Christopher M. Leavitt

Cryogenic Ion Chemistry and Spectroscopy

The use of mass spectrometry in macromolecular analysis is an incredibly important technique and has allowed efficient identification of secondary and tertiary protein structures. Over 20 years ago, Chemistry Nobelist John Fenn and co-workers revolutionized mass spectrometry by developing ways to non-destructively extract large molecules directly from solution into the gas phase. This advance, in turn, enabled rapid sequencing of biopolymers through tandem mass spectrometry at the heart of the burgeoning field of proteomics. In this Account, we discuss how cryogenic cooling, mass selection, and reactive processing together provide a powerful way to characterize ion structures as well as rationally synthesize labile reaction intermediates. This is accomplished by first cooling the ions close to 10 K and condensing onto them weakly bound, chemically inert small molecules or rare gas atoms. This assembly can then be used as a medium in which to quench reactive encounters by rapid evaporation of the adducts, as well as provide a universal means for acquiring highly resolved vibrational action spectra of the embedded species by photoinduced mass loss. Moreover, the spectroscopic measurements can be obtained with readily available, broadly tunable pulsed infrared lasers because absorption of a single photon is sufficient to induce evaporation. We discuss the implementation of these methods with a new type of hybrid photofragmentation mass spectrometer involving two stages of mass selection with two laser excitation regions interfaced to the cryogenic ion source. We illustrate several capabilities of the cryogenic ion spectrometer by presenting recent applications to peptides, a biomimetic catalyst, a large antibiotic molecule (vancomycin), and reaction intermediates pertinent to the chemistry of the ionosphere. First, we demonstrate how site-specific isotopic substitution can be used to identify bands due to local functional groups in a protonated tripeptide designed to stereoselectively catalyze bromination of biaryl substrates. This procedure directly reveals the particular H-bond donor and acceptor groups that enforce the folded structure of the bare ion as well as provide contact points for noncovalent interaction with substrates. We then show how photochemical hole-burning involving only vibrational excitations can be used in a double-resonance mode to systematically disentangle overlapping spectra that arise when several conformers of a dipeptide are prepared in the ion source. Finally, we highlight our ability to systematically capture reaction intermediates and spectroscopically characterize their structures. Through this method, we can identify the pathway for water-network-mediated, proton-coupled transformation of nitrosonium, NO(+) to HONO, a key reaction controlling the cations present in the ionosphere. Through this work, we reveal the critical role played by water molecules occupying the second solvation shell around the ion, where they stabilize the emergent product ion in a fashion reminiscent of the solvent coordinate responsible for the barrier to charge transfer in solution. Looking to the future, we predict that the capture and characterization of fleeting intermediate complexes in the homogeneous catalytic activation of small molecules like water, alkanes, and CO2 is a likely avenue rich with opportunity.

Determination of Noncovalent Docking by Infrared Spectroscopy of Cold Gas-Phase Complexes

Ties That Bind Almost by definition, effective catalysts bind their substrates for a very short time—releasing them quickly after helping them react and then moving on to bind new, as yet unreacted, substrates. This property engenders an efficient cycle, but it hinders study of the binding motif. Garand et al. (p. 694, published online 19 January; see the Perspective by Zwier) devised a technique to extract bound complexes from solution and freeze their conformations in cold, gas-phase clusters. Probing these clusters by vibrational spectroscopy in conjunction with theoretical calculations then allowed the sites of hydrogen bonding that hold the complexes together to be pinpointed. Conformationally freezing a weakly bound complex in the gas phase sheds light on its likely binding motifs in solution. Multidentate, noncovalent interactions between small molecules and biopolymer fragments are central to processes ranging from drug action to selective catalysis. We present a versatile and sensitive spectroscopic probe of functional groups engaged in hydrogen bonding in such contexts. This involves measurement of the frequency changes in specific covalent bonds upon complex formation, information drawn from otherwise transient complexes that have been extracted from solution and conformationally frozen near 10 kelvin in gas-phase clusters. Resonances closely associated with individual oscillators are easily identified through site-specific isotopic labeling, as demonstrated by application of the method to an archetypal system involving a synthetic tripeptide known to bind biaryl substrates through tailored hydrogen bonding to catalyze their asymmetric bromination. With such data, calculations readily converge on the plausible operative structures in otherwise computationally prohibitive, high-dimensionality landscapes.

Isomer-Specific IR-IR Double Resonance Spectroscopy of D2-Tagged Protonated Dipeptides Prepared in a Cryogenic Ion Trap.

Isomer-specific vibrational predissociation spectra are reported for the gas-phase GlySarH(+) and SarSarH(+) [Gly = glycine; Sar = sarcosine] ions prepared by electrospray ionization and tagged with weakly bound D2 adducts using a cryogenic ion trap. The contributions of individual isomers to the overlapping vibrational band patterns are completely isolated using a pump-probe photochemical hole-burning scheme involving two tunable infrared lasers and two stages of mass selection (hence IR(2)MS(2)). These patterns are then assigned by comparison with harmonic (MP2/6-311+G(d,p)) spectra for various possible conformers. Both systems occur in two conformations based on cis and trans configurations with respect to the amide bond. In addition to the usual single intramolecular hydrogen bond motif between the protonated amine and the nearby amide oxygen atom, cis-SarSarH(+) adopts a previous unreported conformation in which both amino NHs act as H-bond donors. The correlated red shifts in the NH donor and C═O acceptor components of the NH···O═C linkage to the acid group are unambiguously assigned in the double H-bonded conformer.

Cerium Oxyhydroxide Clusters: Formation, Structure and Reactivity

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [Ce(x)O(y)] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [Ce(x)O(y)(OH)(z)](-) for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x = 1 to a value slightly greater than 1 for x >or= 8. The Ce(3) and Ce(6) species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and 4. Conversely, the x = 6 cluster is significantly less hydroxylated than its x = 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce(3) oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce(6) oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce(13), which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.

Addition of H2O and O2 to Acetone and Dimethylsulfoxide Ligated Uranyl(V) Dioxocations

Gas-phase complexes of the formula [UO2(lig)]+ (lig = acetone (aco) or dimethylsulfoxide (dmso)) were generated by electrospray ionization (ESI) and studied by tandem ion-trap mass spectrometry to determine the general effect of ligand charge donation on the reactivity of UO2(+) with respect to water and dioxygen. The original hypothesis that addition of O2 is enhanced by strong sigma-donor ligands bound to UO2(+) is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H2O from [UO2(dmso)(H2O)(O2)]+, whereas both H2O and O2 are eliminated from the corresponding [UO2(aco)(H2O)(O2)]+ species. Ligand-addition reaction rates were investigated by monitoring precursor and product ion intensities as a function of ion storage time in the ion-trap mass spectrometer: these experiments suggest that the association of dioxygen to the UO2(+) complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H2O is favored for the analogous complex ion that contains an aco ligand. Experimental rate measurements are supported by density function theory calculations of relative energies, which show stronger bonds between UO2(+) and O2 when dmso is the coordinating ligand, whereas bonds to H2O are stronger for the aco complex.

IRMPD Spectroscopy of Anionic Group II Metal Nitrate Cluster Ions

Anionic group II metal nitrate clusters of the formula [M2(NO3)5]−, where M2 = Mg2, MgCa, Ca2, and Sr2, are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate (“mono-bridging”), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands (“tri-bridging”). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of [Mg2(NO3)5]− and [MgCa(NO3)5]−. However, the profile of the low-frequency ν3 band for the Mg2 complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca2 and Sr2 complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.

Investigations of acidity and nucleophilicity of diphenyldithiophosphinate ligands using theory and gas-phase dissociation reactions

Diphenyldithiophosphinate (DTP) ligands modified with electron-withdrawing trifluoromethyl (TFM) substitutents are of high interest because they have demonstrated potential for exceptional separation of Am (3+) from lanthanide (3+) cations. Specifically, the bis( ortho-TFM) (L 1 (-)) and ( ortho-TFM)( meta-TFM) (L 2 (-)) derivatives have shown excellent separation selectivity, while the bis( meta-TFM) (L 3 (-)) and unmodified DTP (L u (-)) did not. Factors responsible for selective coordination have been investigated using density functional theory (DFT) calculations in concert with competitive dissociation reactions in the gas phase. To evaluate the role of (DTP + H) acidity, density functional calculations were used to predict p K a values of the free acids (HL n ), which followed the trend of HL 3 < HL 2 < HL 1 < HL u. The order of p K a for the TFM-modified (DTP+H) acids was opposite of what would be expected based on the e (-)-withdrawing effects of the TFM group, suggesting that secondary factors influence the p K a and nucleophilicity. The relative nucleophilicities of the DTP anions were evaluated by forming metal-mixed ligand complexes in a trapped ion mass spectrometer and then fragmenting them using competitive collision induced dissociation. On the basis of these experiments, the unmodified L u (-) anion was the strongest nucleophile. Comparing the TFM derivatives, the bis( ortho-TFM) derivative L 1 (-) was found to be the strongest nucleophile, while the bis( meta-TFM) L 3 (-) was the weakest, a trend consistent with the p K a calculations. DFT modeling of the Na (+) complexes suggested that the elevated cation affinity of the L 1 (-) and L 2 (-) anions was due to donation of electron density from fluorine atoms to the metal center, which was occurring in rotational conformers where the TFM moiety was proximate to the Na (+)-dithiophosphinate group. Competitive dissociation experiments were performed with the dithiophosphinate anions complexed with europium nitrate species; ionic dissociation of these complexes always generated the TFM-modified dithiophosphinate anions as the product ion, showing again that the unmodified L u (-) was the strongest nucleophile. The Eu(III) nitrate complexes also underwent redox elimination of radical ligands; the tendency of the ligands to undergo oxidation and be eliminated as neutral radicals followed the same trend as the nucleophilicities for Na (+), viz. L 3 (-) < L 2 (-) < L 1 (-) < L u (-).

Spectroscopic evidence for mobilization of amide position protons during CID of model peptide ions

AbstractInfrared multiple photon dissociation (IRMPD) spectroscopy was used to study formation of b2+ from nicotinyl-glycine-glycine-methyl ester (NicGGOMe). IRMPD shows that NicGGOMe is protonated at the pyridine ring of the nicotinyl group, and more importantly, that b2+ from NicGGOMe is not protonated at the oxazolone ring, as would be expected if the species were generated on the conventional bn+ /yn+ oxazolone pathway, but at the pyridine ring instead. IRMPD data support a hypothesis that formation of b2+ from NicGGOMe involves mobilization and transfer of an amide position proton during the fragmentation reaction.

Infrared Spectroscopy of OH··CH3OH: Hydrogen-Bonded Intermediate Along the Hydrogen Abstraction Reaction Path.

Substantial non-Arrhenius behavior has been previously observed in the low temperature reaction between the hydroxyl radical and methanol. This behavior can be rationalized assuming the stabilization of an association adduct in the entrance channel of the reaction, from which barrier penetration via quantum mechanical tunneling produces the CH3O radical and H2O. Helium nanodroplet isolation and a serial pick-up technique are used to stabilize the hydrogen bonded prereactive OH··CH3OH complex. Mass spectrometry and infrared spectroscopy are used to confirm its production and probe the OH stretch vibrations. Stark spectroscopy reveals the magnitude of the permanent electric dipole moment, which is compared to ab initio calculations that account for wide-amplitude motion in the complex. The vibrationally averaged structure has Cs symmetry with the OH moiety hydrogen bonded to the hydroxyl group of methanol. Nevertheless, the zero-point level of the complex exhibits a wave function significantly delocalized over a bending coordinate leading to the transition state of the CH3O producing reaction.

Spectroscopic investigation of H atom transfer in a gas-phase dissociation reaction: McLafferty rearrangement of model gas-phase peptide ions

Wavelength-selective infrared multiple-photon photodissociation (WS-IRMPD) was used to study isotopically-labeled ions generated by McLafferty rearrangement of nicotinyl-glycine-tert-butyl ester and betaine-glycine-tert-butyl ester. The tert-butyl esters were incubated in a mixture of D(2)O and CH(3)OD to induce solution-phase hydrogen-deuterium exchange and then converted to gas-phase ions using electrospray ionization. McLafferty rearrangement was used to generate the free-acid forms of the respective model peptides through transfer of an H atom and elimination of butene. The specific aim was to use vibrational spectra generated by WS-IRMPD to determine whether the H atom remains at the acid group, or migrates to one or more of the other exchangeable sites. Comparison of the IRMPD results in the region from 1200-1900 cm(-1) to theoretical spectra for different isotopically-labeled isomers clearly shows that the H atom is situated at the C-terminal acid group and migration to amide positions is negligible on the time scale of the experiment. The results of this study suggest that use of the McLafferty rearrangement for peptide esters could be an effective approach for generation of H-atom isotope tracers, in situ, for subsequent investigation of intramolecular proton migration during peptide fragmentation studies.