Threshold Dissociation Energies Research Articles

Competitive hydrogen atom migrations accompanying cascade dissociations of peptide cation-radicals of the z+[rad] type

We report a combined experimental and computational study of energy-resolved collision-induced dissociation (ER-CID) and time-resolved infrared multiphoton dissociation (TR-IRMPD) of z4 ions prepared by electron transfer dissociation of peptide (Ala-Ala-Asn-Ala-Arg+2H)2+ ions. The z4 cation-radicals, ANAR+, undergo competitive dissociations by backbone cleavage and loss of a CONH2 radical from the Asn side chain. The backbone cleavage proceeds by radical-assisted dissociation of the Asn Cα-CO bond, forming an x2 ion intermediate which rapidly dissociates by HNCO elimination to yield a stable z2 fragment ion, AR+. The ER-CID and TR-IRMPD data were consistent with the consecutive nature of the backbone dissociation but showed different branching ratios for the two major fragmentations. The ER-CID data showed branching ratios 0.6–1.0 for the side-chain and backbone cleavages whereas the TR-IRMPD data showed an earlier onset for the latter dissociation. Computational analysis of the potential energy surface with density functional theory and ab initio calculations was carried out to provide structures and energies for the reactant ions as well as several intermediates, products, and transition states. Dissociation pathways for cis and trans amide conformers were distinguished and their energies were evaluated. The threshold dissociation energies for the backbone and side-chain dissociations were similar in accordance with the experimental ER-CID branching ratio. The TR-IRMPD data were interpreted by different absorbances of intermediates produced by hydrogen atom migrations along the dissociation pathways.

Structure and energetics of gas phase halogen-bonding in mono-, bi-, and tri-dentate anion receptors as studied by BIRD

Complexes of mono-, bi- (RB), and tridentate (RT) receptors with a range of anions (Cl(-), Br(-), I(-), NO3(-), H2PO4(-), HSO4(-), and tosylate (TsO(-))) have been studied in the gas phase by both experimental and theoretical methods. Temperature dependent blackbody infrared radiative dissociation (BIRD) experiments were performed on complexes of C8F17I with Br(-) and I(-), RB with I(-), NO3(-), HSO4(-), H2PO4(-), and TsO(-), and RT with I(-), HSO4(-) and TsO(-) and the observed Arrhenius parameters are reported here. Master equation modeling of the BIRD kinetics data was carried out to determine threshold dissociation energies. Geometry optimizations and thermochemistry calculations were performed using the B3LYP/6-31+G(d,p) level of theory. Additional single point energies were calculated using MP2/6-311++G(2d,p). Results were examined in terms of the binding order of various anions as well as the added binding strength from additional halogen bonding (XB) interactions. The relative binding energies of ions were generally consistent with the ordering previously reported from solution phase experiments; however, the relatively strong binding of H2PO4(-) to the bidentate receptor contrasted the solution phase observation of oxoanions having weaker interactions when compared to halides. An increase in the energy required to remove the same anion from the tridentate receptor when compared to the bidentate and monodentate receptors is explained as being due to the increase in halogen bonding interactions. The possibility of mixed halogen and hydrogen bonded complexes were considered.

Appropriate choice of event length in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: Activated ion collision-induced dissociation

Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) is used in Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) to activate ions through multiple, low-energy collisions. This is done by irradiating the ions with RF energy with a frequency offset, Δf, from their resonant frequency. This causes the orbit radius and kinetic energy of the excited ions to fluctuate with a periodicity of 1/Δf. In this paper we demonstrate that the extent of ion activation (and subsequent dissociation) is strongly influenced when SORI-CID event lengths are not integer multiples of 1/Δf. This is due to residual kinetic energy in the ions following the SORI event. These effects are seen over a range of Δf values. Small variations in SORI-CID event lengths can therefore be used to enhance or suppress dissociation, and care must be taken to account for these effects when comparing threshold dissociation energies obtained using these methods.

Structures and energetics of electrosprayed uracilnCa2+clusters (n = 14–4) in the gas phase

Clusters of uracil (U) about a calcium dication, U(n)Ca(2+) (n = 14-4), have been studied in the gas phase by both experimental and theoretical methods. Temperature dependent blackbody infrared radiative dissociation (BIRD) experiments were performed on U(n)Ca(2+) clusters with n = 14-5 and the observed Arrhenius parameters are reported here. Master equation modeling of the BIRD kinetics data was carried out to determine threshold dissociation energies. Initial geometry calculations were performed using the B3LYP density functional and 3-21G(d) basis set. A sample of ten conformations per cluster was obtained through a simulated annealing study. These structures were optimized using B3LYP/6-31G(d) level of theory. Fragment-based hybrid many body interaction (HMBI) MP2/6-311++G(2df,2p)/Amoeba calculations were performed on representative conformations to determine theoretical binding energies. Results were examined in relation to cluster size (n). A significant increase in the energy required to remove uracil from U(6)Ca(2+) when compared to larger clusters supports previous reports that the calcium ion is coordinated by six uracil molecules in the formation of an inner shell. For clusters larger than n = 6, an odd-even alternation in threshold dissociation energies was observed, suggesting that the outer shell uracil molecules bind as dimers to the inner core. Proposed binding schemes are presented. Multiple structures of U(5)Ca(2+) are suggested as being present in the gas phase where the fifth uracil may be either part of the first or second solvation shell.

Water binding energies of [Pb(amino acid-H)H2O]+ complexes determined by blackbody infrared radiative dissociation

The water binding energies (E(0)) of eight deprotonated Pb(2+)-amino acid (Aa) complexes of the form [Pb(Aa-H)H(2)O](+) (Aa = Gly, Ala, Val, Leu, Ile, Phe, Glu, and Lys) were determined using blackbody infrared radiative dissociation (BIRD). A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer was used to trap ions generated by electrospray ionization (ESI) in a "zero"-pressure (~10(-10) torr) cell where dissociation can only occur by absorption of thermally generated photons. Since the [Pb(Aa-H)H(2)O](+) complexes have relatively few vibrational degrees of freedom (36-78) and are within the slow-exchange kinetic limit, the master equation was solved to extract meaningful threshold dissociation energies and thermal unimolecular dissociation rate constants (k(uni)). The master equation analysis uses variable reaction coordinate transition state theory (VRC-TST) to minimize the Rice-Ramsperger-Kassel-Marcus (RRKM) dissociation rate constants. The determined water binding energies range from 76.6 to 113.6 kJ mol(-1), and agree well with 0 K dissociation energies calculated using the B3LYP/6-31+G(d,p) and MP2(full)/6-311++G(2d,2p)//B3LYP/6-31+G(d,p) methods. The relative strengths of the binding energies reflect the known structural isomers (A-, B-, C-, and D-type) of these [Pb(Aa-H)H(2)O](+) complexes.

Measuring Internal Energy Deposition in Collisional Activation Using Hydrated Ion Nanocalorimetry to Obtain Peptide Dissociation Energies and Entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na(+)(H(2)O)(30) results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na(+)(H(2)O)(30), are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.

Investigation of Energy Deposited by Femtosecond Electron Transfer in Collisions Using Hydrated Ion Nanocalorimetry

Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.

Binding Energies of Water to Doubly Hydrated Cationized Glutamine and Structural Analogues in the Gas Phase

The modes of metal-ion and water binding in doubly hydrated complexes of lithiated and sodiated glutamine (Gln) are probed using blackbody infrared radiative dissociation experiments and density functional theory calculations. Threshold dissociation energies, E0, for loss of a water molecule from these complexes are obtained from master-equation modeling of these data. The values of E0 are 36 +/- 1 and 38 +/- 2 kJ/mol for the lithiated and sodiated glutamine complexes, respectively, and are consistent with calculated water binding energies for the nonzwitterionic form of these complexes. Calculated water binding energies for the zwitterionic forms of these complexes are significantly higher. In contrast, calculations indicate that the zwitterionic form of Gln in these complexes is more stable than the nonzwitterionic form by 8 and 15 kJ/mol when lithiated and sodiated, respectively. Doubly hydrated lithiated and sodiated complexes of asparagine methyl ester (AsnOMe), asparagine ethyl ester (AsnOEt), and glutamine methyl ester (GlnOMe) were also studied for comparison to Gln. Although these clusters lack the acidic group of Gln and therefore have different water coordination behavior, these results further support the conclusion that Gln is nonzwitterionic in these clusters. Surprisingly, the complexes containing sodium are more stable than those containing lithium, a result that is attributed to subtle differences in how these two metal ions bind to the amino acid esters in these complexes.

Structures and Hydration Enthalpies of Cationized Glutamine and Structural Analogues in the Gas Phase

The structures of lithiated and sodiated glutamine, both with and without a water molecule, are investigated using experiment and theory. Loss of water from these complexes and from lithiated and sodiated complexes of asparagine methyl ester, asparagine ethyl ester, and glutamine methyl ester is probed with blackbody infrared radiative dissociation experiments performed over a wide temperature range. Threshold dissociation energies, E(o), for loss of a water molecule from these complexes are obtained from master equation modeling of these data. The values of E(o) are 63 +/- 1 and 53 +/- 1 kJ/mol for the lithiated and sodiated glutamine complexes, respectively. These values are similar to those for the nonzwitterionic model complexes and are in excellent agreement with calculated values. In contrast, water binding to the zwitterionic form is calculated to be significantly higher. These results indicate that glutamine in these lithiated and sodiated complexes with a water molecule are nonzwitterionic. Complexes with the asparagine side chain have slightly higher E(o) values than those with the glutamine side chain, a result consistent with more effective solvation of the metal ion due to the slightly longer side chain of glutamine. Calculations indicate that lithiated and sodiated glutamine are nonzwitterionic, with the metal ion interacting with the amine nitrogen and carbonyl oxygen from the amino acid backbone and the amide oxygen of the side chain. Addition of a water molecule does not affect the lowest-energy structure of lithiated glutamine, whereas, for sodiated glutamine, the lowest-energy zwitterionic and nonzwitterionic structures are essentially isoenergetic.

A Comparison of Data Analysis Methods for Determining Gas Phase Stabilities by CID: Alkali Metal Complexes of Polyether Ionophore Antibiotics

The gas phase stabilities of Group I metal complexes of the polyether ionophore antibiotics lasalocid and monensin were investigated by collision induced dissociation mass spectrometry. Electrospray ionization was used with a triple quadrupole mass spectrometer for the determination of threshold dissociation energies upon application of increasing collision energies. Various data analysis techniques for the determination of dissociation energies are discussed to assess the most suitable method for determining the stabilities of the ionophore-metal complexes studied here. In all cases only the relative stabilities of different complexes may be obtained by the method presented in this study, which does not assess absolute gas phase dissociation energies. Correction factors have been applied, however, to account for the energy conversion during collisions of different metal complexes and the varying degrees of freedom of different sized ligands, allowing for the comparison of the stabilities of different ionophores with like-metals. The measured threshold dissociation energies were compared with respect to the ionic radius of the metal cation, revealing a maximum stability for the K+ complexes of both lasalocid and monensin. A striking decrease in the stabilities of the Rb+ and Cs+ complexes was observed and is believed to be related to a decreasing degree of coordination that the ionophores can accomplish with the larger metals.

Structures of Cationized Proline Analogues: Evidence for the Zwitterionic Form

The structures of lithiated and sodiated alpha-methyl-proline (alpha-Me-Pro) and structural isomers, both with and without a water molecule, are investigated using blackbody infrared radiative dissociation (BIRD) and density functional theory. From the BIRD kinetic data measured as a function of temperature, combined with master equation modeling of these data, threshold dissociation energies for the loss of a water molecule from these clusters are obtained. These energies are 77.5 +/- 0.5 and 53 +/- 1 kJ/mol for lithiated and sodiated alpha-Me-Pro, respectively. For the nonzwitterionic isomer, proline methyl ester, these values are 3.0-4.5 kJ/mol higher. These results provide compelling experimental evidence that alpha-Me-Pro is zwitterionic in these clusters. Theory at the temperature corrected B3LYP/6-311++G**//B3LYP/6-31++G** level indicates that the salt-bridge or zwitterionic forms of lithiated and sodiated alpha-Me-Pro are between 17 and 23 kJ/mol lower in energy than the nonzwitterionic or charge-solvated forms and that attachment of a single water molecule does not significantly change the structure or the relative energies of these clusters. The proton affinity of proline is 8 kJ/mol higher than that of alpha-Me-Pro, indicating that lithiated and sodiated singly hydrated proline should also be zwitterionic.

Threshold dissociation and molecular modeling of transition metal complexes of flavonoids

The relative threshold dissociation energies of a series of flavonoid/transition metal/auxiliary ligand complexes of the type [MII (flavonoid - H) auxiliary ligand]+ formed by electrospray ionization (ESI) were measured by energy-variable collisionally activated dissociation (CAD) in a quadrupole ion trap (QIT). For each of the isomeric flavonoid diglycoside pairs, the rutinoside (with a 1-6 inter-saccharide linkage) requires a greater CAD energy and thus has a higher dissociation threshold than its neohesperidoside (with a 1-2 inter-saccharide linkage) isomer. Likewise, the threshold energies of complexes containing flavones are higher than those containing flavanones. The monoglycoside isomers also have characteristic threshold energies. The flavonoids that are glycosylated at the 3-O- position tend to have lower threshold energies than those glycosylated at the 7-O- or 4'-O- position, and those that are C- bonded have lower threshold energies than the O- bonded isomers. The structural features that substantially influence the threshold energies include the aglycon type (flavanone versus flavone), the type of disaccharide (rutinose versus neohesperidose), and the linkage type (O- bonded versus C- bonded). Various computational means were applied to probe the structures and conformations of the complexes and to rationalize the differences in threshold energies of isomeric flavonoids. The most favorable coordination geometry of the complexes has a plane-angle of about 62 degrees , which means that the deprotonated flavonoid and 2,2'-bipyridine within a complex do not reside on the same plane. Stable conformations of five cobalt complexes and five deprotonated flavonoids were identified. The conformations were combined with the point charges and helium accessible surface areas to explain qualitatively the differences in threshold energies for isomeric flavonoids.

Blackbody infrared radiative dissociation at low temperature: hydration of X 2+(H 2O) n, for X = Mg, Ca

A new apparatus for making blackbody infrared radiative dissociation (BIRD) measurements at below ambient temperature is described, and its use for measuring threshold dissociation energies of weakly bound clusters is demonstrated. Hydration energies are determined for alkaline-earth metal water clusters, X 2+(H 2O) n , X=Mg, Ca and n=8–10. For n=8 and 9, the energies obtained from BIRD measurements are in excellent agreement with values reported previously, but for n=10, the energies are slightly lower than those determined previously using the high-pressure ion source mass spectrometry (HPMS) equilibrium method.

Threshold dissociation energies of protonated amine/polyether complexes in a quadrupole ion trap

Electrospray ionization mass spectrometry (ESI-MS) is increasingly used to probe the nature of noncovalent complexes; however, assessing the relevance of gas-phase results to structures of complexes in solution requires knowledge of the types of interactions that are maintained in a solventless environment and how these might compare to key interactions in solution. This study addresses the factors impacting the strength of hydrogen bonding noncovalent interactions in the gas phase. Hydrogen bonded complexes consisting of ammonium ions bound to polydentate ethers are transported to the gas phase with ESI, and energy-variable collisional activated dissociation (CAD) is used to map the relative dissociation energies. The measured relative dissociation energies are correlated with the gas-phase basicities and steric factors of the amine and polyether constituents. To develop correlations between hydrogen bonding strength and structural features of the donor and acceptor molecules, a variety of amines with different gas-phase basicities and structures were selected, including primary, secondary, and tertiary amines, as well as those that are bidentate to promote intramolecular hydrogen bonding. The acceptor molecules are polydentate ethers, such as 18-crown-6. Four primary factors influence the observed dissociation energies of the polyether/ammonium ion complexes: the gas-phase basicities of the polyether and amine, steric effects of the amines, conformational flexibility of the polyethers, and the inhibition of intramolecular hydrogen bonds of the guest ammonium ions in the resulting ammonium/polyether noncovalent complexes.

Effects of Gas-Phase Basicity on the Proton Transfer between Organic Bases and Trifluoroacetic Acid in the Gas Phase: Energetics of Charge Solvation and Salt Bridges†

The unimolecular dissociation pathways and kinetics of a series of protonated trimer ions consisting of two organic bases and trifluoroacetic acid were investigated using blackbody infrared radiative dissociation. Five bases with gas-phase basicities (GB) ranging from 238.4 to 246.2 kcal/mol were used. Both the dissociation pathways and the threshold dissociation energies depend on the GB of the base. Trimers consisting of the two most basic molecules dissociate to form protonated base monomers with an E(0) ~ 1.4 eV. Trimers consisting of the two least basic molecules dissociate to form protonated base dimers with an E(0) ~ 1.1-1.2 eV. These results indicate that the structures of the trimers change as a function of the GB of the basic molecule. The predominant structure of the protonated trimers consisting of the two most basic molecules is consistent with a salt bridge in which both of the basic molecules are protonated, and the trifluoroacetic acid molecule is deprotonated, whereas the predominant structure of the protonated trimers consisting of the two least basic molecules are consistent with charge-solvated complexes in which the proton is shared. The structure of the trimer consisting of the base of intermediate basicity is less clear; it dissociates to form primarily protonated base dimer, but has an E(0) ~ 1.2 eV. These results are consistent with the structure of this trimer as a salt bridge, but the resulting dissociation A(-). BH(+) product does not appear to be stable as an ion pair in the dissociative transition state.

Dissociation energies of deoxyribose nucleotide dimer anions measured using blackbody infrared radiative dissociation

The dissociation kinetics of deprotonated deoxyribose nucleotide dimers were measured using blackbody infrared radiative dissociation. Experiments were performed with noncovalently bound dimers of phosphate, adenosine (dAMP), cytosine (dCMP), guanosine (dGMP), thymidine (dTMP), and the mixed dimers dAMP · dTMP and dGMP · dCMP. The nucleotide dimers fragment through two parallel pathways, resulting in formation of the individual nucleotide or nucleotide + HPO 3 ion. Master equation modeling of this kinetic data was used to determine threshold dissociation energies. The dissociation energy of (dGMP · dCMP − H) − is much higher than that for the other nucleotide dimers. This indicates that there is a strong interaction between the nucleobases in this dimer, consistent with the existence of Watson–Crick hydrogen bonding between the base pairs. Molecular mechanics simulations indicate that Watson–Crick hydrogen bonding occurs in the lowest energy structures of (dGMP · dCMP − H) −, but not in (dAMP · dTMP − H) −. The trend in gas phase dissociation energies is similar to the trend in binding energies measured in nonaqueous solutions within experimental error. Finally, the acidity ordering of the nucleotides is determined to be dTMP < dGMP < dCMP < dAMP, where dAMP has the highest acidity (largest Δ G acid).

Hydration energies and structures of alkaline earth metal ions, M2+(H2O)n, n = 5-7, M = Mg, Ca, Sr, and Ba.

The evaporation of water from hydrated alkaline earth metal ions, produced by electrospray ionization, was studied in a Fourier transform mass spectrometer. Zero-pressure-limit dissociation rate constants for loss of a single water molecule from the hydrated divalent metal ions, M(2+)(H(2)O)(n) (M = Mg, Ca, and Sr for n = 5-7, and M = Ba for n = 4-7), are measured as a function of temperature using blackbody infrared radiative dissociation. From these values, zero-pressure-limit Arrhenius parameters are obtained. By modeling the dissociation kinetics using a master equation formalism, threshold dissociation energies (E(o)) are determined. These reactions should have a negligible reverse activation barrier; therefore, E(o) values should be approximately equal to the binding energy or hydration enthalpy at 0 K. For the hepta- and hexahydrated ions at low temperature, binding energies follow the trend expected on the basis of ionic radii: Mg > Ca > Sr > Ba. For the hexahydrated ions at high temperature, binding energies follow the order Ca > Mg > Sr > Ba. The same order is observed for the pentahydrated ions. Collisional dissociation experiments on the tetrahydrated species result in relative dissociation rates that directly correlate with the size of the metals. These results indicate the presence of two isomers for hexahydrated magnesium ions: a low-temperature isomer in which the six water molecules are located in the first solvation shell, and a high-temperature isomer with the most likely structure corresponding to four water molecules in the inner shell and two water molecules in the second shell. These results also indicate that the pentahydrated magnesium ions have a structure with four water molecules in the first solvation shell and one in the outer shell. The dissociation kinetics for the hexa- and pentahydrated clusters of Ca(2+), Sr(2+), and Ba(2+) are consistent with structures in which all the water molecules are located in the first solvation shell.

Binding Energies of Hexahydrated Alkaline Earth Metal Ions, M2+(H2O)6, M = Mg, Ca, Sr, Ba: Evidence of Isomeric Structures for Magnesium

Binding Energies of Hexahydrated Alkaline Earth Metal Ions, M²⁺(H₂O)₆, M = Mg, Ca, Sr, Ba: Evidence of Isomeric Structures for Magnesium

Binding Energies of Proton-Bound Dimers of Imidazole and n-Acetylalanine Methyl Ester Obtained by Blackbody Infrared Radiative Dissociation

The dissociation kinetics of protonated n-acetyl-L-alanine methyl ester dimer (AcAlaME(d)), imidazole dimer, and their cross dimer were measured using blackbody infrared radiative dissociation (BIRD). Master equation modeling of these data was used to extract threshold dissociation energies (E(o)) for the dimers. Values of 1.18 +/- 0.06, 1.11 +/- 0.04, and 1.12 +/- 0.08 eV were obtained for AcAlaME(d), imidazole dimer, and the cross dimer, respectively. Assuming that the reverse activation barrier for dissociation of the ion-molecule complex is negligible, the value of E(o) can be compared to the dissociation enthalpy (DeltaH(d) degrees ) from HPMS data. The E(o) values obtained for the imidazole dimer and the cross dimer are in agreement with HPMS values; the value for AcAlaME(d) is somewhat lower. Radiative rate constants used in the master equation modeling were determined using transition dipole moments calculated at the semiempirical (AM1) level for all dimers and compared to ab initio (RHF/3-21G*) calculations where possible. To reproduce the experimentally measured dissociation rates using master equation modeling, it was necessary to multiply semiempirical transition dipole moments by a factor between 2 and 3. Values for transition dipole moments from the ab initio calculations could be used for two of the dimers but appear to be too low for AcAlaME(d). These results demonstrate that BIRD, in combination with master equation modeling, can be used to determine threshold dissociation energies for intermediate size ions that are in neither the truncated Boltzmann nor the rapid energy exchange limit.

Binding Energies of Protonated Betaine Complexes: A Probe of Zwitterion Structure in the Gas Phase

The dissociation kinetics of proton-bound dimers of betaine with molecules of comparable gas-phase basicity were investigated using blackbody infrared radiative dissociation (BIRD). Threshold dissociation energies were obtained from these data using master equation modeling. For bases that have comparable or higher gas-phase basicity, the binding energy of the protonated base.betaine complex is approximately 1.4 eV. For molecules that are approximately 2 kcal/mol or more less basic, the dissociation energy of the complexes is approximately 1.2 eV. The higher binding energy of the former is attributed to an ion-zwitterion structure which has a much larger ion-dipole interaction. The lower binding energy for molecules that are approximately 2 kcal/mol or more less basic indicates that an ion-molecule structure is more favored. Semiempirical calculations at both the AM1 and PM3 levels indicate the most stable ion-molecule structure is one in which the base interacts with the charged quaternary ammonium end of betaine. These results indicate that the measurement of binding energies of neutral molecules to biological ions could provide a useful probe for the presence of zwitterions and salt bridges in the gas phase. From the BIRD data, the gas-phase basicity of betaine obtained from the kinetic method is found to be 239.2 +/- 1.0 kcal/mol. This value is in excellent agreement with the value of 239.3 kcal/mol (298 K) from ab initio calculations at the MP2/6-31+g** level. The measured value is slightly higher than those reported previously. This difference is attributed to entropy effects. The lower ion internal energy and longer time frame of BIRD experiments should provide values closer to those at standard temperature.