Proton Transfer Properties Research Articles

Dynamic Properties of Proton Transfer in the Anharmonic-Interaction Hydrogen Bond Systems**The project supported by Natural Science Foundation of Shandong Province of China (No. Q99A03) and National Natural ScienceFoundation of China under Grant No. 19974034

We analyze the properties of the excited solitary-wave model in the case of anharmonic-interaction of heavy ionic lattice in hydrogen bond systems. In this case, some new phenomena appear. We find different types of solutions for the proton displacement and influences on the kinks and pulse solitary waves by numerical calculation. For each of them we have presented a direct relation with the effective potential of the system.

Dynamic properties of proton transfer in hydrogen-bonded molecular systems

The mechanism of the formation of ionic and bonding defects and the interaction between protons and heavy ions in hydrogen-bonded molecular systems is investigated and a new model is constructed. This model considers two coupled sublattices corresponding to those of protons and heavy ions in order to study the dynamic properties resulting from deformation and local fluctuation of the heavy ion sublattice due to the protonic displacements. As compared with other models, the model emphasizes, in particular, the collective effect of the motion and the mutual correlation between the two sublattices. The equations of motion admit analytic soliton solutions of different types corresponding to two different defects. The combined effect of the two defects and the proton transfer in the system are described by means of these solutions, and the appropriate conditions associated with actual physical processes. Finally, we discuss in detail the characteristics of the proton transfer and the advantages of the model.

Effects of chemical substitution upon excited state proton transfer. Fluoroderivatives of salicylaldimine

Ab initio methods are used to probe the effect of fluorosubstitution upon the excited state proton transfer process in the OH⋯N hydrogen bond contained in the model system salicylaldimine. Two different sites are chosen for substitution, one nearby to the nitrogen on the cycle that contains the intramolecular H-bond, while the other position is located on the aromatic ring close to the oxygen. Many of the computed effects on the proton transfer potential can be considered inductive in nature; the electronegative F makes the proximate N or O atom a stronger acid/weaker base and thereby moderates the preferred position of the bridging proton. The magnitude of the perturbation diminishes as the site of substitution is further removed from the H-bond. This principle also controls the manner in which F affects the geometry and strength of the intramolecular H-bond in both the enol and keto tautomers. There are a number of stretches and contractions that occur in bonds that do not border on the H-bond, patterns that can be understood via the bonding patterns in the orbitals involved in the excitation. Whereas these notions apply fairly consistently to the ground state and excited ππ ∗ singlet and triplet, there are a number of anomalous patterns that emerge in the 1 nπ ∗ state. In general, the effects of fluorosubstitution are smaller in magnitude than the changes that occur in the proton transfer properties as a result of electronic excitation.

Intermolecular proton transfer in catalysis by carbonic anhydrase V

The dehydration of bicarbonate catalyzed by carbonic anhydrase is accompanied by the transfer of a proton from solution to the zinc-bound hydroxide. We have investigated the properties of proton transfer from donors in solution, mostly derivatives of imidazole and pyridine, to a truncated mutant of carbonic anhydrase V with replacements that render the active site cavity less sterically constrained, Tyr 64 →> Ala and Phe 65 →> Ala. Catalysis was measured by determining the rate of exchange of 18O between the CO2-HCO3- system and water, and rate constants for proton transfer were estimated as the rate-limiting step in the release of H218O from the enzyme to solution. Each proton donor enhanced catalytic activity in a saturable manner. The resulting rate constants for proton transfer when compared with the values of pKa of the donor and acceptor gave a Brønsted plot of high curvature. These data could also be described by Marcus theory which showed an intrinsic barrier for intermolecular proton transfer near 0.8 kcal/mol and a work term or thermodynamic contribution to the free energy of reaction near 10 kcal/mol. This low intrinsic kinetic barrier for proton transfer is very similar to nonenzymic bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. However, the significant thermodynamic contribution suggests appreciable involvement of solvent and active-site organization prior to proton transfer. These Marcus parameters are very similar to those describing intramolecular proton transfer from His 64 in carbonic anhydrase, suggesting similarities in the intra- and intermolecular proton transfer processes.Key words: carbonic anhydrase, proton transfer, Marcus theory, carbon dioxide.

Intermolecular proton transfer in catalysis by carbonic anhydrase V

The dehydration of bicarbonate catalyzed by carbonic anhydrase is accompanied by the transfer of a proton from solution to the zinc-bound hydroxide. We have investigated the properties of proton transfer from donors in solution, mostly derivatives of imidazole and pyridine, to a truncated mutant of carbonic anhydrase V with replacements that render the active site cavity less sterically constrained, Tyr 64 →> Ala and Phe 65 →> Ala. Catalysis was measured by determining the rate of exchange of 18O between the CO2-HCO3- system and water, and rate constants for proton transfer were estimated as the rate-limiting step in the release of H218O from the enzyme to solution. Each proton donor enhanced catalytic activity in a saturable manner. The resulting rate constants for proton transfer when compared with the values of pKa of the donor and acceptor gave a Brønsted plot of high curvature. These data could also be described by Marcus theory which showed an intrinsic barrier for intermolecular proton transfer near 0.8 kcal/mol and a work term or thermodynamic contribution to the free energy of reaction near 10 kcal/mol. This low intrinsic kinetic barrier for proton transfer is very similar to nonenzymic bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. However, the significant thermodynamic contribution suggests appreciable involvement of solvent and active-site organization prior to proton transfer. These Marcus parameters are very similar to those describing intramolecular proton transfer from His 64 in carbonic anhydrase, suggesting similarities in the intra- and intermolecular proton transfer processes.Key words: carbonic anhydrase, proton transfer, Marcus theory, carbon dioxide.

The crystal structure of dienoyl-CoA isomerase at 1.5 A resolution reveals the importance of aspartate and glutamate sidechains for catalysis.

The degradation of unsaturated fatty acids is vital to all living organisms. Certain unsaturated fatty acids must be catabolized via a pathway auxiliary to the main beta-oxidation pathway. Dienoyl-coenzyme A (dienoyl-CoA) isomerase catalyzes one step of this auxiliary pathway, the isomerization of 3-trans,5-cis-dienoyl-CoA to 2-trans,4-trans-dienoyl-CoA, and is imported into both mitochondria and peroxisomes. Dienoyl-CoA isomerase belongs to a family of CoA-binding proteins that share the enoyl-CoA hydratase/isomerase sequence motif. The crystal structure of rat dienoyl-CoA isomerase has been determined at 1.5 A resolution. The fold closely resembles that of enoyl-CoA hydratase and 4-chlorobenzoyl-CoA dehalogenase. Dienoyl-CoA isomerase forms hexamers made up of two trimers. The structure contains a well ordered peroxisomal targeting signal type-1 which is mostly buried in the inter-trimer space. The active-site pocket is deeply buried and entirely hydrophobic, with the exception of the acidic residues Asp176, Glu196 and Asp204. Site-directed mutagenesis of Asp204 revealed that this residue is essential for catalysis. In a molecular modeling simulation, a molecule of 3-trans,5-cis-octadienoyl-CoA was docked into the active site. The structural data, supported by the mutagenesis data, suggest a reaction mechanism where Glu196 acts as a proton acceptor and Asp204 acts as a proton donor. Asp176 is paired with Glu196 and is important for optimizing the catalytic proton transfer properties of Glu196. In the predicted mode of substrate binding, an oxyanion hole stabilizes the transition state by binding the thioester oxygen. The presence of a buried peroxisomal targeting signal suggests that dienoyl-CoA isomerase is prevented from reaching its hexameric structure in the cytosol.

Properties of Intramolecular Proton Transfer in Carbonic Anhydrase III

We investigated the efficiency of glutamic acid 64 and aspartic acid 64 as proton donors to the zinc-bound hydroxide in a series of site-specific mutants of human carbonic anhydrase III (HCA III). Rate constants for this intramolecular proton transfer, a step in the catalyzed dehydration of bicarbonate, were determined from the proton-transfer-dependent rates of release of H 2 18O from the enzyme measured by mass spectrometry. The free energy plots representing these rate constants could be fit by the Marcus rate theory, resulting in an intrinsic barrier for the proton transfer of Δ G O ‡ = 2.2 ± 0.5 kcal/mol, and a work function or thermodynamic contribution to the free energy of reaction w r = 10.8 ± 0.1 kcal/mol. These values are very similar in magnitude to the Marcus parameters describing intramolecular proton transfer from His 64 and His 67 to the zinc-bound hydroxide in mutants of HCA III. That result and the equivalent efficiency of Glu 64 and Asp 64 as proton donors in the catalysis by CA III demonstrate a lack of specificity in proton transfer from these sites, which is indirect evidence of a number of proton conduction pathways through different structures of intervening water chains. The dominance of the thermodynamic contribution or work function for all of these proton transfers is consistent with the view that formation and breaking of hydrogen bonds in such water chains is a limiting factor for proton translocation.

Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Studies of X-Irradiated Crystals of Cytosine Hydrochloride. Part I: Free Radical Formation at 10 K after High Radiation Doses

Anhydrous single crystals of cytosine hydrochloride (protonated at N3) have been X-irradiated at 10 K and studied using K-band EPR, ENDOR and FSE spectroscopy. At least seven radicals were present at 10 K after X irradiation with a dose of about 150 kGy. Two different protonation states of the one-electron reduced cytosine cation were observed: an amino-protonated species (R1) and the pristine one-electron reduced species (R2) with zero net charge. Apparently three deprotonated versions of the one-electron oxidized cytosine cation were formed: the amino-deprotonated cation (R3), an N3-deprotonated cation (R4) and an N1-deprotonated cation (R5). Finally, two products formed by net hydrogen addition to the cytosine base were observed: a C5 hydrogen-addition radical (R6) and a C6 hydrogen-addition radical (R7). The crystalline lattice of cytosine hydrochloride is characterized in part by a cytosine base initially protonated at the N3-position, thus forming a cytosine base cation, and in part by an extended network of hydrogen bonding involving the chlorine anions. Proton transfer properties of pristine one-electron oxidation and reduction base products in this lattice are discussed and are suggested as explanations of the unusual multitude of positions for deprotonation of the one-electron oxidized species as well as for the two protonation states of the reduction product observed. The magnetic parameters for the amino-protonated species R1 agree well with those extracted from previous studies of cytosine derivatives in frozen solutions and in various glasses.

Effect of Substitution on the Photoinduced Intramolecular Proton Transfer in Salicylic Acid

The influence of methyl and methoxy substitution in the para position of the phenolic OH functional group on the intramolecular proton-transfer properties of electronically excited salicylic acid (ESIPT) has been investigated both in solution and in the isolated gas-phase conditions provided by supersonic cooling. The dual fluorescence observed for 5-methylsalicylic acid (5-MeSA) in alkane solutions has been attributed for its blue part to the excited tautomer resulting from the intramolecular proton-transfer process and for its UV component to the dimer. A single fluorescence emission peaking at 400 nm is observed in alkane solutions of 5-methoxysalicylic acid (5-MeOSA). In the presence of proton acceptors such as diethyl ether, the 5-MeSA solution emits only in the blue region while 5-MeOSA exhibits two fluorescence bands at 400 and 475 nm. This behavior shows that the ESIPT process is promoted by complexation with proton-accepting molecules. In the supersonic expansion, the excitation and dispersed emi...

On the Quantum Nature of the Shared Proton in Hydrogen Bonds

The relative influence of thermal and quantum fluctuations on the proton transfer properties of the charged water complexes H5O2+ and H3O2- was investigated with the use of ab initio techniques. These small systems can be considered as prototypical representatives of strong and intermediate-strength hydrogen bonds. The shared proton in the strongly hydrogen bonded H5O2+ behaved in an essentially classical manner, whereas in the H3O2- low-barrier hydrogen bond, quantum zero-point motion played a crucial role even at room temperature. This behavior can be traced back to a small difference in the oxygen-oxygen separation and hence to the strength of the hydrogen bond.

THE NONLINEAR EXCITATIONS AND THEORY OF PROTON TRANSFER IN HYDROGEN BONDED MOLECULAR SYSTEMS

Based on the analysis of properties of protonic excitation and displacements in the hydrogen bonded molecular systems we proposed here a new model Hamiltonian and theory for studying the properties of nonlinear collective excitation and proton transfer resulting from the localized fluctuation of protons and the deformation of structure of heavy ionic sublattice owing to the protonic displacements.We give the equations of motion and corresponding solutions of solitons,discuss the features and meanings of these solutions and the advantages of our model,and explain completely motion of two kinds of defects and proton transfer in the hydrogen bonded molecular systems.

Proton Transfer Properties of Imidazole

The imidazole molecule is paired with NH3 in order to examine the proton transfer properties of the former by ab initio methods. The primary minimum on the surface is ImH+···NH3 wherein the inter-nitrogen distance in the H bond is 2.89 Å. A second well appears in the surface, corresponding to Im···+HNH3, and the barrier between the two minima is rapidly enlarged, when the latter distance is elongated. When the NH3 is displaced from the N lone pair direction of the imidazole in the plane of the latter, the greater proton-attracting power of the imidazole relative to NH3 is enhanced; the opposite is observed when the NH3 is pulled out of the imidazole plane. This distinction is explained simply on the basis of the dipole and quadrupole moments of imidazole. The ability of imidazole to act as a proton shuttle from one molecule to another is examined by placing one NH3 molecule on either side. Taking H3NH+···ImH···NH3 as a starting point, the simultaneous transfer of two protons to form H3N···HIm···H+NH3 must overcome a large energy barrier. A stepwise process, passing through the H3N···HImH+···NH3 intermediate, is greatly favored energetically. If the central imidazole is permitted the freedom to translate between the two NH3 molecules, it is possible for the latter to be quite some distance apart. The imidazole will first approach within about 2.65 Å of the donor H3NH+ ion. The transfer to imidazole can then take place with little or no energy barrier. The protonated imidazole will then move close to the receptor NH3 before depositing the proton with it. In most cases, the largest energy barrier is associated not with the proton transfers, but with the motion of the imidazole cation. The barrier for this translation grows as the ultimate donor and acceptor NH3 molecules are moved further apart.

18] Proton transfer in carbonic anhydrase measured by equilibrium isotope exchange

Kinetic and structural studies demonstrate the pathways of inter- and intramolecular proton transfer in the hydration of CO2 catalyzed by carbonic anhydrase. A histidine residue at position 64 is the most efficient of the carbonic anhydrases, carbonic anhydrase II, and acts as a transfer group shuttling protons between the buffer in solution and the zinc-bound water at the active site. This conclusion is confirmed by mutations to replace His-64 that results in the inactivation of catalysis. Mutations to introduce His-64 in the less efficient carbonic anhydrase III results in activation, with the mutant showing proton transfer properties, similar to the efficient form of the enzyme. In a series of site-specific mutants of isozyme III, the rate constants for proton transfer obeys a Brønsted correlation and show sharp curvature characteristic of facile proton transfers. Application of Marcus rate theory shows that this proton transfer has the small intrinsic energy barrier (near 1.5 kcal/mol) characteristic of rapid bimolecular proton transfer between nitrogen and oxygen acids and bases in solution. The observed overall energy barrier (near 10 kcal/mol) for proton transfer in the catalysis indicates the involvement of accompanying energy-requiring processes, such as solvent reorganization or conformational change. Some of this barrier is likely to involve the formation of hydrogen-bonded water in the active-site cavity through the intramolecular proton transfer that occurs.

Proton inclusion properties of a new azamacrocycle. Synthesis, characterization and crystal structure of [H3L][Cl]3·2H2O (L = 4,10-dimethyl-1,4,7,10-tetraazabicyclo [5.5.2] tetradecane)

Abstract Macrocyclic compounds have been extensively investigated, the interest for this huge class of compounds spans from fundamental research to specific industrial applications. 1 – 8 We have recently published a few papers on small macrocycles having cage-like molecular topology.9 – 19 They present special proton-transfer properties and in few cases they are strong, selective lithium binders in aqueous solution.14,15,17 The three-dimensional cavity present in these compounds is achieved by connecting two secondary nitrogen atoms of a tetraazamacrocycle with different bridging groups (see Fig 1). Molecular topology and nature of donor atoms are the most important elements influencing the chemical properties of this class of compounds. For these reasons we are continuing our investigations on these compounds by systematically changing the bridging unit; we report here the synthesis and chemical properties of the new cage 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]-tetradecane, abbreviated as L.

Synthesis and characterization of an aza-cage, basicity behaviour and crystal structure of its diprotonated species

The synthesis and characterization of the macrobicycle 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.5]-heptadecane (L) is reported. The proton-transfer properties of L in aqueous solution have been, investigated by potentiometry (25 °C, I= 0.15 mol dm–3) and NMR spectroscopy (1H and 13C). L behaves as a fairly strong base in the first protonation step (log K1= 11.55), and as a moderate base in the second step (log K2= 6.94). 1H and 13C resonances have been assigned for both [HL]+and [H2L]2+ species. NMR experiments also indicate the stepwise protonation sites. Crystals of [H2L]-[ClO4]2 are orthorhombic, space group Pca21 with a= 13.130(1), b= 16.185(4), c= 20.320(6)Å, and Z= 8; final R value of 0.086 (Rw= 0.079) for 2383 unique observed reflections with I > 3σ(I). Two independent [H2L]2+ cations are present in the unit cell; they differ from each other in the conformation of the hydrocarbon chain connecting the two unmethylated nitrogen atoms. In both cases the overall cage conformation is due to the presence of two internal hydrogen bonds, connecting the two bridgehead nitrogens and the two methylated nitrogens.

A theoretical analysis of excited state proton transfer in 3-hydroxyflavone. Promoting effect of a low frequency bending mode

The dynamical properties of excited state intramolecular proton transfer in 3-hydroxyflavone have been analyzed on the basis of the time evolution of the quantum states of the two isomeric forms. Potential energy surfaces have been computed at the MNDO/AM1 level. The results shed light on the essential features of the proton transfer mechanism: in particular, the rapidity of the process is to be attributed to the promoting effect of a low frequency bending vibration, which shortens the distance between donor and acceptor atoms.

Novel Multichromophoric Dyes for Chemical, and Physical Switching

Abstract Functionalized photochromic compounds and their applications in chemical and physical switching processes are under investigation. In this report exclusive consideration is given to the dihydroazulene @HA)/vinylheptafulvene (VHF) system ½. A short description of the photochromism of ½. is presented. Three topics of potential switching behaviour released by the photochromism of ½. are pointed out: (i) change of the pka and of proton transfer properties in anilinium derivatives, (ii) change of the redox potential in compounds with electron transfer active subunits, (iii) increase of second‐order nonlinear optical properties by irradiation of 1.

Synthesis and characterization of the aza-cage 4-benzyl-10,15-dimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L). Its proton transfer properties and lithium complex. The crystal structure of the monoprotonated salt [HL][BPh4

The synthesis and characterization of the small, new benzyl aza-cage 4-benzyl-10,15-dimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L) are reported. The basicity constants have been determined in aqueous solution by potentiometry (25 °C, [NaCl] 0.15 mol dm–3). L behaves as a biprotic base: log K1= 11.8, log K2= 8.3. The crystal structure of the monoprotonated salt [HL][BPh4] has been determined by single-crystal X-ray analysis. The compound crystallizes in an orthorhombic unit cell (space group Pc21n, Z= 4) with lattice constants a= 11.400(4), b= 16.893(3), c= 20.537(6)Å. Least-squares refinement converged at R= 0.078 (Rw= 0.056) for 1290 unique reflections with I > 3σ(I). The structure of [HL]+ shows the five nitrogens atoms in an endo configuration, indicating that the proton is inside the cage cavity. The cage L encapsulate Li+ and the complex formation equilibrium for [LiL]+ has been investigated by potentiometry [log K= 3.0(1)] and 7Li NMR techniques.

Small Synthetic Macrocyclic Cages. Proton Transfer Properties and Metal Complexes

Abstract The synthesis, characterization and ligational properties of a series of small macrocyclic cages are discussed.