Proton release complex

Photoisomerization of retinal from the all-trans to the 13-ct.v form leads to a cyclic photoreaction with intermediates, bR, K, L, M, N, and O, which are subsequently formed before recovery of the initial state, as schematically illustrated, with the absorption wavelength in the suffix (Fig. 5), together with their individual lifetimes.50 51 The first proton transfer in the photocycle of the all-trans, 15-anti isomer is from the retinal Schiff base to Asp 85 at the central part of the protein. Then, a proton is released from the proton release complex (PRC) consisting of Glu 204, 194 and a bound water molecule at the extracellular surface, as illustrated by arrow A. This is followed by reprotonation of... 

It was suggested that translocation of a proton from the cytoplasmic surface to the extracellular surface is associated with a protein conformational change.It has been recognized that the first proton transfer in the photocycle of bR is from the protonated Schiff base to the anionic Asp 85, in the L to M reaction. This protonation induces proton release from the proton release complex (PRC),... 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

Figure 6. Pathways of protons and electrons during mitochondrial oxidations. The diagrams show the pathways of electrons which enter the electron chain at the level ofcomplexi (a)orcomplex II (b). Complexes I, III, and IV usethefreeenergy of electron transport to pump protons out of the matrix. This diagram also distinguishes formally between protons released by dehydrogenation and those which are pumped out of the matrix, although they all enter or leave the same pool.

First of all, the mesomerism of HBI is rendered complex by the presence of several protonable groups actually, HBI might exist, depending on pH, under cationic, neutral, zwitterionic, anionic, and possibly enolic forms (Fig. 3a). The experimental p/sTa s of model analogs of HBI in aqueous solutions have been studied. Titration curves follow two macroscopic transitions at pH 1.8 and pH 8.2, each corresponding to a single proton release [69]. Comparison of theoretical... 

The Kurbatov plot is a convenient tool to display in a simple way adsorption (surface complex formation) data. But care must be exercised in the interpretation of the data, because n varies with pH and may vary with the adsorption coverage. For an exact analysis of the proton release stoichiometry, see Hohl and Stumm (1976) or Honeyman and Leckie (1986). 

SOH represents any surface site unassociated with any species of M, SOM a metal/surface-site complex and x the apparent ratio of moles of protons released or consumed per mole of adsorbate removed from solution. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

Cation adsorption is accompanied by release of protons with the number of protons released per cation being termed z. Ideally, z should be one for mononuclear complexes and two for binuclear complexes, but in fact, the value is often found to be between one and two. In addition, the value of z may also vary between these limits as the pH rises. The occurrence of intermediate values has been attributed to the simultaneous formation of binuclear complexes and mononuclear species and to the presence of different surface sites (Hohl Stumm, 1976 Benjamin Leckie, 1891 Schindler, 1984). 

Mn(III) r Mn(IV) transition, 33 226-227 Mn-K edge spectra, 33 228-229 Oj-evolving complex, 33 221 proton release pattern, 33 235 rebinding, 33 223 spin echo studies, 33 233-234 S state model, 33 221 temperature dependence of EPR signals, 33 231-232... 

Completely different behavior toward liquid NH3 is shown by the three iron carbonyls Fe(CO)s, Fe3(CO)9, and Fes(CO),2 (98, 99) and the two cobalt carbonyls Co2(CO)8 and Co4(CO)i3 (100). Between -21 and 0°C, Fe(CO)5 and liquid NH3 give a homogeneous, pale-yellow solution from which Fe(CO)5 may be recovered on evaporating off the NH3. The solution contains the carbamoyl complex NHJfOC Fe—CONHJ which cannot be isolated and which is formed by nucleophilic attack of an NH3 molecule on a CO ligand, followed by proton release (101). At 20°C after 14 days of reaction, (NHJ FefCOlJ and CO(NH2)2 are obtained (99) ... 

The fate of the Mn fragment is not clear, but it is conceivable that if an unobserved Mn-H species forms it could be immediately proto-nated to the H2 complex by the protons released from silane hydrolysis. The labile H2 could then in turn be displaced by CH2C12 solvent to regenerate [Mn(CO)3 P(OCH2)3CMe 2(CH2Cl2)]+ [Eq. (38)]. 

Fig. 6 a, b. Acceleration of dissociation of poly(carboxylic acid)s according to the complex formation with various polycations, (a) Potentiometric titration of poly(methacrylic add) (PMAA) and its complexes (b) Amount of protons released from poly(carboxy]ic add) in the formation of polyelectrolyte complexes. Polyanions PMAA and poly(acrylic add) (PAA). Polycations and their low molecular weight analogues ... 

A profound change of metal binding is observed when nucleosides are added to solutions containing transition metal ions. The free bases have lost one proton-releasing nitrogen atom because of the attached ribose residue. The formation of metal chelates is disturbed. The ribose moiety may cause steric hindrance as well as an inductive effect on the heterocyclic base (41). Therefore, the stability constants of the resulting complexes are expected to be much smaller. Table 2 presents the numeric values of the stability constants of some nucleoside transition metal complexes. 

The electrochemical transformation of a molybdenum nitrosyl complex [Mo(NO)(dttd)J [dttd = 1,2-bis(2-mercaptophenylthio)ethane] (30) is rather interesting (119). Ethylene is released from the backbone of the sulfur ligand upon electrochemical reduction. The resulting nitrosyl bis(dithiolene) complex reacts with O2 to give free nitrite and a Mo-oxo complex. Multielectron reduction of 30 in the presence of protons releases ethylene and the NO bond is cleaved, forming ammonia and a Mo-oxo complex (Scheme 15). The proposed reaction mechanism involves successive proton-coupled electron-transfer steps reminiscent of schemes proposed for Mo enzymes (120). 

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