Proton release group

Brown, L., Sasaki, J., Kandori, H., Maeda, A., Needleman, R., and Lanyi, J. K. (1995). Glutamic acid 204 is the terminal proton release group at the extracellular surface of bacteriorhodopsin./. Biol. Chem. 270, 27122-27126. 

Table 135.1 shows the protonation states of some of the key residues through the primary and branched photocycle intermediates in BR. Isomerization of the chromophore creates an electrostatic environment that destabilizes the protonated Schifif base. The Schiff base donates a proton to a nearby aspartic acid residue, Asp-85 (D85). Concomitant with the protonation of Asp-85 is a release of a proton from the proton release group, collectively referred to as XH, because the process is still poorly understood. Deprotonation of XH involves the participation of Arg-82, Glu-194, Glu-204, and a hydrogen-bonded network of internal water molecules. The proton movement from the Schiff base to Asp-85 generates the M state, which is blue-shifted, because the Schiff base is unprotonated. The O state is characterized by a deprotonated leaving group and an ah-tram chromophore. The chromophore is reprotonated in the M — N transition by transfer of a proton from another nearby aspartic acid, Asp-96. 

Why is proton release retarded with respect to the oxidation of Z We have found that the apparent pK of the proton-releasing group is 5 (not shown). An acid of pK which is in contact with water is expected to deprotonate much faster (t. see H.Eigen, 1963) than we observe... 

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... 

A Hammett correlation was not observed, due to enhanced conjugation (4) between the electron-releasing groups and the protonated heterocyclic nitrogen. ... 

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. 

The ease of the above reactions is controlled by the electronic effect of substituents and, as one might expect, electron-releasing groups usually promote and electron-withdrawing groups suppress reaction. In the case of an a-substituent the crucial factor usually proves to be its size one or two large a-groups can prevent quaternization by all except protons. 

Recently Benkovic and Schrayl28b and Clark and Kirby,26c have investigated the hydrolysis of dibenzylphosphoenolpyruvic acid and mono-benzylphospho-enolpyruvic acid which proceed via stepwise loss of benzyl alcohol (90%) and the concomitant formation of minor amounts (10%) of dibenzylphosphate and monobenzylphosphate, respectively. The pH-rate profiles for release of benzyl alcohol reveal that the hydrolytically reactive species must involve a protonated carboxyl group or its kinetic equivalent. In the presence of hydroxylamine the course of the reaction for the dibenzyl ester is diverted to the formation of dibenzyl phosphate (98%) and pyruvic acid oxime hydroxamate but remains unchanged for the monobenzyl ester except for production of pyruvic acid oxime hydroxamate. The latter presumably arises from phosphoenolpyruvate hydroxamate. These facts were rationalized according to scheme (44) for the dibenzyl ester, viz. 

Furans substituted with electron releasing groups usually undergo polymerization with mineral acids due to facile protonation at the 2-position. Aluminum chloride also causes resinification of furan but benzo[6 ]furan and compounds with electron withdrawing groups are, however, more stable. The reversion to type so characteristic of the electrophilic substitution of benzene is by no means prevalent in the chemistry of furan and benzo[6]furan and many apparent electrophilic substitutions in reality proceed by addition. 

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