is Cl, Br, or I) obey a Curie law with values of 1.37 (Cl),• 1.26 (Br), and 1.05 (I) BM. The low moments for these complexes were accounted for on the basis of distortion from octahedral symmetry and electron delocalization. It is quite reasonable that similar effects are present in Nb(NCS)4(py)2 and are responsible for the observed moment.

Oxygen-1 7 line broadening measurements have been made on the MnATPz-complex in aqueous solution. The results are interpreted in terms of three kinetically equivalent waters being present in the inner sphere of the complex. The kinetic parameters for water exchange (per water molecule) are: k,,(25") = 5.0 x 107 sec-'; AH* = 9.6 kcal mol-'; AS* = 8.8 cal mol-! deg-1. These parameters are compared with published kinetic results for the substitution of 8-hydroxyquinoline into the inner sphere of MnATP2-and it is R ecent temperature-jump studies on the kinetics of manganese(I1) (Hague and Zetter, 1970; Hague et a/., 1972) and magnesium (Hague and Eigen, 1966; Hague et a / . , 1972) ternary complex formation and dissociation revealed several interesting results which may be significant in the understanding of Mg2+ and Mnz+ activation of enzymes. Of particular interest were the reactions of MnATP2-and MgATP2-with the anion of 8-hydroxyquinoline (Ox-), i.e.where M is Mg2+ or Mn2+. The rate of formation in this type of reaction is generally described (Hewkin and Prince, 1970) in terms of where KO, is a n outer sphere formation constant, dependent, in part, on the charge product of the reacting species, and k,, is the rate constant for the exchange of a solvent molecule between a metal ion (or complex) and the bulk solvent. For reaction 1, kf and the associated thermodynamic activation parameters for MgATP2-were consistent with eq 2 if it was assumed that k,, for MgATP2-was similar to that measured for Mg(H20)62+ (Neely and Connick, 1970). This was not the case for MnATP2-; kf was lower and the activation enthalpy higher than expected, suggesting that either k,, was lower than that measured for Mn(Hz0)62+ (Grant er al., 1971) or some other process-possibly a change in the type of coordination of the ATP to the Mn2T-was rate determining.The structure of the MnATP2-complex in aqueous solution has been the subject of much discussion. The picture that has emerged from numerous nmr studies indicates (1) that the a-, 8-, and y-phosphate groups (Cohn and Hughes, 1962; concluded that the generally considered mechanism for metal complex formation does not apply in this case. The discrepancy between these two sets of kinetic data is discussed in terms of possible binding differences of ATP in MnATP2-and 8-hydroxyquinoline-MnATP3-. It is suggested that these proposed differences can rationalize, in part, the similar behavior of Mn2+ and Mg2+ in the enzymatic activation of transphosphorylation reactions involving ATP.Sternlicht et al., 1965) and the adenine ring are bound to manganese, the latter being separated from the metal by an inner-sphere water molecule (Glassman er a/., 1971), and (2) that the complex contains three rapidly exchanging waters (Glassman et al., 1971).An I7O nmr study can further our knowledge of the properties of the MnATP2-complex in aqueous solution. Firstly, this method can sometimes give an indication of the number of water molecules in the complex. Secondly, it is sometimes possible to distinguish betw...

No abstract

The temperaturejump relaxation method has been used to measure the activation parameters for the formation and dissociation of the 1 : 1 complexes between magnesium and manganesem) and 8-hydroxyquinoline (oxine) and of the ternary complexes between oxine and the magnesium and manganese@) complexes of nitrilotriacetate, uramil-NN-diacetate, adenosine-S-diphosphate, adenosine-5'-triphosphate and polytriphosphate. The results strengthen the previous conclusion that the first ligand has comparatively little influence on the kinetics of the reaction of magnesium with a secondligand,whereas manganesem) shows similarities to zinc in the reiactivityof its complexes. The enzymatic behaviour of magnesium and manganese(I1) is discussed in the light of these results.

The temperature-jump relaxation method has been used to measure the kinetics of formation and dissociation of the 1 : 1 complex between manganese(l1) and 8-hydroxyquinoline (oxine) and of the ternary complexes between oxine and the manganese(1T)-nitrilotriacetate, -uramil NN diacetate, -adenosine-5'-triphosphate and -poIytriphosphate complexes. In all cases the data are consistent with a two-path mechanism involving the addition to the manganese species of the oxine anion or the oxine molecule. The first ligand has comparatively little influence on the rate at which manganese reacts with and dissociates from the second ligand. The observed enzymatic behaviow of manganese(II), magnesium and calcium is discussed in the light of these results.