A kinetic study of labile metal complexes: a model enzyme system.

The mixed ligand (ternary) complexes considered in this thesis are intended to represent an enzyme-metal-substrate system. Both zinc(II) and magnesium(II) occur widely in hydrolytic enzymes and in these enzymes ternary complex formation is often important. In this work an attempt has been made to compare the kinetics of ternary complex formation for magnesium and zinc and to investigate the modifying nature of bound ligands on the kinetics of complex formation.

The following reactions have been investigated in aqueous solution:

LZⁿ⁺ + PADA ⇌ ^kf _kb LZn(PADA)ⁿ⁺

where PADA is the neutral bidentate ligand pyridine-2-azo-pdimethylaniline and L is one of the following ligands: diethylenetriamine (dien), triethylenetetramine (trien), 2,2',2" triaminotriethylamine (tren), nitrilotriacetate (NTA³⁻), cystein (cyst²⁻), iminodiacetate (IDA²⁻), ethylenediaminediacetate (EDDA²⁻) and tripolyphosphate (TP⁵⁻). The reaction between aquo zinc and PADA has also been investigated. The reaction between 5-nitrosalicylic acid (NSA) and the ZnL complexes where L represents uramil NN diacetate (UDA³⁻) and N,N'-bis(2-aminoethyl)-1,3-propanediamine(2-3-2 tet) and the ligands used for the PADA work except cysteine have been investigated. No evidence for ternary complex formation was found for the reaction between Zntren2+ and PADA or NSA. The reactions of aquomagnesium, magnesium-NTA, -UDA, -adenosines' -triphosphate (ATP²⁻) and -TP with NSA have also been investigated.

The kinetics of the reaction of NSA with metal ions are consistent with a two path mechanism involving the addition of the metal species to either the NSA dianion or the monoprotonated species.

The temperature-jump relaxation method was used to measure the formation and dissociation rate constants over a range of temperature, and the activation enthalpies (ΔS‡) and entropies (ΔS‡) associated with these rate constants have been determined.

The pKs of NSA have been determined spectrophotometrically as have the stability constants of LZn(PAM) and LM(NSA) for the majority of the above systems.

In general, the formation rate constants and the associated activation parameters for the magnesium systems are consistent with a diffusion-controlled pre -equilibrium step involving the formation of an outer sphere complex and the rate limiting dissociation of a water molecule from the inner coordination sphere of the metal« The data for the zinc systems are not consistent with such a mechanism« Here the rate at which the ligand enters the inner-sphere of the metal and the rate of closure of the chelate ring are thought to be important. The results are discussed in terms of the number and types of group bound to the metal in the ligand, L, and hence the stereochemistry of the ML complex.

The stereochemistry of a number of zinc ammines has been investigated using an empirical correlation between the zinc-nitrogen stretching frequency and the coordination number. The frequency of the principal Zn-N stretching mode at about 450cm⁻¹, as determined by the laser Raman method,, is reported for complexes of zinc containing the ligands ethylenediamine, diethylenetriamine, triethylenetetramine, 2 ,2 ',2 "-triaminotriethylamine, tris(2-dimethyl(aminoethyl)- amine, N,N?-dimethylethylenediamine and N,Nf-bis(2-aminoethyl)- 1 ,3-propanediamine(2-3-2 tet). The correlation between the frequency of the Zn-N stretching mode seems to be independent of the phase, the nature of the other groups in the inner coordination sphere of the metal and the other groups bound to the coordinating nitrogen.

It has been suggested that the reason why Co(II) can be substituted into a number of Zn(II) metalloenzymes without complete loss of activity is due to the ability of Co(II) like Zn(II) to adopt a number of different configurations of its inner coordination sphere. Although no concrete conclusions may be drawn from a comparison of the kinetics of ternary complex formation of Co(II) and Zn(II) species it is interesting to note the similar pattern found for the formation rate constants of Co(II) and Zn(II) species with PADA