Polar solvent fluctuations drive proton transfer in hydrogen bonded complexes of carboxylic acid with pyridines: NMR, IR and ab initio MD study.

We study a series of intermolecular hydrogen-bonded 1 : 1 complexes formed by chloroacetic acid with 19 substituted pyridines and one aliphatic amine dissolved in CD2Cl2 at low temperature by 1H and 13C NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (O-H-N+) ones, while NMR spectra show the formation of short strong hydrogen bonds in intermediate cases. Analysis of C[double bond, length as m-dash]O stretching and asymmetric CO2- stretching bands in FTIR spectra reveal the presence of proton tautomerism. On the basis of these data, we construct the overall proton transfer pathway. In addition to that, we also study by use of ab initio molecular dynamics the complex formed by chloroacetic acid with 2-methylpyridine, surrounded by 71 CD2Cl2 molecules, revealing a dual-maximum distribution of hydrogen bond geometries in solution. The analysis of the calculated trajectory shows that the proton jumps between molecular and zwitterionic forms are indeed driven by dipole-dipole solvent-solute interactions, but the primary cause of the jumps is the formation/breaking of weak CHO bonds from solvent molecules to oxygen atoms of the carboxylate group.

Efficient design of multituned transmission line NMR probes: the electrical engineering approach.

Transmission line-based multi-channel solid state NMR probes have many advantages regarding the cost of construction, number of RF-channels, and achievable RF-power levels. Nevertheless, these probes are only rarely employed in solid state-NMR-labs, mainly owing to the difficult experimental determination of the necessary RF-parameters. Here, the efficient design of multi-channel solid state MAS-NMR probes employing transmission line theory and modern techniques of electrical engineering is presented. As technical realization a five-channel ((1)H, (31)P, (13)C, (2)H and (15)N) probe for operation at 7 Tesla is described. This very cost efficient design goal is a multi port single coil transmission line probe based on the design developed by Schaefer and McKay. The electrical performance of the probe is determined by measuring of Scattering matrix parameters (S-parameters) in particular input/output ports. These parameters are compared to the calculated parameters of the design employing the S-matrix formalism. It is shown that the S-matrix formalism provides an excellent tool for examination of transmission line probes and thus the tool for a rational design of these probes. On the other hand, the resulting design provides excellent electrical performance. From a point of view of Nuclear Magnetic Resonance (NMR), calibration spectra of particular ports (channels) are of great importance. The estimation of the π/2 pulses length for all five NMR channels is presented.

Carboxylic acid-doped SBA-15 silica as a host for metallo-supramolecular coordination polymers.

The adsorption of a metallo-supramolecular coordination polymer (Fe-MEPE) in the cylindrical pores of SBA-15 silica with pure and carboxylic acid (CA) carrying pore walls has been studied. Fe-MEPE is an intrinsically stiff polycation formed by complexation of Fe(II)-acetate with an uncharged ditopic bis-terpyridine ligand. The adsorption affinity and kinetics of the Fe-MEPE chains is strongly enhanced when the pore walls are doped with CA, and when the pH of the aqueous medium or temperature is increased. The initial fast uptake is connected with a decrease of pH of the aqueous solution, indicating an ion-exchange mechanism. It is followed by a slower (presumably diffusion-controlled) further uptake. The maximum adsorbed amount of Fe-MEPE in the CA-doped material corresponds to a monolayer of Fe-MEPE chains disposed side-by-side along the pore walls. The stoichiometry of Fe-MEPE in the pores (determined by XPS) was found to be independent of the loading and similar to that of the starting material. The mean chain length of Fe-MEPE before and after embedding in the CA-doped matrix was studied by solid-state 15N NMR using partially 15N-labeled Fe-MEPE. It is shown that the average chain length of Fe-MEPE is reduced when the complex is incorporated in the pores.

Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion.

In this paper a quantitative explanation for a diamagnetic ortho/para H2 conversion is given. The description is based on the quantum-mechanical density matrix formalism originally developed by Alexander and Binsch for studies of exchange processes in NMR spectra. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a three spin system, the reactions of the hydrogen gas with the catalysts are treated as a phenomenological rate process, described by a rate constant. Numerical calculations reveal that for nearly all possible geometrical arrangements of the three spin system an efficient spin conversion is obtained. Only in the chemically improbable case of a linear group H-X-H no spin conversion is obtained. The efficiency of the spin conversion depends strongly on the lifetime of the H-X-H complex and on the presence of exchange interactions between the two hydrogens. Even moderate exchange couplings cause a quench of the spin conversion. Thus a sufficiently strong binding of the dihydrogen to the S spin is necessary to render the quenching by the exchange interaction ineffective.

Evidence of microphase separation in controlled pore glasses.

The phase separation of a mixture of water and isobutyric acid (iBA) confined in the pore space of Controlled Pore Glass (CPG) 10-75 has been studied by 1H NMR relaxometry and 1H-pulsed field gradient (PFG) diffusion measurements. For an acid-rich mixture (mass fraction 54 wt% iBA), evidence of a phase separation process in the pores was obtained, which occurs in a temperature window between 32 and 39 degrees C, as indicated in the PFG data by an anomalous temperature dependence of the diffusion coefficient and in the relaxation data by a bi-exponential magnetization decay. The phase separation temperature of the mixture in the pore is slightly lower than in the bulk mixture of the same composition (41 degrees C) and extends over a finite temperature range. A qualitative model of the phase separation process in the pores is developed, which assumes a temperature-dependent domain-like structure of the liquid below the phase transition temperature and a breakdown of these domains upon reaching the transition temperature.

A solid-state NMR, X-ray diffraction, and ab initio computational study of hydrogen-bond structure and dynamics of pyrazole-4-carboxylic acid chains.

Using high-resolution solid-state (15)N CMAS NMR, X-ray crystallography, and ab initio calculations, we have studied the structure of solid pyrazole-4-carboxylic acid (1). The crystal structure was determined at 295 and 150 K. Molecules of 1 are located on a two-fold axis, implying proton disorder of the NH and OH groups; no phase transition was observed between these two temperatures. The compound forms quasi-linear ribbons in which the molecules are linked by cyclic hydrogen bonds between pyrazole and carboxylic acid groups with disordered hydrogen-bonded protons. Crystallography is unable to decide whether the disorder is dynamic or static. NMR shows that this disorder is dynamic, that is, consisting of very fast degenerate double proton transfers between two rapidly interconverting O-H.N and O.H-N hydrogen bridges. However, at low temperature, NMR shows a proton disorder-order transition where the protons are preferentially localized on given nitrogen and oxygen atoms. An amorphous phase exhibiting proton order is observed when the compound is precipitated rapidly. In this case, the defects are annealed by moderate heating. Ab initio calculations performed on oligomers of 1 show that the O-H.N hydrogen bridge is about 0.064 A shorter and less bent ( approximately 171 degrees ) than the O.H-N hydrogen bridge ( approximately 150 degrees ). For an isolated ribbon, this result leads to structures with localized protons, either to a cycle with about 200 molecules, or to a quasi-linear ribbon involving an undulated structure, or to a combination of both motifs. Only the undulated structure is compatible with the linear ribbon observed by X-ray crystallography, where the fast proton transfer in the high-temperature phase is assisted by the motions of the undulated chain. A disordered structure is assigned to the amorphous phase, which exhibits the combination of the curved and the undulated motifs.

Hydrogen bonds and proton transfer in general-catalytic transition-state stabilization in enzyme catalysis.

The question of the nature of the proton bridge involved in general acid-base catalysis in both enzymic and non-enzymic systems is considered in the light of long-known but insufficiently appreciated work of Jencks and his coworkers and of more recent results from neutron-diffraction crystallography and NMR spectroscopic studies, as well as results from isotope-effect investigations. These lines of inquiry lead toward the view that the bridging proton, when between electronegative atoms, is in a stable potential at the transition state, not participating strongly in the reaction-coordinate motion. Furthermore they suggest that bond order is well-conserved at unity for bridging protons, and give rough estimates of the degree to which the proton will respond to structural changes in its bonding partners. Thus if a center involved in general-catalytic bridging becomes more basic, the proton is expected to move toward it while maintaining a unit total bond order. For a unit increase in the pK of a bridging partner, the other partner is expected to acquire about 0.06 units of negative charge. The implications are considered for charge distribution in enzymic transition states as the basicity of catalytic residues changes in the course of molecular evolution or during progress along a catalytic pathway.

Evidence of surface diffusion of water molecules on proteins of rabbit lens by 1H NMR relaxation measurements.

In this work, we propose a relaxation model for the interpretation of NMR proton spinlattice and spin-spin relaxation times of mammalian lenses. The framework for this model is based on nuclear magnetic spin-lattice relaxation measurements as a function of temperature at different Larmor frequencies for whole rabbit lenses and fragments of the lens. According to this model, two different dynamic processes of the water molecules determine the relaxation behaviour, namely rotational diffusion and translational surface diffusion. These dynamic processes in conjunction with a two site exchange model give a good explanation of all the measured relaxation data. From the experimental data, we were able to obtain the activation parameters for rotational and translational diffusion of bound lens water. Correlation times of 2.1 x 10(-11) sec and 2.5 x 10(-9) sec and activation energies of 20.5 kJ/mol and 22.5 kJ/mol respectively were found at 308K. At low Larmor frequencies (< or = 100 MHz) the longitudinal relaxation is mainly determined by translational surface diffusion of bound water with a mean square displacement of 1.5 nm, whereas at higher frequencies (> or = 300 MHz), rotational diffusion is the main relaxation mechanism. The spin-spin relaxation is determined by translational diffusion over the whole frequency range and therefore shows only a very small dispersion. By our model it is possible to explain: 1) the strikingly large difference between the T1 value and the T2A and T2B values observed in the lens and 2) the different values of the activation energies measured at different fields for the lens.

1H NMR and calorimetric measurements on rabbit eye lenses.

The dynamic properties of water molecules in the rabbit lens were studied by proton nuclear magnetic resonance line shape analysis, measurements of relaxation times as a function of temperature and calorimetric measurements. The experiments prove, as already suggested by other authors, that there are two types of water in the lens of rabbit eyes, namely bound unfreezable hydration water and bulk freezable water. Line shape analysis and relaxometry showed, that this two types of water exist in two different environments, which may be identified as the nucleus and the cortex of the lens. The line shape analysis showed furthermore that water molecules in the rabbit lens has a common spin lattice relaxation time (T1), but two different transverse relaxation times (T2A and T2B). The tentative model of fast water exchange on the T1 time scale and slow water exchange on the T2 time scale, was used to explain experimental proton relaxation data of the rabbit lens. An estimation for this exchange rate kex by comparing it to the relaxation times is given (T1(-1) << kex << T1(-1)). It has also been shown by a calorimetric measurements, that the lenses can be easily under-cooled to temperatures well below the freezing point of water. The achievable maximum undercooling temperature of the lens is a function of the cooling rate KC, therefore it has to be considered as an experimentally adjustable parameter which is not characteristic for the investigated sample. Thus it must be noted that any previous discussions about the specific value of the temperature of water crystallisation in biological systems need to be carefully reconsidered.

15N and 13C solid-state nuclear magnetic resonance study of 5-thiomethyltetrazole.

15N and 13C cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of 5-thiomethyltetrazole have been measured. Two kinds of tetrazole molecules are found in the hydrogen bond chain system depending on the configuration of the S-CH3 group in relation to the ring hydrogen atom. The two-dimensional 15N CP-MAS experiment excludes the possibility of proton tautomeric dynamics in this system.

Evidence for tautomerism in nucleic acid base pairs. 1H NMR study of 15N labeled tRNA.

The imino proton resonances of 15N labeled tRNA appear as asymmetric doublet signals, the asymmetry being dependent on the applied magnetic field strength. Assuming a tautomerism of the type N-H...N not equal to N...H-N in the base pairs the line shapes can be simulated. The most important parameters fitted in the simulation are the rate constants of the proton transfer and the mole fractions of either tautomeric state. The rate constants are of the order of 100s-1 and the mole fractions of the non dominant tautomer about 0.1 depending on the temperature and on the nature of the base pairing. The observations are attributed to a double proton transfer in the base pairs. The unexpectedly slow rates of the double proton transfer process may be connected with a concomitant conformational change of the duplex structure.