Proton conduits

Proton conduits can also catalyze isomerization. A biologically interesting example is the isomerization of DNA bases such as guanine, as illustrated in Fig. 29.17. These molecules occur is several isomeric forms that differ in the position of one or more of the protons. Water or alcohol molecules can form an OH bridge connecting an occupied with an unoccupied position, which may lead to formation of an isomer. For the isomerization depicted in Fig. 29.17, a single water mol- 

The dashed lines represent the rate constants obtained by transition state theory. The two (close) sets of data for the mixed isotope combinations HD and DH reflect the asymmetry of the barrier. 

To illustrate the relative efficiency of long and short proton conduits, we return to the 7-azaindole, molecule, whose dimer was briefly discussed in Section 29.7. As pointed out in Section 29.7, excitation of this molecule to the fluorescent state redistributes the charge between the two nitrogens. This redistribution renders the tautomeric form produced by the excitation unstable relative to the form in 

In the dimer the excitation is essentially localized on one partner and the unexcited partner serves as an effective proton conduit. 

The same occurs in the monomer if water is present. Water molecules form hydrogen-bonded complexes with 7-azaindole, two of which are illustrated in Fig. 

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These calculations illustrate two aspects of the role of hydrogen-bonding solvents in proton transfer processes. On the one hand they catalyze these processes by forming proton conduits between donor and acceptor atoms, and on the other they provide a dielectric medium that stabilizes ionic or highly polar intermediates. Representation of the solvent by a dielectric continuum cannot account for this dual role. To treat the transfer process adequately, it will be necessary to introduce a primary shell of discrete solvent molecules into the calculations. 

A small isotope effect for proton transport through a water chain is also observed for catalytic conversion of CO2 to HCO by carbonic anhydrase II [68, 69]. The rate-determining step in this process is the transfer of a proton from the H2O ligand of a four-coordinated zinc ion to a histidine residue located at a distance of about 8 A. The proton conduit is known to consists of water molecules located in a pocket that can contain several such molecules, which are freely exchanged with... 

Conservation of the triad in methionine synthases suggested its possible role as a proton conduit (2). Reduction of cob(II)alamin MetH to cob(I)alamin (38) or attack on the methylcobalamin species by Hey (22) is accompanied by uptake of a proton, and proton uptake is perturbed by mutation of Asp757 (22). Protonation of the His-Asp pair serves to facilitate the dissociation of the histidine ligand that must occur in formation of the intermediate cob(I)alamin... 

En particulier, on a ie conduit 4 penser que, pour differents donneurs de protons les frequences de valence V(XRb mesurdes par spectroscopic infrarouge en solution diluee dans un accepteur , eorrespondaient aux complexes donneur-accepteur , Nous avons entrepris dans cette perspective de completer nos travaux anterieura par une etude plus general des derives substitu s du benzene. 

The cytochrome c oxidase protein is thought to consist of two heme iron centers (heme a with two axial histidines and heme 03 with one axial histidine (analogous to myoglobin)) and two copper centers (Cua with two histidine, two cysteine, and one water/tyrosine ligand in its oxidized state and Cub with three histidine, one methionine, and one H2O/HO" ligands). The CuA/heme a pair constitute two coupled, one-electron redox couples (low potential, 0.4V) that facilitate (a) electron transfer from cytochrome c(Fe ) at the matrix side of the inner mitochondrial membrane as well as (b) proton transfer from the mitochondrial matrix across the inner membrane to the cytosol. At the cytosol side of the inner mitochondrial membrane, the CuB/heme a- pair constitute the binding site for O2 as well as the conduit for its high-potential four-electron, four-proton reduction to two H2O molecules. 

In the basic transport unit of Fig. 1, Mq and Mi were required to be permeable to and water and to enable H" to enter them and move along their internal surfaces by the specific transport mechanism described in Section 3.1. In Section 3.5, the apoplasm between Mq and Mq was also assumed to show proton conduction. Thus two xylem conduits with their walls in contact with each other constitute, together with the pits between them as the organismal capillaries, a variation of the basic transport unit. The wall phase without the pits may be considered as having an appreciably lower conductance for H", K", and water, while the pits form... 

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