Pyrophosphate, formation constants with

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

Weak complexes form between vanadate and phosphate, 9,99 pyrophosphate,59 arsenate,59 and chromate.106 The formation constant for the simple phosphovanadate is about 20 M 1." Structural characterization has been reported for interesting cluster structures of vanadium with organophosphonates107 and with sulfate.108 Divanadium(V) complexes with either monodentate or bidentate nitrate (N03 ) groups have been reported.109... 

The complexation of Ni with partially protonated pyrophosphate ion was studied as a function of ionic strength (KNO3, 0.02 M to 0.20 M), temperature (5 to 35°C) for 0.1 M KNO3 in solutions with pH values from 4.5 to 7.0. Values are reported for the formation constants of NiPjO, and NiHPjO,. ... 

Figure A-24 Variation of formation constants from Perlmutter-Hayman and Secco [73PER/SEC] with ionic strength. For the first and second protonation constants of pyrophosphate, = - 8 and - 6 for Reactions (A.50) and (A.51), = - 16 and - 12.

Adenosine 5 -triphosphate (ATP) hydrolyzes via the diphosphate (rate constant k ) and monophosphate ( 2)) ot directly to the monophosphate with the pyrophosphate formation (k ). One mole of pyrophosphate then hydrolyzes to 2 mol of phosphate ( 4) ... 

Vitamin B2 (thiamine) and the thiamine pyrophosphate undergo typical complex formation with tryptophan and other indoles [121-123]. Association constants of such complexes increase as electron-donating groups are substituted, while they decrease with increasing ionic strength, which indicates little dative character in the ground state [38,121-123]. 

There is a general requirement for pyridoxal-5-phosphate (24, 25, 27, 44) although not all of the activity lost on dialysis is restored by adding the cofactor. This requirement explains the inhibition by hydroxylamine and hydrazine (24, 25). The reaction is a typical pyridoxal-5-phosphate catalyzed a,/ -elimination with a mechanism similar to serine dehydrase and cysteine desulfhydrase (45). The coenzyme is probably bound as a Schiff base with an amino group of the enzyme since there is an absorption maximum at 415 nm in solutions of the purified garlic enzyme (40). The inhibition by L-cysteine is presumably caused by formation of a thiazolidine with the coenzyme (46). Added pyridoxal-5-phosphate also combines directly with the substrate. The dissociation constant for the complex is about 5 X lO M. When this is taken into account, the dissociation constant of the holoenzyme can be shown to be about 5 X 10 M (47). The higher enzyme activity in pyrophosphate buflFer than in Tris or phosphate may be explained by pyrophosphate chelation of metal ions which otherwise form tighter complexes with the substrate and coenzyme (47). This decreases the availability of added coenzyme. 

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