Proton covalently bound

Scheme 4.10 Protons covalently bound to the matrix in potyfvinylphosphonic acid).

Figure 12.S Schematic diagram of the bacteriorhodopsin molecule illustrating the relation between the proton channel and bound retinal in its tram form. A to E are the seven transmembrane helices. Retinal is covalently bound to a lysine residue. The relative positions of two Asp residues, which are important for proton transfer, are also shown. (Adapted from R. Henderson et al.,...

Complex 1 850 kDa (probably a dimer in membrane) About 40 1 FMN covalently bound, bound 16-24 Fe-S atoms in 5 to 7 centers Spans membrane, NADH site on matrix face, UQ site in membrane 0.06 UQ Pumps protons out of matrix during electron transporl/2e"... 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

The following theoretical and experimental data is presented to support the idea for activation of the BPDEs in an acid catalyzed Sjj2 reaction for trans addition (90) with some contribution to car-bonium ion formation in an Sjjl reaction for both trans and cis addition. The non-covalently bound BPDE adducts to DNA formed initially are intercalation complexes (H6,51-55) Meehan et al. (U6) and Geacintov et al. (5 0 reported that the BPDE intercalates into DNA on a millisecond time scale while the BPDE alkylates DNA on a time scale of minutes. Because the electrostatic potential is most negative in the grooves, positive ions are attracted to these regions (93,9, 95) Consequently, protonation of the BPDE l(+) to form the... 

On coordination, the porphyrin macrocycle loses two protons (to yield a neutral complex when the central metal ion is divalent). The extensive electron delocalization throughout the ligand will normally extend to the central metal when the latter is covalently bound to the porphyrin. As expected, such complexes are extremely stable this is undoubtedly important to the biological role of these complexes. 

The mechanism of NO release from N-diazeniumdiolates is depicted in Fig. 3.7. If R>, of the generic structure shown at the top is a cation, NO is generated spontaneously on protonation of the anionic portion along with the formation of dialkylamine. If R3 is covalently bound, it must be removed first to free the anion before spontaneous... 

Bridged polysilsesquioxanes having covalently bound acidic groups, introduced via modification of the disulfide linkages within the network, were studied as solid-state electrolytes for proton-exchange fuel cell applications.473 Also, short-chain polysiloxanes with oligoethylene glycol side chains, doped with lithium salts, were studied as polymer electrolytes for lithium batteries. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

The topologically defined region(s) on an enzyme responsible for the binding of substrate(s), coenzymes, metal ions, and protons that directly participate in the chemical transformation catalyzed by an enzyme, ribo-zyme, or catalytic antibody. Active sites need not be part of the same protein subunit, and covalently bound intermediates may interact with several regions on different subunits of a multisubunit enzyme complex. See Lambda (A) Isomers of Metal Ion-Nucleotide Complexes Lock and Key Model of Enzyme Action Low-Barrier Hydrogen Bonds Role in Catalysis Yaga-Ozav /a Plot Yonetani-Theorell Plot Induced-Fit Model Allosteric Interaction... 

A catalytic cycle is composed of a series of elementary processes involving either ionic or nonionic intermediates. Formation of covalently bound species in the reaction with surface atoms may be a demanding process. In contrast to this, the formation of ionic species on the surface is a facile process. In fact, the isomerization reaction, the hydrogenation reaction, and the H2-D2 equilibration reaction via ionic intermediates such as alkyl cation, alkylallyl anion, and (H2D)+ or (HD2)+ are structure-nonrequirement type reactions, while these reactions via covalently bound intermediates are catalyzed by specific sites that fulfill the prerequisites for the formation of covalently bound species. Accordingly, the reactions via ionic intermediates are controlled by the thermodynamic activity of the protons on the surface and the proton affinity of the reactant molecules. On the other hand, the reactions via covalently bound intermediates are regulated by the structures of active sites. 

Lanthanide complexes of mono- and tetra-amide /1-cyclodextrin derivatives of DOTA have been characterized [140]. The proton NMR spectra of the Eu3+ complexes in methanol-d, show that, while the tetra-amide complex occurs in solution exclusively as a C4-symmetry SAP structure, the mono-amide complex, with less than C4-symmetry, occurs predominantly as two SAP isomers (A/XXXX and Al8885), with the presence of a small amount of the twisted SAP isomer. Luminescence and relaxivity measurements confirm that the Eu3+, Tb3+ and Gd3+ complexes of the eight-coordinate mono-amide ligand possess one bound water molecule, while the tetra-amide complexes have q = 0. The relaxivity of the /LCD mono-amide Gd3+ complex is enhanced when non-covalently bound to a second Gd3+ complex bearing two phenyl moieties (MS-325, AngioMARK , EPIX/Mallinckrodt). 

Activation reactions catalyzed by serine proteases (including kallikreins) are an example of limited proteolysis in which the hydrolysis is limited to one or two particular peptide bonds. Hydrolysis of peptide bonds starts with the oxygen atom of the hydroxyl group of the serine residue that attacks the carbonyl carbon atom of the susceptible peptide bond. At the same time, the serine transfers a proton first to the histidine residue of the catalytic triad and then to the nitrogen atom of the susceptible peptide bond, which is then cleaved and released. The other part of the substrate is now covalently bound to the serine by an ester bond. The charge that develops at this stage is partially neutralized by the third (asparate) residue of the catalytic triad. This process is followed by deacylation, in which the histidine draws a... 

The proton donated from the OH group of Ser 195 to His 57 is then donated to the N atom of the scissile bond, cleaving the C-N peptide bond (or the C-O ester bond) to produce the amine and the acyl-enzyme intermediate. The amine is that part of the substrate which follows the scissile bond in the sequence the acyl-enzyme intermediate is the remaining fragment covalently bound to Ser 195. 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

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