Hydroxoaquo(3,3′,3″-triaminotripropylamine)cobalt(III) ion. A highly effective reagent for promoting of hydrolysis of phosphate species.

Abstract

In earlier work we demonstrated the effectiveness of tn 2Co III in promoting the hydrolysis of phosphate esters [1] and polyphosphate [2–4] [tn = H 2N(CH 2) 3NH 2; (aq) = (OH 2) 2, (OH)(OH 2) or (OH) 2 depending on pH; charges omitted]. Such studies can provide insight into possible roles for metal centers in biological phosphoryl transfer. Under appropriate conditions tn 2Co III(aq) produces, on direct addition to phosphate species, accelarations in hydrolysis of up to ∼10 5. Using the tripodal ligand trpn [=N(CH 2CH 2CH 2NH 2) 3] we have now prepared trpnCo III(aq); this reagent is significantly more effetive than tn 2Co III(aq) in promothing hydrolysis of phosphate esters and polyphosphates. Here we compare the reactivities of trpnCo III(aq) and tn 2Co III(aq) towards adenosine 5′-diphosphate (ADP), bis(2,4-dinitrophenyl) phosphate [2,4-DNPP] [5]. For hydrolysis studies on ADP (20 °C) the N 4Co III)ADP 1:1 complex was pre-formed at pH 4.0 prior to addition of further N 4Co III(aq) and pH adjustment to 7.0: reaction was followed by 31P NMR analysis of solutions quenched with NaOH [3,4]. Hydrolysis studies on (2,4-DNP) 2P and 2,4-DNPP were at 25 °C (μ = 0.50 M) with direct mixing of ester and N 4Co III(aq) solutions;reactions was followed spectrophotometrically from production of 2,4-dinitrophenolate. Table 1 compares rates for hydrolysis of ADP to AMP and P i for the two cobalt reagents. In both cases the 1:1 complex is formed almost quantitatively and is of low reactivity, while excess N 4Co III(aq) leads to marked increases in rate. The most noticeable new feature is the much higher reactivity observed for trpnCo III(aq) compared to tn 2Co III(aq). At the highest ratio studied (8:1) the half-life for the trpn system is <1 min, which corresponds to an accelaration over the unpromoted reaction of >1 × 10 6 [7]. t001 ADP hydrolysis Promoted by N 4Co III a Co/ADP k × 10 4 (sec −1) k trpn/k tn 2 trpnCo(III)(aq) tn 2Co(III) (aq) b 1:1 ⩽0.2 0.2 ⩽1 2:1 8.2 2.3 3.6 4:1 42.0 7.3 5.8 8:1 >150.0 13.4 >10.0 a ADP = 0.020 M, 20.0 °C, pH 7.0 1:1 complex pre-formed at pH 4.0 and 20 °C for 60 min at a concentration 2.3 M for both reactants. b Results from ref. 4. Figure 1 summarize results for the hydrolysis of 2,4-DNP and (2,4-DNP) 2P in the presence of the trpnCo III(aq). At the pH corresponding to maximum rates [which approximates the pH at which trpnCo(OH)(OH 2) 2+ is maximized in solutions of trpnCo III(aq)] the diester reacts 25 times faster than in the presence of tn 2Co III(aq) at its pH maximum (otherwise comparable conditions); a similar comparison for the monoester trpn over tn 2 by a factor of 30 [8]. ▪ It may be emphasized that in these experiments the rate is not close to saturation for the trpnCo III(aq) concentration used. The results therefore imply specific rate constants for the more active complexes which are much larger than the rate constant reported here. The extraordinary effectiveness of trpnCo(OH)(OH 2) 2+ in promoting these phosphoryl transfer reaction relates to: (i) the cis requirements for the coordinated waters; (ii) the rapidity with which complexation takes place through water substitution; (iii) the involvement of complexes containing more than one cobalt center; (iv) the availability and reactivity of coordinated hydroxide on a suitably positioned cobalt.