on the electron density on the central Co(III) centers. Some of these trends were supported by DFT-based computational studies. The study highlights to which extend porphyrins vary from chlorins and chlorin analogues in their coordination abilities and to which extraordinary degree meso-thienyl-substituents influence the electronic structure of porphyrins. The study also defines further the scope and limits CID experiments can be used to interrogate the electronic structures of metalloporphyrin complexes.
Journal of the American Society for Mass Spectrometry, Sep 1, 1999
Direct associative equilibrium provides probably the best way to measure the thermochemical prope... more Direct associative equilibrium provides probably the best way to measure the thermochemical properties of ion-neutral complexes. This approach has been widely used in high pressure mass spectrometry (HPMS). We describe the establishment and observation of equilibrium at low pressure in the Fourier-transform ion cyclotron resonance (FT-ICR) spectrometer, using the hydration reaction of protonated 18-crown-6 as a test case. The measured enthalpy and entropy of monohydration are in agreement with the prior HPMS results, with the advantage that the present measurements were possible in a temperature range nearly 100°C lower. The low pressure regime of the FT-ICR technique makes accessible complexes bound by 10-15 kcal higher than for a corresponding HPMS experiment. Modeling of the equilibrium experiment was carried out using the Standard Hydrocarbon estimates of association kinetics. The strongly coupled domains of pressure, binding energy, temperature, and molecular size were mapped out for which it should be possible to establish and observe equilibrium under FT-ICR conditions. The particular difficulties raised by small molecule size and high binding energy were noted, and the possibility was noted that use of a high pressure of inert bath gas could alleviate these problems by accelerating the attainment of equilibrium.
Journal of Liquid Chromatography & Related Technologies, Apr 15, 2019
Nucleotides and other phosphate-containing compounds are integral to enzymatic reactions such as ... more Nucleotides and other phosphate-containing compounds are integral to enzymatic reactions such as those of the methylerythritol phosphate (MEP) pathway and glycolysis. Traditional chromatographic analysis of phosphates is often plagued by long run times and/or lack of MS compatibility. This study compares separation of five enzymatically-important nucleotides using ion-pair reversed phase (IP-RP), strong anion exchange (SAX), and hydrophilic interaction (HILIC) methods. These three methods were evaluated and compared based on separation parameters describing retention, resolution, efficiency, peak symmetry, selectivity, and inter-and intraday peak drift. Use of the FructoShell-N HILIC column led to separation of the five nucleotides isocratically with the shortest run time of all three methods tested. Additionally, the FructoShell HILIC method yielded a very low intraday variability and low peak asymmetry, issues that are often observed with HILIC separations on other stationary phases. To our knowledge, this column has not been applied to the separation of phosphates in biological samples and future work will focus on in vitro and in vivo analysis as well as broadening the applicability to other pathways. To this end, we have shown that the column will retain fructose bisphosphate, the substrate of the aldolase enzyme, under the same chromatographic conditions used for nucleotides.
The formation of gas-phase complexes by radiative association is a strongly temperature-dependent... more The formation of gas-phase complexes by radiative association is a strongly temperature-dependent process, whose modeling provides a good test of theoretical approaches to modeling the kinetics and whose predictability is useful for temperature extrapolations. The temperature dependence of the low-pressure association rate constants, measured in the Fourier transform ion cyclotron resonance mass spectrometer, was considered for four systems, acetone/(acetone)H + , acetone-d 6 /(acetone-d 6)D + , butanone/(butanone)H + , and NO + /3-pentanone. For the first two systems, the experimentally measured temperature range was extended down to 245 K to complement data already available for room and higher temperatures. The data for the third system above room temperature are new, while data for the final system, already available from our earlier experiments, are reconsidered here. Modeling was done by variational transition-state theory (VTST), incorporating ab initio calculations of vibrational frequencies and infrared emission intensities. The VTST-based approach gave excellent agreement with the measured values of the rate constants and their temperature dependences. The results suggest that VTST-based modeling provides an adequate description of the radiative association kinetics and can serve as an accurate approach for making binding energy estimates from experimental association results.
Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A... more Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A general formalism for evaluating the effective rate constant for radiative and collisional association is reviewed. The implementation of variable reaction coordinate transition state theory estimates within this formalism is described. A detailed discussion is given of the limiting cases of high and low stabilization efficiency. The basic validity of the algorithm is illustrated through sample calculations for the high efficiency limit. The low efficiency limit allows for the determination of binding energies which are independent of any transition state model. The relation between the predicted and observed temperature dependence in the low efficiency limit is explored. Sample calculations employing the general formalism illustrate the usefulness of this modeling in estimating the binding energy of the complex. Modest levels of quantum chemistry (e.g., MP2/6-31G*) are found to provide satisfactory estimates of the vibrational frequencies and intensities required in the modeling. Overall, the modeling provides estimated binding energies for the protonated acetone dimer, NO+...3-pentanone, and Al+...C6H6 complexes which agree with the available literature values to within 2 kcal/mol.
International Journal of Mass Spectrometry, Nov 1, 2015
The effects of hydrogen bonding on the gas-phase reactivity of phenoxyl oxygen radicals were inve... more The effects of hydrogen bonding on the gas-phase reactivity of phenoxyl oxygen radicals were investigated experimentally and theoretically in model systems and the dipeptide LysTyr. Gasphase ion-molecule reactions were carried out between radical cations of several aromatic nitrogen bases with the neutrals nitric oxide and n-propyl thiol. The variation in the structure of the model compounds allowed the four-, five-, and six-membered ring to be formed between the protonated nitrogen and the phenoxyl oxygen. The hydrogen bond length was calculated to decrease in the series (1-4), which coincided with the decrease in reaction rates towards both nitric oxide and n-propyl thiol. A control radical cation with no hydrogen-bonding capability displayed faster reactivity. DFT calculations found that the lowest energy structure of the distonic radical cation of the dipeptide [LysTyr(O •)] + has a short hydrogen bond between the protonated Lys side chain and the phenoxyl oxygen, 1.70 Å, which is consistent with its low reactivity.
International Journal of Mass Spectrometry and Ion Processes, Nov 1, 1997
Large but regular changes in the rates of formation of gas-phase complexes by radiative associati... more Large but regular changes in the rates of formation of gas-phase complexes by radiative association occur as the number of degrees of freedom of the complex changes. In interpreting measurements of such rates for purposes like bonding energy comparisons, it is important to have a confident and predictively useful understanding of these size effects; the present work was carried out to provide useful sets of comparative data for two homologous series of ketone complexes, and to match the observations with our present theoretical understanding. Improved measurements were made in the ion cyclotron resonance ion trap for the series of NO÷/ketone complexes from acetone to pentanone. Also, the series of complexes of protonated ketones with their parent ketones was extended upward from the well-studied acetone case, by new measurements for the butanone, pentanone and hexanone molecules. For both series, the systematic dependence of the association rate on the number of degrees of freedom of the complex was considered in the context of the semiquantitative generic theoretical approach termed the 'standard hydrocarbon' approach. For both series, the standard hydrocarbon predictions were in excellent agreement with the relative values observed for the different sized members of the series. The model also gave reasonable predictions for the absolute rates of the NO÷/ketone series, but was much too low for the protonated ketone/ketone series. The latter discrepancy is due in large part to emission rates from the protonated ketone/ketone complexes being exceptionally high relative to typical hydrocarbon systems. The results are encouraging to the use of the standard hydrocarbon approach for comparisons and predictions of radiative association for series of related molecules, while also reinforcing the warning against expecting better than order-of-magnitude predictions of absolute rates from this approach.
Journal of the American Society for Mass Spectrometry, Mar 20, 2013
In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric ... more In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations-a distonic indolyl N-radical (H3N(+) - TrpN(•)) and a canonical aromatic π (Trp(•+)) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [Cu(II)(terpy)(Trp)](•2+) complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm(-1) region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
European journal of mass spectrometry, Feb 1, 2019
Rob Dunbar, who passed away on 31 October 2017 at the age of 74, was one of the most respected an... more Rob Dunbar, who passed away on 31 October 2017 at the age of 74, was one of the most respected and beloved ion chemists. The fundamental understanding gained from his work on ion spectroscopy, dissociation rates, and infrared emission processes arc his lasting legacy, which will continue to influence our field in the coming years. Those lucky enough to have known Rob personally remember his devotion to the pursuit of ever-deepening understanding of ion chemistry, his modesty, generosity, and his ability to balance his scientific work with his love for the arts and literature.
Journal of the American Society for Mass Spectrometry, Jun 12, 2012
The Co(II) complexes of twelve meso-tetraaryl-porphyrins,-chlorins, and chlorin analogues contain... more The Co(II) complexes of twelve meso-tetraaryl-porphyrins,-chlorins, and chlorin analogues containing non-pyrrolic heterocycles were synthesized and converted in situ to the corresponding Co(III) complexes coordinated to one or two imidazoles. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) in conjunction with the energy-variable collision-induced dissociation (CID) technique was used to compare the relative gas-phase binding strength of the axially coordinated imidazoles to the octahedral and square planar Co(III) porphyrinoid complex ions. The observed binding energies of these ligands were rationalized in terms of the effects of porphyrinoid core structure and meso-substitution on the electron density on the central Co(III) centers. Some of these trends were supported by DFT-based computational studies. The study highlights to which extend porphyrins vary from chlorins and chlorin analogues in their coordination abilities and to which extraordinary degree meso-thienyl-substituents influence the electronic structure of porphyrins. The study also defines further the scope and limits CID experiments can be used to interrogate the electronic structures of metalloporphyrin complexes.
Journal of the American Society for Mass Spectrometry, Sep 1, 1999
Direct associative equilibrium provides probably the best way to measure the thermochemical prope... more Direct associative equilibrium provides probably the best way to measure the thermochemical properties of ion-neutral complexes. This approach has been widely used in high pressure mass spectrometry (HPMS). We describe the establishment and observation of equilibrium at low pressure in the Fourier-transform ion cyclotron resonance (FT-ICR) spectrometer, using the hydration reaction of protonated 18-crown-6 as a test case. The measured enthalpy and entropy of monohydration are in agreement with the prior HPMS results, with the advantage that the present measurements were possible in a temperature range nearly 100°C lower. The low pressure regime of the FT-ICR technique makes accessible complexes bound by 10-15 kcal higher than for a corresponding HPMS experiment. Modeling of the equilibrium experiment was carried out using the Standard Hydrocarbon estimates of association kinetics. The strongly coupled domains of pressure, binding energy, temperature, and molecular size were mapped out for which it should be possible to establish and observe equilibrium under FT-ICR conditions. The particular difficulties raised by small molecule size and high binding energy were noted, and the possibility was noted that use of a high pressure of inert bath gas could alleviate these problems by accelerating the attainment of equilibrium.
Journal of Liquid Chromatography & Related Technologies, Apr 15, 2019
Nucleotides and other phosphate-containing compounds are integral to enzymatic reactions such as ... more Nucleotides and other phosphate-containing compounds are integral to enzymatic reactions such as those of the methylerythritol phosphate (MEP) pathway and glycolysis. Traditional chromatographic analysis of phosphates is often plagued by long run times and/or lack of MS compatibility. This study compares separation of five enzymatically-important nucleotides using ion-pair reversed phase (IP-RP), strong anion exchange (SAX), and hydrophilic interaction (HILIC) methods. These three methods were evaluated and compared based on separation parameters describing retention, resolution, efficiency, peak symmetry, selectivity, and inter-and intraday peak drift. Use of the FructoShell-N HILIC column led to separation of the five nucleotides isocratically with the shortest run time of all three methods tested. Additionally, the FructoShell HILIC method yielded a very low intraday variability and low peak asymmetry, issues that are often observed with HILIC separations on other stationary phases. To our knowledge, this column has not been applied to the separation of phosphates in biological samples and future work will focus on in vitro and in vivo analysis as well as broadening the applicability to other pathways. To this end, we have shown that the column will retain fructose bisphosphate, the substrate of the aldolase enzyme, under the same chromatographic conditions used for nucleotides.
The formation of gas-phase complexes by radiative association is a strongly temperature-dependent... more The formation of gas-phase complexes by radiative association is a strongly temperature-dependent process, whose modeling provides a good test of theoretical approaches to modeling the kinetics and whose predictability is useful for temperature extrapolations. The temperature dependence of the low-pressure association rate constants, measured in the Fourier transform ion cyclotron resonance mass spectrometer, was considered for four systems, acetone/(acetone)H + , acetone-d 6 /(acetone-d 6)D + , butanone/(butanone)H + , and NO + /3-pentanone. For the first two systems, the experimentally measured temperature range was extended down to 245 K to complement data already available for room and higher temperatures. The data for the third system above room temperature are new, while data for the final system, already available from our earlier experiments, are reconsidered here. Modeling was done by variational transition-state theory (VTST), incorporating ab initio calculations of vibrational frequencies and infrared emission intensities. The VTST-based approach gave excellent agreement with the measured values of the rate constants and their temperature dependences. The results suggest that VTST-based modeling provides an adequate description of the radiative association kinetics and can serve as an accurate approach for making binding energy estimates from experimental association results.
Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A... more Various aspects of the theory and modeling of ion–molecule radiative association are discussed. A general formalism for evaluating the effective rate constant for radiative and collisional association is reviewed. The implementation of variable reaction coordinate transition state theory estimates within this formalism is described. A detailed discussion is given of the limiting cases of high and low stabilization efficiency. The basic validity of the algorithm is illustrated through sample calculations for the high efficiency limit. The low efficiency limit allows for the determination of binding energies which are independent of any transition state model. The relation between the predicted and observed temperature dependence in the low efficiency limit is explored. Sample calculations employing the general formalism illustrate the usefulness of this modeling in estimating the binding energy of the complex. Modest levels of quantum chemistry (e.g., MP2/6-31G*) are found to provide satisfactory estimates of the vibrational frequencies and intensities required in the modeling. Overall, the modeling provides estimated binding energies for the protonated acetone dimer, NO+...3-pentanone, and Al+...C6H6 complexes which agree with the available literature values to within 2 kcal/mol.
International Journal of Mass Spectrometry, Nov 1, 2015
The effects of hydrogen bonding on the gas-phase reactivity of phenoxyl oxygen radicals were inve... more The effects of hydrogen bonding on the gas-phase reactivity of phenoxyl oxygen radicals were investigated experimentally and theoretically in model systems and the dipeptide LysTyr. Gasphase ion-molecule reactions were carried out between radical cations of several aromatic nitrogen bases with the neutrals nitric oxide and n-propyl thiol. The variation in the structure of the model compounds allowed the four-, five-, and six-membered ring to be formed between the protonated nitrogen and the phenoxyl oxygen. The hydrogen bond length was calculated to decrease in the series (1-4), which coincided with the decrease in reaction rates towards both nitric oxide and n-propyl thiol. A control radical cation with no hydrogen-bonding capability displayed faster reactivity. DFT calculations found that the lowest energy structure of the distonic radical cation of the dipeptide [LysTyr(O •)] + has a short hydrogen bond between the protonated Lys side chain and the phenoxyl oxygen, 1.70 Å, which is consistent with its low reactivity.
International Journal of Mass Spectrometry and Ion Processes, Nov 1, 1997
Large but regular changes in the rates of formation of gas-phase complexes by radiative associati... more Large but regular changes in the rates of formation of gas-phase complexes by radiative association occur as the number of degrees of freedom of the complex changes. In interpreting measurements of such rates for purposes like bonding energy comparisons, it is important to have a confident and predictively useful understanding of these size effects; the present work was carried out to provide useful sets of comparative data for two homologous series of ketone complexes, and to match the observations with our present theoretical understanding. Improved measurements were made in the ion cyclotron resonance ion trap for the series of NO÷/ketone complexes from acetone to pentanone. Also, the series of complexes of protonated ketones with their parent ketones was extended upward from the well-studied acetone case, by new measurements for the butanone, pentanone and hexanone molecules. For both series, the systematic dependence of the association rate on the number of degrees of freedom of the complex was considered in the context of the semiquantitative generic theoretical approach termed the 'standard hydrocarbon' approach. For both series, the standard hydrocarbon predictions were in excellent agreement with the relative values observed for the different sized members of the series. The model also gave reasonable predictions for the absolute rates of the NO÷/ketone series, but was much too low for the protonated ketone/ketone series. The latter discrepancy is due in large part to emission rates from the protonated ketone/ketone complexes being exceptionally high relative to typical hydrocarbon systems. The results are encouraging to the use of the standard hydrocarbon approach for comparisons and predictions of radiative association for series of related molecules, while also reinforcing the warning against expecting better than order-of-magnitude predictions of absolute rates from this approach.
Journal of the American Society for Mass Spectrometry, Mar 20, 2013
In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric ... more In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations-a distonic indolyl N-radical (H3N(+) - TrpN(•)) and a canonical aromatic π (Trp(•+)) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [Cu(II)(terpy)(Trp)](•2+) complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm(-1) region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.
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