Free solution electrophoresis of homopolyelectrolytes

We investigate the behavior of single polyelectrolytes in multivalent salt solutions under the action of electric fields through computer simulations. The chain is unfolded in a strong electric field and aligned parallel to the field direction, and the chain size shows a sigmoidal transition. The unfolding electric field E* depends on the salt concentration and scales as V (-1/2) with V being the ellipsoidal volume occupied by the chain. The magnitude of the electrophoretic mobility of chain drastically increases during the unfolding. The fact that E* depends on the chain length provides a plausible mechanism to separate long charged homopolymers by size in free solution electrophoresis via the unfolding transition of globule polyelectrolytes condensed by multivalent salt.     

Femtosecond dynamics on 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole: solvent polarity in the excited-state proton transfer.

Detailed insights into the excited-state enol(N*)-keto(T*) intramolecular proton transfer (ESIPT) reaction in 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (HABT) have been investigated via steady-state and femtosecond fluorescence upconversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2'-hydroxyphenyl)benzothiazole (>2.9+/-0.3 x 10(13) s(-1)), HABT undergoes a relatively slow rate (approximately 5.4+/-0.5 x 10(11) s(-1)) of ESIPT. In polar aprotic solvents competitive rate of proton transfer and rate of solvent relaxation were resolved in the early dynamics. After reaching the solvation equilibrium in the normal excited state (N(eq)*), ESIPT takes place with an appreciable barrier. The results also show N(eq)*(enol)<-->T(eq)*(keto) equilibrium, which shifts toward N(eq)* as the solvent polarity increases. Temperature-dependent relaxation dynamics further resolved a solvent-induced barrier of 2.12 kcal mol(-1) for the forward reaction in CH(2)Cl(2). The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between N(eq)* and T(eq)*, while the dipolar vector for the enol form in the ground state (N) is in between that of N(eq)* and T(eq)*. Upon N-->N* Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the solvation relaxation process. Upon reaching equilibrium configurations N(eq)* and T(eq)*, forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between N(eq)* and T(eq)*.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n -butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N * ) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC * ) mesophase. The metal complexes with n -butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N * phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C * phase of the two ligands.     

The guanine cation radical: investigation of deprotonation states by ESR and DFT

This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.     

Novel chiral dimesogenic bidentate ligands and their Cu(II) and Pd(II) metal complexes

The synthesis and characterization of cholesterol-based dimesogenic bidentate ligands and their Cu(II) and Pd(II) metallomesogens are reported in detail. To understand structure-property relationships in these materials the terminal alkoxy chains and the central metal atom have been varied. Our studies reveal that chiral dimesogenic bidentate ligands with n-butyloxy chains exhibit smectic A (SmA), twist grain boundary and chiral nematic (N*) mesophases while substitution with either n -decyloxy or 3,7-dimethyloctyloxy chains also show a ferroelectrically switchable chiral smectic C (SmC*) mesophase. The metal complexes with n-butyloxy chains show only the SmA phase whereas higher chain length derivatives exhibit N* phase irrespective of the metal atom present. The ligands are thermally stable whereas their metal complexes, especially Pd(II) systems, seem to be heat sensitive. Spontaneous polarization, response time and tilt angle measurements have been carried out in the smectic C* phase of the two ligands.     

Photoinduced energy transfer coupled to charge separation in a Ru(II)-Ru(II)-acceptor triad

The bichromophoric system Ru-Ru(C)-PI ([(bpy)3Ru-Ph-Ru(dpb)(Metpy-PI)][PF6]3, where bpy is 2,2'-bipyridine, Hdpb is 1,3-di(2-pyridyl)-benzene, Metpy is 4'-methyl-2,2':6',2' '-terpyridine and PI is pyromellitimide) containing two Ru(II) polypyridyl chromophores with a N6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes Ru(C)-PI ([Ru(dpb)(Metpy-PI)][PF6]), Ru(C)-phi-PI ([Ru(dpb)(ttpy-PI)][PF6], ttpy is 4'-p-tolyl-2,2':6',2' '-terpyridine), Ru(C)-phi ([Ru(dpb)(ttpy)][PF6]), and Ru(C) ([Ru(dpb)(Metpy)][PF6]). Excitation of the Ru(C) unit in the dyads leads to oxidative quenching, forming the Ru(C)(III)-phi-PI*- and Ru(C)(III)-Pl.- charge-separated (CS) states with k(f)(ET) = 7.7 x 10(7) s(-1) (CH3CN, 298 K) in the tolyl-linked Ru(C)-phi-PI and k(f)(ET) = 4.4 x 10(9) s(-1) (CH2Cl2, 298 K) in the methylene-linked Ru(C)-PI. In the Ru-Ru(C)-PI triad, excitation of the Ru(C) chromophore leads to dynamics similar to those in the Ru(C)-PI dyad, generating the Ru(II)-Ru(C)(III)-PI*- CS state, whereas excitation of the Ru unit results in an initial energy transfer (k(EnT) = 4.7 x 10(11) s(-1)) to the cyclometalated Ru(C) unit. Subsequent electron transfer to the PI acceptor results in the formation of the same Ru(II)-Ru(C)(III)-PI*- CS state with k(f)(ET) = 5.6 x 10(9) s(-1) that undergoes rapid recombination with k(b)(ET) = 1 x 10(10) s(-1) (CH2Cl2, 298 K). The fate of the Ru(II)-Ru(C)(III)-PI*- CS state upon a second photoexcitation was studied by pump-pump-probe experiments in an attempt to detect the fully charge-separated Ru(III)-Ru(C)(II)-PI*- state.     

A theoretical study on fullerene-dizincocene hybrids

Using the density functional theory, we investigated the possible formation of fullerene-dizincocene hybrids, specifically C(60)*-Zn-Zn-Cp*, C(60)*-Zn-Zn-C(60)*, C(70)*-Zn-Zn-Cp*, and C(70)*-Zn-Zn-C(70)*, where C(60)*, Cp*, and C(70)* represent C(60)(CH(3))(5), C(5)(CH(3))(5), and C(70)(CH(3))(5) radicals. Our calculation shows that these hybrids have HOMO-LUMO gaps which are larger than has been experimentally identified for C(60)*-Fe-Cp. In addition, the strength of the Zn--Zn bonds is similar to that in Cp*-Zn-Zn-Cp* which was also synthesized recently. Furthermore, heterohybrids, C(60)*-Zn-Zn-Cp* and C(70)*-Zn-Zn-Cp* are expected to exist in equilibria with homohybrids, C(60)*-Zn-Zn-C(60)* and C(70)*-Zn-Zn-C(70)*, in which heterohybrids are much more favored. On the other hand, another hybrid involving Sc(3)N@C(68) as a fullerene unit is not highly probable.     

Charge-induced unfolding of multiply charged polyethylene glycol ions

The electrical mobility of mass-selected single poly(ethylene glycol) (PEG) chains of mass m (<14 kDalton) and charge state z (+1 to +5) reveals a near-spherical shape above a critical mass m(z) approximately z(2). The abrupt unfolding observed at m < m(z) shows that the polymer molecules behave as liquid drops upon reaching the Rayleigh limit, with an apparent surface energy of 0.026 N/m at ion diameters from 1.7 to 3.2 nm. Other nonspherical shape families with structures independent of charge, and with charge-dependent stability domains, are observed. Highly charged ions adopt approximately linear highly stretched configurations where the mobility depends only on m/z, independently of z. An operational definition of the surface energy of a single long chain molecule that is computable and agrees with the measured surface energy is provided.     

Layered and pillared metal carboxyethylphosphonate hybrid compounds

A series of carboxyethylphosphonate hybrid materials has been prepared: Mn(II)(O3PCH2CH2COOH) *H2O (1), Mn(III)(OH)(O3PCH2CH2COOH)*H2O (2), Al3(III)(OH)3(O3PCH2CH2CO2)2 *3H2O (3) and Cr2(III)(OH)3(O3PCH2CH2CO2) *3H2O (4). Compounds 1 and 2 were synthesized from Mn(III)(CH3COO)3 *2H2O under hydrothermal, or refluxing treatments, respectively. The crystal structures of the manganese-bearing solids have been solved ab initio from laboratory X-ray powder diffraction data and refined by the Rietveld method. 1 crystallises in a orthorhombic cell and 2 in monoclinic symmetry. Both solids have inorganic 2D layered structures with the acid carboxylic groups pointing towards the interlayer space, and the layers linked only through hydrogen bonds. The inorganic layers of these compounds are formed by manganese atoms in distorted octahedral environments linked together by the phosphonate groups. The crystal structure of 3 has been solved ab initio from synchrotron X-ray powder diffraction data. This solid shows a pillared structure with the phosphonate and carboxylate groups cross-linking the inorganic layers. These layers contain chains of aluminium octahedra running parallel to each other. 4 is amorphous and the IR-UV-VIS spectra suggest a framework with Cr(III) cations in octahedral environments. Thermal, spectroscopic and magnetic data for manganese and chromium compounds as well as the structural details of these solids are discussed.     

Organically directed iron sulfate chains: structural diversity based on hydrogen bonding interactions

Six organically directed 1-D iron sulfates hydrated and hydrolyzed to different extents have been prepared hydrothermally. [C2H10N2]1.5[Fe(SO4)(3)].2H2O (I), [C2H10N2][Fe(SO4)2(OH)].H2O (II), [C6H18N2]0.5[Fe(SO4)2(H2O)2] (III), and [C6H18N2]0.5[Fe2(SO4)(H2O)4(OH)].H2O (V) possess the linear topological structures observed in ferrinatrite, sideronatrite, kr?hnkite, and copiapite minerals, respectively. [C4H12N2][Fe2(SO4)3(OH)2(H2O)2].H2O (IV) shows a novel linear structure that can be regarded as a hybrid of the tancoite and butlerite types. [C6N4H22]0.5[Fe(SO4)2(OH)].2H2O (VI) adopts a cis configuration, compared with II, to give a rare inorganic helical iron sulfate chain which is a new member of the organically directed transitional metal sulfates. The results reveal that the starting molar proportion of the reactants and the type of amines are critical for the structural motif. There is an obvious relationship between the constitution of chains and the type of amino groups, involving the amount of N-H...O hydrogen bonds.     

Electroclinic effect in the chiral smectic A and cholesteric phases at the proximity of a N*–SmA–SmC* multicritical point

We studied the electro-optic and dielectric properties of three pure ferroelectric liquid crystal materials (C10, C11 and C12) of the same series exhibiting cholesteric (N*), smectic A (SmA) and chiral smectic C (SmC*) phases. From electro-optic investigations, the tilt angle and spontaneous polarisation were determined as a function of temperature. In the dielectric measurements carried out without a dc bias field, we studied the soft-mode relaxation in the SmA phase. From experimental data and using the results of a Landau model, we evaluated the soft-mode rotational viscosity and the electroclinic coefficient in the SmA phase. A soft-mode like mechanism was also observed in the N* phase for compounds with shorter chains (C10 and C11). This relaxation process is not detected for the homologue with a longer chain (C12). The observation of this mechanism is related to smectic order fluctuations within N* phase whose amplitude is increased when approaching the SmC*–SmA–N* multicritical point.     

Effects of substituents on the stabilities of phosphonyl radicals and their hydroxyphosphinyl tautomers

High-level ab initio quantum chemical methods have been used to calculate the radical stabilization energies (RSEs) of phosphonyl radicals XYP(=O)* bearing a range of substituents X and Y. The main influences on these radicals' stabilities are sigma-effects. Due to the high positive charge on phosphorus, sigma-withdrawal is destabilizing, and sigma-donation is stabilizing. The pyramidal geometry at phosphorus minimizes the effect of stabilization by pi-delocalization, while the potentially stabilizing effect of lone-pair donation is outweighed by concomitant sigma-withdrawal. Thus, the calculated RSEs of phosphonyl radicals XHP(=O)* increase in the order X = F < Me(3)N+ < MeO < CF3 < tBu < Me(2)N < NC < H < Ph < MeS < Me(3)Si. The tautomeric hydroxyphosphinyl radicals X(OH)P. exhibit a different set of substituent effects, with RSEs increasing in the order X = CF3 < Me(2)N < Me(3)N+ < MeO < (t)Bu < H < MeS < Me(3)Si < F < NC < Ph. In these radicals, both the sigma- and pi-properties of the X substituent influence stability, in tandem with those of the OH group. A comparison of the absolute enthalpies of isomeric phosphonyl and hydroxyphosphinyl radicals indicates that the hydroxyphosphinyl radicals X(OH)P* are more stable than the phosphonyl radicals XYP(=O)*. This is not a common situation in phosphorus chemistry. It is primarily attributed to the greater phosphorus p character of the singly occupied molecular orbital (SOMO) in the hydroxyphosphinyl radicals compared with the phosphonyl tautomers. As in closed-shell phosphorus species, the magnitude of the effect is modulated by the electronegativity of the substituent X.     

Cyano-bridged Mn(III)-M(III) single-chain magnets with M(III)=Co(III), Fe(III), Mn(III), and Cr(III)

A series of isostructural cyano-bridged Mn(III)(h.s.)-M(III)(l.s.) alternating chains, [Mn(III)(5-TMAMsalen)M(III)(CN)(6)]?4H(2)O (5-TMAMsalen(2-)=N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate), Mn(III)(h.s.)=high-spin Mn(III), M(III)(l.s.)=low-spin Co(III), Mn-Co; Fe(III), Mn-Fe; Mn(III), Mn-Mn; Cr(III), Mn-Cr) was synthesized by assembling [Mn(III)(5-TMAMsalen)](3+) and [M(III)(CN)(6)](3-). The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [-Mn(III)-NC-M(III)-CN-] repeating motif, for which the -NC-M(III)-CN- motif is provided by the [M(III)(CN)(6)](3-) moiety adopting a trans bridging mode between [Mn(III)(5-TMAMsalen)](3+) cations. The Mn(III) and M(III) ions occupy special crystallographic positions: a C(2) axis and an inversion center, respectively, forming a highly symmetrical chain with only one kind of cyano bridge. The Jahn-Teller axis of the Mn(III)(h.s.) ion is perpendicular to the N(2)O(2) plane formed by the 5-TMAMsalen tetradentate ligand. These Jahn-Teller axes are all perfectly aligned along the unique chain direction without a bending angle, although the chains are corrugated with an Mn-N(axis) -C angle of about 144°. In the crystal structures, the chains are well separated with the nearest inter-chain M???M distance being relatively large at 9?? due to steric hindrance of the bulky trimethylammoniomethyl groups of the 5-TMAMsalen ligand. The magnetic properties of these compounds have been thoroughly studied. Mn-Fe and Mn-Mn display intra-chain ferromagnetic interactions, whereas Mn-Cr is characterized by an antiferromagnetic exchange that induces a ferrimagnetic spin arrangement along the chain. Detailed analyses of both static and dynamic magnetic properties have demonstrated without ambiguity the single-chain magnet (SCM) behavior of these three systems, whereas Mn-Co is merely paramagnetic with S(Mn)=2 and D/k(B)=-5.3?K (D being a zero-field splitting parameter). At low temperatures, the Mn-M compounds with M=Fe, Mn, and Cr display remarkably large M versus H hysteresis loops for applied magnetic fields along the easy magnetic direction that corresponds to the chain direction. The temperature dependence of the associated relaxation time for this series of compounds systematically exhibits a crossover between two Arrhenius laws corresponding to infinite-chain and finite-chain regimes for the SCM behavior. These isostructural hetero-spin SCMs offer a unique series of alternating [-Mn-NC-M-CN-] chains, enabling physicists to test theoretical SCM models between the Ising and Heisenberg limits.     

Structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy) phosphazene]

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.     

The decomposition of C4H8 complexes at controlled internal energies

A coincidence technique is used to study the influence of the internal energy of the reactant ion on the cross section of the ion-molecule reactions in the C₂H₄⁺ + C₂H₄ system. The experiment is performed at thermal collision energies. In the ion-molecule reactions of C₂H₄⁺ + C₂H₄ our measurements indicate a barrier between the initially formed collision complex (C₂H₄)₂⁺* and a tight complex (C₄H₈⁺)*. Using an extension of our earlier developed statistical model, now including a potential barrier between the initially formed loose complex (C₂H₄)₂⁺* and the tight complex (C₄H₈⁺)*, our experimental data can be reproduced. For comparison also the internal energy dependence of the unimolecular decomposition of photoionised 1-C₄H₈⁺ is measured. Assuming that the photoionised 1-C₄H₈⁺ is identical with the tight (C₄H₈⁺)* complex, the model applied to the ion-molecule reactions describes also the unimolecular decay of 1-C₄H₈⁺ correctly, using the same set of parameters.     

Potential interstellar molecules. Formation of neutral C(6)CO from C(6)CO(-*) in the gas phase

Computations at the RCCSD(T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory indicate that neutral C(6)CO is a stable species. The ground state of this neutral is the singlet cumulene oxide :C=C=C=C=C=C=C=O. The adiabatic electron affinity and dipole moment of singlet C(6)CO are 2.47 eV and 4.13 D, respectively, at this level of theory. The anion (C(6)CO)-* should be a possible precursor to this neutral. It has been formed by an unequivocal synthesis in the ion source of a mass spectrometer by the S(N)2(Si) reaction between (CH(3))(3)Si-C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-CMe(3) and F(-) to form (-)C(triple bond)C-C(triple bond)C-C(triple bond)C-CO-Me(3) which loses Me(3)C* in the source to form C(6)CO(-)*. Charge stripping of this anion by vertical Franck-Condon oxidation forms C(6)CO, characterised by the neutralisation-reionisation spectrum (-NR(+)) of C(6)CO(-*), which is stable during the timeframe of this experiment (10(-6) s).     

Active oxygen species generated from photoexcited fullerene (C60) as potential medicines: O2-* versus 1O2

To characterize fullerenes (C(60) and C(70)) as photosensitizers in biological systems, the generation of active oxygen species, through energy transfer (singlet oxygen (1)O(2)) and electron transfer (reduced active oxygen radicals such as superoxide anion radical O(2)(-)* and hydroxyl radical *OH), was studied by a combination of methods, including biochemical (DNA-cleavage assay in the presence of various scavengers of active oxygen species), physicochemical (EPR radical trapping and near-infrared spectrometry), and chemical methods (nitro blue tetrazolium (NBT) method). Whereas (1)O(2) was generated effectively by photoexcited C(60) in nonpolar solvents such as benzene and benzonitrile, we found that O(2)(-)* and *OH were produced instead of (1)O(2) in polar solvents such as water, especially in the presence of a physiological concentration of reductants including NADH. The above results, together with those of a DNA cleavage assay in the presence of various scavengers of specific active oxygen species, indicate that the active oxygen species primarily responsible for photoinduced DNA cleavage by C(60) under physiological conditions are reduced species such as O(2)(-)* and *OH.     

The impact of protonation and deprotonation of 3-methyl-2'-deoxyadenosine on N-glycosidic bond cleavage

The enzyme-substrate contacts that are believed to be involved in depurination by proton transfer have been modelled by protonation and deprotonation of 3-methyl-2'-deoxyadenosine (3-MDA) using quantum mechanical calculations in the gas-phase and solution media. The change in the charge distribution on the sugar ring and nucleobase that is introduced by the protonation and deprotonation strongly affects the N-glycosidic bond length. The unimolecular cleavage and hydrolysis of the N-glycosidic bond, involving D(N)*A(N) and A(N)D(N) pathways, have been considered at several levels of theory. The trend in the energy barriers is A(N)D(N) > cleavage > D(N)*A(N). All probable proton transfer reactions resulting from enzyme-substrate contacts do not facilitate the N-glycosidic bond cleavage of 3-MDA. The deprotonation of 3-MDA that may result from the interaction between H6 and enzyme do not facilitate bond cleavage. The protonation at N7 induces more positive charge on the sugar ring and further facilitates the depurination relative to the protonation at N1. The changes in the charges calculated on the ribose and nucleobase are in good relationship with the C1'-C2', C1'-O4', and N-glycosidic bond lengths along the cleavage. The change in energy barrier ΔE of glycosidic bond cleavage from the gas-phase to solution media strongly depends on the charge of the species.     

Infrared spectroscopy of cationized arginine in the gas phase: direct evidence for the transition from nonzwitterionic to zwitterionic structure

The gas-phase structures of protonated and alkali metal cationized arginine (Arg) and arginine methyl ester (ArgOMe) are investigated with infrared spectroscopy and ab initio calculations. Infrared spectra, measured in the hydrogen-stretch region, provide compelling evidence that arginine changes from its nonzwitterionic to zwitterionic form with increasing metal ion size, with the transition in structure occurring between lithium and sodium. For sodiated arginine, evidence for both forms is obtained from spectral deconvolution, although the zwitterionic form is predominant. Comparisons of the photodissociation spectra with spectra calculated for low-energy candidate structures provide additional insights into the detailed structures of these ions. Arg*Li+, ArgOMe*Li+, and ArgOMe*Na+ exist in nonzwitterionic forms in which the metal ion is tricoordinated with the amino acid, whereas Arg*Na+ and Arg*K+ predominately exist in a zwitterionic form where the protonated side chain donates one hydrogen bond to the N terminus of the amino acid and the metal ion is bicoordinated with the carboxylate group. Arg*H+ and ArgOMe*H+ have protonated side chains that form the same interaction with the N terminus as zwitterionic, alkali metal cationized arginine, yet both are unambiguously determined to be nonzwitterionic. Calculations indicate that for clusters with protonated side chains, structures with two strong hydrogen bonds are lowest in energy, in disagreement with these experimental results. This study provides new detailed structural assignments and interpretations of previously observed fragmentation patterns for these ions.