Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation

The gas phase structures of cationized histidine (His), including complexes with Li(+), Na(+), K(+), Rb(+), and Cs(+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) (Li(+), Na(+), and K(+) complexes) and B3LYP/HW*/6-311+G(d,p) (Rb(+) and Cs(+) complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li(+)(His) and Na(+)(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N(α),N(1)], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K(+)(His), Rb(+)(His), and Cs(+)(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li(+)(His) and Na(+)(His). For these complexes, the bidentate [CO,N(1)] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.     

IRMPD Action Spectroscopy of Alkali Metal Cation–Cytosine Complexes: Effects of Alkali Metal Cation Size on Gas Phase Conformation

The gas-phase structures of alkali metal cation-cytosine complexes generated by electrospray ionization are probed via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical calculations. IRMPD action spectra of five alkali metal cation–cytosine complexes exhibit both similar and distinctive spectral features over the range of ~1000–1900 cm^-1. The IRMPD spectra of the Li⁺(cytosine), Na⁺(cytosine), and K⁺(cytosine) complexes are relatively simple but exhibit changes in the shape and shifts in the positions of several bands that correlate with the size of the alkali metal cation. The IRMPD spectra of the Rb⁺(cytosine) and Cs⁺(cytosine) complexes are much richer as distinctive new IR bands are observed, and the positions of several bands continue to shift in relation to the size of the metal cation. The measured IRMPD spectra are compared to linear IR spectra of stable low-energy tautomeric conformations calculated at the B3LYP/def2-TZVPPD level of theory to identify the conformations accessed in the experiments. These comparisons suggest that the evolution in the features in the IRMPD action spectra with the size of the metal cation, and the appearance of new bands for the larger metal cations, are the result of the variations in the intensities at which these complexes can be generated and the strength of the alkali metal cation-cytosine binding interaction, not the presence of multiple tautomeric conformations. Only a single tautomeric conformation is accessed for all five alkali metal cation–cytosine complexes, where the alkali metal cation binds to the O2 and N3 atoms of the canonical amino-oxo tautomer of cytosine, M⁺(C₁).

Infrared multiphoton dissociation spectroscopy of cationized serine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li+(Ser) and Na+(Ser) are similar and relatively simple, whereas Cs+(Ser) includes distinctive new IR bands, and K+(Ser) and Rb+(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Li+ and Na+, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1[N,CO,OH]. For the cesiated complex, a band corresponding to a zwitterionic structure, ZW[CO2-], is clearly visible. K+(Ser) and Rb+(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3[COOH], in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally.     

Encapsulation of metal cations by the PhePhe ligand: a cation-π ion cage

Structures and binding thermochemistry are investigated for protonated PhePhe and for complexes of PhePhe with the alkaline-earth ions Ba(2+) and Ca(2+), the alkali-metal ions Li(+), Na(+), K(+), and Cs(+), and the transition-metal ion Ag(+). The two neighboring aromatic side chains open the possibility of a novel encapsulation motif of the metal ion in a double cation-π configuration, which is found to be realized for the alkaline-earth complexes and, in a variant form, for the Ag(+) complex. Experimentally, complexes are formed by electrospray ionization, trapped in an FT-ICR mass spectrometer, and characterized by infrared multiple photon dissociation (IRMPD) spectroscopy using the free electron laser FELIX. Interpretation is assisted by thermochemical and IR spectral calculations using density functional theory (DFT). The IRMPD spectrum of protonated PhePhe is reproduced with good fidelity by the calculated spectrum of the most stable conformation, although the additional presence of the secondmost stable conformation is not excluded. All metal-ion complexes have charge-solvated binding modes, with zwitterion (salt bridge) forms being much less stable. The amide oxygen always coordinates to the metal ion, as well as at least one phenyl ring (cation-π interaction). At least one additional chelation site is always occupied, which may be either the amino nitrogen or the carboxy carbonyl oxygen. The alkaline-earth complexes prefer a highly compact caged structure with both phenyl rings providing cation-π stabilization in a "sandwich" configuration (OORR chelation). The alkali-metal complexes prefer open-cage structures with only one cation-π interaction, except perhaps Cs(+). The Ag(+) complex shows a unique preference for the closed-cage amino-bound NORR structure. Ligand-driven perturbations of normal-mode frequencies are generally found to correlate linearly with metal-ion binding energy.     

Experimental and theoretical investigation of alkali metal cation interactions with hydroxyl side-chain amino acids

Absolute bond dissociation energies of serine (Ser) and threonine (Thr) to alkali metal cations are determined experimentally by threshold collision-induced dissociation of M+AA complexes, where M+=Li+, Na+, and K+ and AA=Ser and Thr, with xenon in a guided ion beam tandem mass spectrometer. Experimental results show that the binding energies of both amino acids to the alkali metal cations are very similar to one another and follow the order of Li+>Na+>K+. Quantum chemical calculations at three different levels, B3LYP, B3P86, and MP2(full), using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d,p) level show good agreement with the experimental bond energies. Theoretical calculations show that all M+AA complexes have charge-solvated structures (nonzwitterionic) with [CO, N, O] tridentate coordination.     

Infrared multiphoton dissociation spectroscopy of cationized threonine: effects of alkali-metal cation size on gas-phase conformation

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.     

A simple model for metal cation-phosphate interactions in nucleic acids in the gas phase: Alkali metal cations and trimethyl phosphate

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of complexes of alkali metal cations to trimethyl phosphate, TMP. Endothermic loss of the intact TMP ligand is the only dissociation pathway observed for all complexes. Theoretical calculations at the B3LYP/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of neutral TMP and the M+(TMP) complexes. Theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) level using the B3LYP/6-31G* optimized geometries. The agreement between theory and experiment is reasonably good for all complexes except Li+(TMP). The absolute M+-(TMP) BDEs are found to decrease monotonically as the size of the alkali metal cation increases. No activated dissociation was observed for alkali metal cation binding to TMP. The binding of alkali metal cations to TMP is compared with that to acetone and methanol.     

Calix[4]quinones derived from double calix[4]arenes: synthesis, complexation, and electrochemical properties toward alkali metal ions

Bis(calix[4]diquinones) 1 and 2 and double calix[4]diquinone 3 have been synthesized from their corresponding double calix[4]arenes 4, 5, and 6, respectively. Compounds 4-6 have been prepared from one-pot and stepwise syntheses under high pressure. Complexation studies of ligands 1-3 with alkali metal ions such as Li+, Na+, K+, and Cs+ were carried out by 1H NMR titrations. Receptors 1 can selectively form 1:1 complexes with Na+. Ligand 2 prefers to form 1:1 complexes with K+ and Cs+. Receptor 3 retained the cone conformation of the calix[4]arene unit upon binding K+ but changed the conformation when complexing Li+ and Na+. Electrochemical studies using cyclic voltammetry and square wave voltammetry showed significant changing of voltammograms of 2 and 3 in the presence of alkali metal ions. Receptor 3 showed the electrochemically switched binding property toward Na+ and K+.     

The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues

The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.     

Infrared spectroscopy of cationized lysine and epsilon-N-methyllysine in the gas phase: effects of alkali-metal ion size and proton affinity on zwitterion stability

The gas-phase structures of protonated and alkali-metal-cationized lysine (Lys) and epsilon-N-methyllysine (Lys(Me)) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. IRMPD spectra of Lys.Li(+) and Lys.Na(+) are similar, but the spectrum for Lys.K(+) is different, indicating that the structure of lysine in these complexes depends on the metal ion size. The carbonyl stretch of a carboxylic acid group is clearly observed in each of these spectra, indicating that lysine is nonzwitterionic in these complexes. A detailed comparison of these spectra to those calculated for candidate low-energy structures indicates that the bonding motif for the metal ion changes from tricoordinated for Li and Na to dicoordinated for K, clearly revealing the increased importance of hydrogen-bonding relative to metal ion solvation with increasing metal ion size. Spectra for Lys(Me).M(+) show that Lys(Me), an analogue of lysine whose side chain contains a secondary amine, is nonzwitterionic with Li and zwitterionic with K and both forms are present for Na. The proton affinity of Lys(Me) is 16 kJ/mol higher than that of Lys; the higher proton affinity of a secondary amine can result in its preferential protonation and stabilization of the zwitterionic form.     

Dissociation of alkaliated alanine in the gas phase: the role of the metal cation

The dissociation of prototypical metal-cationized amino acid complexes, namely, alkaliated alanine ([Ala+M]+, M+ = Li+, Na+, K+), was studied by energy-resolved tandem mass spectrometry with an ion-trap mass analyzer and by density functional theory. Dissociation leads to formation of fragment ions arising from the loss of small neutrals, such as H2O, CO, NH3, (CO+NH3), and the formation of Na+/K+. The order of appearance threshold voltages for different dissociation pathways determined experimentally is consistent with the order of critical energies (energy barriers) obtained theoretically, and this provides the necessary confidence in both experimental and theoretical results. Although not explicitly involved in the reaction, the alkali metal cation plays novel and important roles in the dissociation of alkaliated alanine. The metal cation not only catalyzes the dissociation (via the formation of loosely bound ion-molecule complexes and by stabilizing the more polar intermediates and transition structures), but also affects the dissociation mechanisms, as the cation can alter the shape of the potential energy surfaces. This compression/expansion of the potential energy surface as a function of the alkali metal cation is discussed in detail, and how this affects the competitive loss of H2O versus CO/(CO+NH3) from [Ala+M]+ is illustrated. The present study provides new insights into the origin of the competition between various dissociation channels of alkaliated amino acid complexes.     

Cation-pi interactions with a model for the side chain of tryptophan: structures and absolute binding energies of alkali metal cation-indole complexes

Threshold collision-induced dissociation techniques are employed to determine bond dissociation energies (BDEs) of mono- and bis-complexes of alkali metal cations, Li+, Na+, K+, Rb+, and Cs+, with indole, C8H7N. The primary and lowest energy dissociation pathway in all cases is endothermic loss of an intact indole ligand. Sequential loss of a second indole ligand is observed at elevated energies for the bis-complexes. Density functional theory calculations at the B3LYP/6-31G level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes. Theoretical BDEs are determined from single point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31G* geometries. The agreement between theory and experiment is very good for all complexes except Li+ (C8H7N), where theory underestimates the strength of the binding. The trends in the BDEs of these alkali metal cation-indole complexes are compared with the analogous benzene and naphthalene complexes to examine the influence of the extended pi network and heteroatom on the strength of the cation-pi interaction. The Na+ and K+ binding affinities of benzene, phenol, and indole are also compared to those of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan to elucidate the factors that contribute to the binding in complexes to the aromatic amino acids. The nature of the binding and trends in the BDEs of cation-pi complexes between alkali metal cations and benzene, phenol, and indole are examined to help understand nature's preference for engaging tryptophan over phenylalanine and tyrosine in cation-pi interactions in biological systems.     

Influence of metal cations on the intramolecular hydrogen-bonding network and pKa in phosphorylated compounds

The binding of the most common metal cations of cytoplasm (Li+, Na+, K+, Mg2+ and Ca2+) to a model molecule having an intramolecular hydrogen-bonding network, myo-inositol-2-monophosphate, was studied using first principles. A strong correlation between the conformation of metal inositol phosphate complexes with the type of metal cation, degree of deprotonation state, and the surrounding environment has been observed. On the basis of the hydrogen-bonding network analysis of the cation-phosphate complexes (Mn+-Ins(2)P1), the alkali cations show little effect on the conformational preference while the conformational preference for the binding of the alkaline earth cations is pH-dependent and solvent-dependent. For example, these calculations predict that Mg2+-Ins(2)P1(0) and Mg2+-Ins(2)P1(2-) favor the 1a/5e form while Mg2+-Ins(2)P1(1-) favors the 5a/1e conformation. The Ca2+-Ins(2)P1(2-) complex prefers the 1a/5e conformation in the gas phase and in a nonpolar protein environment, but inverts to the 5a/1e conformation upon entering the polar aqueous phase. The binding affinities of the cations and the pK(a) values for the cation-phosphate complexes are derived from thermodynamical analysis.     

Effects of alkaline earth metal ion complexation on amino acid zwitterion stability: results from infrared action spectroscopy

The structures of isolated alkaline earth metal cationized amino acids are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theory. These results indicate that arginine, glutamine, proline, serine, and valine all adopt zwitterionic structures when complexed with divalent barium. The IRMPD spectra for these ions exhibit bands assigned to carboxylate stretching modes, spectral signatures for zwitterionic amino acids, and lack bands attributable to the carbonyl stretch of a carboxylic acid functional group. Structural and spectral assignments are strengthened through comparisons with absorbance spectra calculated for low-energy structures and the IRMPD spectra of analogous ions containing monovalent alkali metals. Many bands are significantly red-shifted from the corresponding bands for amino acids complexed with monovalent metal ions, owing to increased charge transfer to divalent metal ions. The IRMPD spectra of arginine complexed with divalent strontium and barium are very similar and indicate that arginine adopts a zwitterionic form in both ions. Calculations indicate that nonzwitterionic forms of arginine are lowest in free energy in complexes with smaller alkaline earth metal cations and that zwitterionic forms are preferentially stabilized with increasing metal ion size. B3LYP and MP2 calculations indicate that zwitterionic forms of arginine are lowest in free energy for M = Ca, Sr, and Ba.     

Structures of aliphatic amino acid proton-bound dimers by infrared multiple photon dissociation spectroscopy in the 700-2,000 cm(-1) region

Structural aspects of proton-bound dimers composed of amino acids with aliphatic side chains are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. Features in the IRMPD spectra in the 700-2,000 cm-1 range are due primarily to C=O stretching, NH2 bending, and COH bending. It was possible to distinguish between isomeric structures by comparing the experimental IRMPD spectra and those predicted using B3LYP/6-31+G(d,p). It was possible, based on the calculations and IRMPD spectra, to assign the experimental spectrum of the glycine proton-bound dimer to a structure which was slightly different from that assigned by previous spectroscopic investigations and in agreement with recent thermochemical studies. Since all proton-bound dimers studied here, composed of the different amino acids, have very similar spectra, it is expected that they also have very similar lowest-energy structures including the mixed alanine/glycine proton-bound dimer. In fact, the spectra are so similar that it would be very challenging to distinguish, for example, the glycine proton-bound dimer from the alanine or valine proton-bound dimers in the 700-2,000 cm-1 range. According to the calculated IR spectra it is shown that in the approximately 2,000-3,200 cm-1 range differentiating between different structures as well as different proton-bound dimers may be possible. This is due mainly to differences in the asymmetric stretch of the binding proton which is predicted to occur in this region.     

Modeling metal cation-phosphate interactions in nucleic acids in the gas phase via alkali metal cation-triethyl phosphate complexes

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of complexes of alkali metal cations, Na+, K+, Rb+, and Cs+, to triethyl phosphate (TEP). The primary and lowest energy dissociation pathway in all cases is the endothermic loss of the neutral TEP ligand. Theoretical electronic structure calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level of theory are used to determine the structures, molecular parameters, and theoretical estimates for the BDEs of these complexes. For the complexes to Rb+ and Cs+, theoretical calculations were performed using hybrid basis sets in which the effective core potentials and valence basis sets of Hay and Wadt were used to describe the alkali metal cation, while the standard basis sets were used for all other atoms. The agreement between theory and experiment is excellent for the complexes to Na+ and K+ and is somewhat less satisfactory for the complexes to the heavier alkali metal cations, Rb+ and Cs+, where effective core potentials were used to describe the cation. The trends in the binding energies are examined. The binding of alkali metal cations to triethyl phosphate is compared with that to trimethylphosphate.     

Self-assembled organometallic [12]metallacrown-3 complexes

The reaction of [(arene)RuCl2]2 (arene = C6H6, cymene, C6H3Et3, or C6Me6) or [Cp*RhCl2]2 with 3-hydroxy-2-pyridone in the presence of Cs2CO3 gives trinuclear metallamacrocyclic complexes. The self-assembly process was shown to be completely diastereoselective, and a racemic mixture of complexes with M(R)M(R)M(R) or MsMsMs (M=Ru, Rh) configuration was obtained. Plausible mononuclear intermediates of the formula [(arene)RuCl(C5H4NO2)] (arene = cymene, C6Me6) have been isolated and characterized. A structurally related trimer was synthesized by using [(cymene)RuCl2]2 and 3-acetamido-2-pyridone instead of 3-hydroxy-2-pyridone. The macrocycles were shown to be highly potent ionophores for Na+ and/or Li+ with negligible affinities for the larger cation K+. The selectivities of the receptors depend on the pi-ligand present: whereas the (C6H6)Ru- and (cymene)Ru complexes bind both Li+ and Na+, the (C6Me6)Ru-, (C6H3Et3)Ru-, and Cp*Rh complexes bind exclusively Li+. For all receptors, the presence of alkali metal ions can be detected electrochemically: the peak potential is shifted by > 300 mV toward anionic potential upon binding. This behavior was utilized to detect Li+ and Na+ colorimetrically. Single crystal X-ray analyses have been carried out on eight complexes, four of which are bound to an alkali metal halide ion pair. Structural parameters, which affect the affinity and selectivity are discussed. A computational study on [[MX][12]crown-3] complexes (M =Li, Na; X=Cl, Br, I) was performed in order to compare relevant bond lengths and angles of the energy-minimized structures with those of the organometallic receptors.     

Structure and energetics of poly(ethylene glycol) cationized by Li(+), Na(+), K(+) and Cs(+): a first-principles study

Density functional theoretical methods, including several basis sets and two functional, were used to collect information on the structure and energetic parameters of poly(ethylene glycol) (PEG), also referred to as poly(ethylene oxide) (PEO), coordinated by alkali metal ions. The oligomer chain is found to form a spiral around the alkali cation, which grows to roughly two helical turns when the oligomer size increases to about the decamer for each alkali ion. Above this size, the additional monomer units do not build the spiral further for Li(+) and Na(+); instead, they form less organized segments outside or next to the initial spiral. The distance of the first layer of co-ordinating O atoms from the alkali cation is 1.9-2.15 ? for Li(+), 2.3-2.5 ? for Na(+), 2.75-3.2 ? for K(+) and 3.5-3.8 ? for Cs(+) complexes. The number of O atoms in the innermost shell is five, six, seven and eleven for Li(+), Na(+), K(+) and Cs(+). The collision cross sections with He increase linearly with the oligomer to a very good approximation. No sign of leaning towards the 2/3 power dependence characterizing spherical particles is observed. The binding energy of the cation to the oligomer increases up to polymerization degree of about 10, where it levels off for each alkali-metal ion, indicating that this is approximately the limit of the oligomer size that can be influenced by the alkali cation. The binding energy-degree of polymerization curves are remarkably parallel for the four cations. The limiting binding energy at large polymerization degrees is about 544 kJ mol(-1), 460 kJ mol(-1), 356 kJ mol(-1) and 314 kJ mol(-1) for Li, Na, K and Cs, respectively. The geometrical features are compared with the X-ray and neutron diffraction data on crystalline and amorphous phases of conducting polymers formed by alkali-metal salts and PEG. The implications of the observations concerning collision cross sections and binding energies to ion mobility spectroscopy and mass spectrometry are discussed.     

Unexpected structural diversity in alkali metal azide-crown ether complexes: syntheses, X-ray structures, and quantum-chemical calculations

A series of alkali metal azide-crown ether complexes, [Li([12]crown-4)(N3)], [Na([15]crown-5)(N3)], [Na([15]crown-5)(H2O)2]N3, [K([18]crown-6)(N3)(H2O)], [Rb([18]crown-6)(N3)(H2O)], [Cs([18]crown-6)(N3)]2, and [Cs([18]crown-6)(N3)(H2O)(MeOH)], has been synthesised. In most cases, single crystals were obtained, which allowed X-ray crystal structures to be derived. The structures obtained have been compared with molecular structures computed by density functional theory (DFT) calculations. This has allowed the effects of the crystal lattice on the structures to be investigated. Also, a study of the M-N(terminal) metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (N(terminal)) in these complexes has allowed the nature of the metal-azide bond to be probed in each case. The bonding in these complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre and minimising ligand-ligand repulsions. The structures of the crown ether complexes determined in this work show the subtle interplay of such factors. The significant role of hydrogen bonding is also demonstrated, most clearly in the structures of the K and Rb dimers, but also in the chain structure of the hydrated Cs complex.     

Theoretical and gas-phase studies of specific cationized purine base quartet

Guanine tetraplexes are biological non-covalent systems stabilized by alkali cations. Thus, self-clustering of guanine, xanthine and hypoxanthine with alkali cations (Na(+), K(+) and Li(+)) is investigated by electrospray ionization mass spectrometry (ESI-MS) in order to provide new insights into G-quartets, hydrogen-bonded complexes. ESI assays displayed magic numbers of tetramer adducts with Na(+), Li(+) and K(+), not only for guanine, but also for xanthine bases. The optimized structures of guanine and xanthine quartets have been determined by B3LYP hybrid density functional theory calculations. Complexes of metal ions with quartets are classified into different structure types. The optimized structures obtained for each quartet explain the gas-phase results. The gas-phase binding sequence between the monovalent cations and the xanthine quartet follows the order Li(+) > Na(+) > K(+), which is consistent with that obtained for the guanine quartet in the literature. The smallest stabilization energy of K(+) and its position versus the other alkali metal ions in guanine and xanthine quartets is consistent with the fact that the potassium cation can be located between two guanine or xanthine quartets, for providing a [gua(or (xan))(8)+K](+) octamer adduct. Even if an abundant octamer adduct with K(+) for xanthine was detected by ESI-MS, it was not the case for guanine.