Intramolecular proton transfer

[structure: see text] Reversal of the normal kinetic protonation stereochemistry results as a consequence of intramolecular delivery.     

Correlated proton motion in hydrogen bonded systems: tuning proton affinities

The theorem of matching proton affinities (PA) has been widely used in the analysis of hydrogen bonds. However, most experimental and theoretical investigations have to cope with the problem that the variation of the PA of one partner in the hydrogen bond severely affects the properties of the interface between both molecules. The B3LYP/d95+(d,p) analysis of two hydrogen bonds coupled by a 5-methyl-1H-imidazole molecule showed that it is possible to change the PA of one partner of the hydrogen bond while maintaining the properties of the interface. This technique allowed us to correlate various properties of the hydrogen bond directly with the difference in the PAs between both partners: it is possible to tune the potential energy surface of the bonding hydrogen atom from that of an ordinary hydrogen bond (localized hydrogen atom) to that of a low barrier hydrogen bond (LBHB, delocalized hydrogen atom) just by varying the proton affinity of one partner. This correlation shows clearly that matching PAs are of lesser importance for the formation of a LBHB than the relative energy difference between the two tautomers of the hydrogen bond.     

An optically-pumped proton maser

The nuclear polarization of protons in a Yb-doped yttrium ethyl sulfate crystal is found to invert by simply illuminating the crystal with near-infrared radiation. A nuclear enhancement of −15 is observed at 1.4 K and H = 15 kOe. This novel dynamic polarization effect is apparently caused by optical pumping of the paramagnetic Yb³⁺ spin. When the crystal is placed in a high-Q NMR circuit at 60 MHz, maser oscillations are induced by the incident light.     

Broadband proton-decoupled proton spectra

We present a new method for recording broadband proton-decoupled proton spectra with absorption mode lineshapes and substantially correct integrals; in both these respects, the new method has significant advantages over conventional J-spectroscopy. In our approach, the decoupled spectrum is simply obtained from the 45 degrees projection of the diagonal-peak multiplets of an anti z-COSY spectrum. This method is straightforward to apply, and does not require any unusual data processing. However, there is a significant reduction in sensitivity when compared to a conventional proton spectrum. The method is demonstrated for typical medium-sized molecules, and it is also shown how such a decoupled spectrum can be used to advantage in measurements of diffusion constants (DOSY), the measurement of relaxation parameters, and the analysis of complex mixtures.     

Heteronuclear proton assisted recoupling

We describe a theoretical framework for understanding the heteronuclear version of the third spin assisted recoupling polarization transfer mechanism and demonstrate its potential for detecting long-distance intramolecular and intermolecular (15)N-(13)C contacts in biomolecular systems. The pulse sequence, proton assisted insensitive nuclei cross polarization (PAIN-CP) relies on a cross term between (1)H-(15)N and (1)H-(13)C dipolar couplings to mediate zero- and∕or double-quantum (15)N-(13)C recoupling. In particular, using average Hamiltonian theory we derive effective Hamiltonians for PAIN-CP and show that the transfer is mediated by trilinear terms of the form N(±)C(?)H(z) (ZQ) or N(±)C(±)H(z) (DQ) depending on the rf field strengths employed. We use analytical and numerical simulations to explain the structure of the PAIN-CP optimization maps and to delineate the appropriate matching conditions. We also detail the dependence of the PAIN-CP polarization transfer with respect to local molecular geometry and explain the observed reduction in dipolar truncation. In addition, we demonstrate the utility of PAIN-CP in structural studies with (15)N-(13)C spectra of two uniformly (13)C,(15)N labeled model microcrystalline proteins-GB1, a 56 amino acid peptide, and Crh, a 85 amino acid domain swapped dimer (MW=2×10.4 kDa). The spectra acquired at high magic angle spinning frequencies (ω(r)∕2π>20 kHz) and magnetic fields (ω(0H)∕2π=700-900 MHz) using moderate rf fields, yield multiple long-distance intramonomer and intermonomer (15)N-(13)C contacts. We use these distance restraints, in combination with the available x-ray structure as a homology model, to perform a calculation of the monomer subunit of the Crh protein.     

Modeling energy landscapes of proton motion in nonaqueous, tethered proton wires

We have modeled structures and energetics of anhydrous proton-conducting wires: tethered hydrogen-bonded chains of the form ···HX···HX···HX···, with functional groups HX = imidazole, triazole, and formamidine; formic, sulfonic, and phosphonic acids. We have applied density functional theory (DFT) to model proton wires up to 19 units long, where each proton carrier is linked to an effective backbone to mimic polymer tethering. This approach allows the direct calculation of hydrogen bond strengths. The proton wires were found to be stabilized by strong hydrogen bonds (up to 50 kJ/mol) whose strength correlates with the proton affinity of HX [related to pK(b)(HX)] and not to pK(a)(HX) as is often assumed. Geometry optimizations and ab initio molecular dynamics near 400 K on imidazole-based proton wires both predict that adding a proton to the end of such wires causes the excess charge to embed into the interior segments of these wires. Proton translocation energy landscapes for imidazole-based wires are sensitive to the imidazole attachment point (head or feet) and to wire architecture (linear or interdigitated). Linear imidazole wires with head-attachment exhibit low barriers for intrawire proton motion, rivaling proton diffusion in liquid imidazole. Excess charge relaxation from the edge of wires is found to be dominated by long-range Grotthuss shuttling for distances as long as 42 ?, especially for interdigitated wires. For imidazole, we predict that proton translocation is controlled by the energetics of desorption from the proton wire, even for relatively long wires (600 imidazole units). Proton desorption energies show no correlation with functional group properties, suggesting that proton desorption is a collective process in proton wires.     

470—The proton pump at work: Part I. The basic concept of excess proton/defect proton translocation

The Nagle-Morowitz proton pump, which is based on proton transport in water and ice, is shown to be inapplicable to weakly H-bonded proton transport systems. It is demonstrated that in weakly or non H-bonded systems protons can migrate either as excess protons H using a high-lying proton conduction band or as defect protons H′ using intermediate energy levels. As H? the protons act as positive charge carriers endowed with a very high mobility. As H′ they act as negative charge carriers. The H′ transport mechanism involves thermally activated proton tunneling. Owing to the very large mobility differences between H? and H′, > 10⁶, very high potentials can be self-generated in any situation which creates a concentration gradient. Proton conductivity data on inorganic model compounds are presented. Applying these results to proton transport across biomembranes, transmembrane potentials U_m, acidification Δ pH and transient phenomena can be explained as result of H? and H′ translocation.     

The proton affinities and proton transfer in imine, amidine and guanidine series

The enthalpies of formation of neutral and protonated alkyl imines, amidine and guanidine are obtained by ab initio theoretical means using an isodesmic reaction technique. The corresponding proton affinities are obtained and discussed in terms of thermochemical stabilization energies. Their relation to the proton transfer process is examined and discussed. To this end, the structure and properties of various molecular complexes obtained between these molecules of interest and formic acid are explored. The sensitivity of this process to the presence of a neighbour water molecule is commented.     

Pyridine as proton acceptor in the concerted proton electron transfer oxidation of phenol

Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation.     

Enhancing proton conductivity of proton exchange membrane with SPES nanofibers containing porous organic cage

Effective proton conducting sites and establishing proton channels are two critical factors in developing high‐performance proton exchange membranes. This study first establishes a strategy in designing effective proton conducting channels for Nafion by using solution blowing of sulfonated polyethersulfone (SPES) nanofibers containing CC3, which is an emerging porous organic cage that possesses the advantages of dissolvable organic solvents and high proton conduction from its interconnected three‐dimensional pore structure. Our strategy results in SPES nanofiber networks with CC3 uniformly involved in and composite membranes with Nafion‐filled interfiber voids. Benefiting from such structural features, the composite membrane exhibits high proton conductivity (0.315 S cm^?1 at 80°C and 100% RH), low methanol permeability (0.69 × 10^?7 cm² S^?1), excellent water absorption, thermal and dimensional stability, and single‐cell performance. This study provides not only a valuable reference for the application of CC3 but also a new idea for establishment of proton transfer channels.     

A proton sponge-based fluorescent switch

A proton-triggered fluorescent switch was developed through direct connection of a proton sponge, 1,8-bis(dimethylamino)naphthalene to a fluorophore, 4-aminonaphthalimide. The molecular fluorescent switch was based on a photoinduced electron transfer (PET) mechanism. The fluorescent switch exhibited significant fluorescence enhancement upon binding the low-concentration protons in high pH aqueous solution. The high pK_a value (around 11) of the fluorescent switch could be ascribed to the nature of the proton sponge.     

Estimation of proton donor ability of surface hydroxy groups in terms of proton affinity

A method to estimate the proton donor ability of hydroxy groups in terms of proton affinity (PA) is suggested. The PA of hydroxy groups of aerosil and of decationated zeolite type Y have been determined.

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(PA). PA .

     

Conformation-coupled proton migration in tetraphenylporphin

The proton migration process in tetraphenylporphin is found to be strongly coupled to the symmetry-breaking deformation of the porphin skeletal structure. The adiabatic potential for the proton motion suggests that the in-plane N-H bending vibration will make an important contribution to the proton migration.     

Decarboxylation in proton acceptor solvents

Heating 3,8-dinitro-10-carboxy-6H-dibenzo[b,d]pyran-6-one in DMSO, DMF, or HMPTA leads to decarboxylation and the replacement of the carboxyl group by a hydroxy group with the formation of 3,8-dinitro-6H-dibenzo[b,d]pyran-6-one and 3,8-dinitro-10-hydroxy-6H-dibenzo [b,d]pyran-6-one. The decarboxylation of 2,7-dinitro-5,10-dioxo-4,5,9,10-tetrahydro-4,9-dioxapyrene in HMPTA is preceded by opening of the two lactone rings and the formation of a 1:4 molecular complex of 4,4-dinitro-6,6-dihydroxy-2,2-dicarboxybiphenyl with HMPTA, whose structure was established by x-ray diffraction structural analysis.Translated from Khimiya Geterotsiklicheskik Soedinenii, No. 2, pp. 164–170, February, 1989.     

Tunnel effect in proton transfer

Semiempirical computations were carried out to determine the tunneling rates in the case of coupled motion of two protons along the reaction coordinate. The following molecular systems were studied for medium intermolecular distances (A—B = 2.72 or 2.75 Å); ⁺AH—BH—A, where A was NH₃ or H₂O and BH was HF or H₂O. In the cases where the bridge was HF, solvation was modeled with just one water molecule attached to each side of the perpendicular axis through HF at 2.75 Å. Coupled motion of three protons was also included in the case of H₃O—H₂O—H₂O—H₂O.     

Stereochemistry-Activity Relationships in Olfaction. Odorants containing a proton donor/proton acceptor unit

A novel class of odorants is described where the odor is associated with the interaction of two functional groups, one being an H-donor (AH function), and the other an H -acceptor ( B function). Generally, odor occurs only if the distance between the two structural elements (AH/B system) is less than 3 Å. Bifunctional derivatives of the p-menthane and iridane series served as models for deriving this rule. The stereospecificity of odor perception was an important prerequisite for its establishment.