Udo H. Verkerk

Electrospray: from ions in solution to ions in the gas phase, what we know now.

There is an advantage for users of electrospray and nanospray mass spectrometry to have an understanding of the processes involved in the conversion of the ions present in the solution to ions in the gas phase. The following processes are considered: Creation of charge droplets at the capillary tip; Electrical potentials required and possibility of gas discharges; Evolution of charged droplets, due to solvent evaporation and Coulomb explosions, to very small droplets that are the precursors of the gas phase ions; Production of gas phase ions from these droplets via the Ion Evaporation and Charge residue models; Analytical uses of ESIMS of small ions, qualitative and quantitative analysis; Effects of the ESI mechanism on the analysis of proteins and protein complexes; Determination of stability constants of protein complexes; Role of additives such as ammonium acetate on the observed mass spectra.

Conformation Switching in Gas-Phase Complexes of Histidine with Alkaline Earth Ions

Infrared multiple photon dissociation spectroscopy of gas-phase doubly charged alkaline earth complexes of histidine reveals a transition from dominance of the zwitterion (salt bridge, SB) conformation with Ba2+ to substantial presence of the canonical (charge-solvated, CS) conformation with Ca2+. This result is a clear illustration of the importance of metal-ion size in governing the delicate balance between these two modes of complexation of gas-phase amino acids. The two conformational motifs are clearly distinguished by characteristic spectral features, confirmed by density functional theory simulated IR spectra of the low-energy conformers. As a further illustration of histidine complexation possibilities, the spectrum of the Na+His complex shows purely CS character and emphasizes the greater tendency toward SB character induced by the higher charge in the alkaline earth complexes. Calculation of the complete series of alkaline earth/histidine complexes confirms the increasing stability of the SB conformations relative to CS with increasing metal ion size, as well as showing that among SB conformations the most highly chelated conformation (SB3) is favored for small metals, whereas the most extended conformation (SB1) is favored for large metals. A decomposition of the binding thermochemistry shows that these thermochemical trends versus metal-ion size are due to differences in electrostatic binding energies, with relatively little contribution from the deformation and rearrangement energy costs of distorting the ligand framework.

Structure of the Observable Histidine Radical Cation in the Gas Phase: A Captodative α-Radical Ion†

Protein-based radicals play crucial roles in some of the greatest biosynthetic challenges in nature, including photosynthesis and substrate oxidation. Radical centers have been located on aromatic and sulfur-containing amino acid residues, as well as glycine residues. Invariably these charged or neutral radical species are generated through involvement of an adjacent metal cofactor. The positions of charge and spin in the radical cations are paramount for reactivity modulation and proton-coupled electron transfer, but obtaining structural details is difficult even for the simplest models. 2] Experiments in vacuo permit the investigation of intrinsic properties of radical cations in the absence of a reactivity-modulating environment. Radical cations of amino acids and peptides have been produced in vacuo by one-electron transfer in collision-induced dissociations (CIDs) of a ternary complex system comprising copper(II), an auxiliary ligand, and the amino acid or peptide. Such ternary complexes are efficiently generated by electrospray ionization, and probed downstream by using mass spectrometry (MS). Under appropriate conditions, CID of the complex yields the radical cation of the amino acid or peptide that can be isolated and trapped for spectroscopic interrogation. Herein, we report the first infrared multiple photon dissociation (IRMPD) spectroscopic experiments on a prototypical amino acid radical cation, HisC, and its ternary complex ion. In a recent article, Ke et al. showed that, by judicious choice of the auxiliary ligand, HisC of different stabilities are formed through CID of the ternary complex ion. In particular, the use of 2,2’:6’,2’’-terpyridine (tpy) as the ligand leads primarily to a HisC that is stable on the MS timescale and can be isolated and fragmented at a subsequent MS stage; by contrast, employing acetone as the ligand results in a metastable HisC and only its fragment ions are observed. Furthermore, the former, relatively stable HisC fragments by losing a water molecule to give [b1-H]C + and then CO to give [a1-H]C , whereas the latter, metastable HisC dissociates spontaneously by losing first CO2 to give the 4-ethaniminoimidazole radical cation, which then loses methanimine to give the 4-methyleneimidazole radical cation. Density functional theory (DFT) calculations at the (unrestricted) UB3LYP/6-311 + + G(d,p) level of theory predicted five low-energy HisC structures. Scheme 1 shows these structures with additional, new information on the barriers against their interconversions (see Figures S2 and S3 in the Supporting Information for details). Ke et al. postulated that the stable and metastable HisC are His5 (the structure at the global minimum) and His2, respectively. His5 is a captodative aradical ion that differs from the canonical His1 structure in having the a-CH hydrogen migrated to the imino nitrogen of the imidazole ring; His2 is best described as a 4-ethaniminoimidazole radical cation solvated by CO2. His2–His5 are all unconventional structures, and experimental verification of the HisC structure is highly desirable for confirmation of the key roles played by spin and charge delocalization in HisC stabilization. Figure 1 compares the experimental IRMPD spectrum collected for HisC with the DFT-predicted IR spectra of His1–His5. It is apparent that only one predicted IR spectrum, that of His5, resembles the measured IRMPD spectrum. In particular, His5 is the only isomer predicted to exhibit two bands, 1596 and 1653 cm , which are assigned as NH2 scissoring and C=O stretching, respectively, that match the 1606 and 1666 cm 1 bands in the IRMPD spectrum. The lack of a strong band at around 1780–1790 cm 1 in the IRMPD spectrum rules out the presence of a significant fraction of His3 and His4. Similarly, His1 can be ruled out by the presence of the doublet, 1606 and 1666 cm , and the absence of spectroscopic details in the region of 1077– 1320 cm . His2 can be eliminated by the absence of peaks at around 810–820 cm 1 and by the low endothermicity against loss of the solvating CO2 (5 kcalmol ). We interpret the excellent match between the experimental IRMPD spectrum and the predicted IR spectrum of His5 to indicate that His5 is the only abundant species present. This degree of selectivity is feasible as His5 is positioned at the bottom of a deep well on the potential-energy surface of HisC. The barriers against His5 converting into the other His isomers and dissociating into [b1-H]C + are high (Scheme 1), [*] Dr. J. Zhao, Dr. C.-K. Siu, Y. Ke, Dr. U. H. Verkerk, Prof. A. C. Hopkinson, Prof. K. W. M. Siu Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street Toronto, ON M3J 1P3 (Canada) E-mail: kwmsiu@yorku.ca

Metal Ion Complexes with HisGly: Comparison with PhePhe and PheGly

Gas-phase complexes of five metal ions with the dipeptide HisGly have been characterized by DFT computations and by infrared multiple photon dissociation spectroscopy (IRMPD) using the free electron laser FELIX. Fine agreement is found in all five cases between the predicted IR spectral features of the lowest energy structures and the observed IRMPD spectra in the diagnostic region 1500-1800 cm(-1), and the agreement is largely satisfactory at longer wavelengths from 1000 to 1500 cm(-1). Weak-binding metal ions (K(+), Ba(2+), and Ca(2+)) predominantly adopt the charge-solvated (CS) mode of chelation involving both carbonyl oxygens, an imidazole nitrogen of the histidine side chain, and possibly the amino nitrogen. Complexes with Mg(2+) and Ni(2+) are found to adopt iminol (Im) binding, involving the deprotonated amide nitrogen, with tetradentate chelation. This tetradentate coordination of Ni(II) is the preferred binding mode in the gas phase, against the expectation under condensed-phase conditions that such binding would be sterically unfavorable and overshadowed by other outcomes such as metal ion hydration and formation of dimeric complexes. The HisGly results are compared with corresponding results for the PheAla, PheGly, and PhePhe ligands, and parallel behavior is seen for the dipeptides with N-terminal Phe versus His residues. An exception is the different chelation pattern determined for PhePhe versus HisGly, reflecting the intercalation-type cation binding pocket of the PhePhe ligand. The complexes group into three well-defined spectroscopic patterns: nickel and magnesium, calcium and barium, and potassium. Factors leading to differentiation of these distinct spectroscopic categories are (1) differing propensities for choosing the iminol binding pattern, and (2) single versus double charge on the metal center. Nickel and magnesium ions show similar gas-phase binding behavior, contrasting with their quite different patterns of peptide interaction in condensed phases.

Enhanced performance of potassium CHEMFETs by optimization of a polysiloxane membrane

In a way to enhance performance of CHEMFETs based on chemically modified polysiloxane membrane matrix, the effect of the polarity of polysiloxane material on the membrane homogeneity in relation to the covalent binding of the ionophore was investigated. Potassium sensors with the membrane based on the new polysiloxane showed a response with a Nernstian slope in a whole investigated range of cation activity. A new polysiloxane as the membrane matrix improved its homogeneity and increased the yield of functioning CHEMFETs.

Binding Energies of the Silver Ion to Alcohols and Amides: A Theoretical and Experimental Study

The silver ion binding energies to alcohols (methanol, ethanol, n-propanol, i-propanol, and n-butanol) and to amides (acetamide, N-methylacetamide, N, N-dimethylacetamide, formamide, N-methylformamide, and N, N-dimethylformamide) have been calculated using density functional theory (DFT) and measured using the threshold collision-induced dissociation (TCID) method. For DFT, the combined basis sets of ECP28MWB for silver and 6-311++G(2df,2pd) for the other atoms were found to be optimal using a series of test calculations on Ag (+) binding to methanol and to formamide. In addition, the Ag (+) binding energies of all ligands were evaluated with nine functionals after full geometric optimizations. TCID binding energies were measured using a triple quadrupole mass spectrometer. Reasonable to good agreements were obtained between the calculated and experimental silver(I) binding energies. Ligation of Ag (+) to the alcohols was primarily via the oxygen, although n-propanol and n-butanol exhibited additional, bidentate coordination via the CH hydrogens. By contrast, silver(I) binding to the amides was all monodentate via the carbonyl oxygen. There appears to be strong correlations between the binding energies and the polarizabilities of the ligands.

Structure of the [M+H-H2O](+) Ion from Tetraglycine: A Revisit by Means of Density Functional Theory and Isotope Labeling

Collision-induced dissociations of protonated (18)O-labeled tetraglycines labeled separately at either the first or the second amide bond established that water loss from the backbone occurs from the N-terminal residue. Density functional theory at B3LYP/6-311++G(d,p) predicted that the low-energy [G(4) + H - H(2)O](+) product ion is an N(1)-protonated 3,5-dihydro-4H-imidazol-4-one. The ion at the lowest energy, III, is 24.8 kcal mol(-1) lower than the protonated oxazole structure, II, proposed by Bythell et al. (J. Phys. Chem A2010, 114, 5076-5082). In addition, structure III has a predicted IR spectrum that provides a better match with the published experimental IRMPD spectrum than that of structure II.

Multi-ion sensing device for horticultural application based upon chemical modification and special packaging of ISFETs

A multi-ion sensing device, based on ISFETs, is described for horticultural applications. The advantages of a new packaging technique based on back-side contacted ISFETs are outlined. The basic pH-sensitive ISFETs can be modified to, e.g., potassium-sensitive devices by using neutral carrier-based membranes and an intermediate hydrogel. Both membrane and hydrogel are chemically anchored to the ISFET surface and the ionophore is chemically bonded to the membrane. It is shown that these chemical modifications of the membrane system lead to an increased lifetime for the sensor.

Bond Dissociation Energies of Solvated Silver(I)—Amide Complexes: Competitive Threshold Collision-Induced Dissociations and Calculations

Using competitive threshold collision-induced dissociation (TCID) measurements, experimental bond dissociation energies have been evaluated for the water, methanol, and acetonitrile adducts of silver(I)-amide complexes. The influence of the solvent molecules on the binding energy of silver(I) to acetamide, N-methylacetamide, and N,N-dimethylacetamide was investigated. Experimental results show that solvents decrease the amide binding energy by 4-6 kcal mol(-1). Using density functional theory (DFT), binding energies were evaluated using nine functionals, after full geometry optimizations with the ECP28MWB basis set for silver and the 6-311++G(2df,2pd) basis set for the other atomic constituents of the ligands. In addition, calculations employing the DZVP basis set for Ag and DZVP2 for C, H, N, and O atoms at the B3LYP and MP2 levels of theory were used to investigate the influence of the basis set on the theoretical bond energies. A comparison of the experimental and theoretical silver(I)-ligand bond dissociation energies enables an assessment of the limitations in the basis sets and functionals in describing the energetics of the metal-solvent interaction and the metal-amide interaction. No single functional/basis set combination was found capable of predicting binding energies with a sufficiently high level of accuracy for the silver(I)-amide solvent complexes.

Infrared Multiple-Photon Dissociation Spectroscopy of Tripositive Ions: Lanthanum-Tryptophan Complexes

Collision-induced charge disproportionation limits the stability of triply charged metal ion complexes and has thus far prevented successful acquisition of their gas-phase IR spectra. This has curtailed our understanding of the structures of triply charged metal complexes in the gas phase and in biological environments. Herein we report the first gas-phase IR spectra of triply charged La(III) complexes with a derivative of tryptophan (N-acetyl tryptophan methyl ester), and an unusual dissociation product, a lanthanum amidate. These spectra are compared with those predicted using density functional theory. The best structures are those of the lowest energies that differ by details in the π-interaction between La(3+) and the indole rings. Other binding sites on the tryptophan derivative are the carbonyl oxygens. In the lanthanum amidate, La(3+) replaces an H(+) in the amide bond of the tryptophan derivative.