Second sphere complexes

Here MX, Y designates an outer sphere or second sphere complex. There is every reason to suppose that formation and dissociation of MX, Y occurs at rates approaching the diffusional-control limit so that the slow conversion to MY is a negligible perturbation on the equilibrium of the first step. There is a similarity here the Langmuir, the Michaelis-Menten and the Lindemann-Hinshelwood schemes. 

Phosphoglucomutase catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate via a phosphoenzyme intermediate in which the phosphate is bound to a serine residue at the active site. The Mg2+ is not bound to the phosphate, but studies with the Nin-substituted enzyme suggest the presence of a second-sphere complex between phosphate and metal, which could ensure correct positioning of the phosphoryl group for nucleophilic attack, as shown in Figure 14.278... 

The second-sphere complex between Mg2 and substrate in phosphoglucomutase... 

Fig. 1. Manganese to phosphorus distances in inner sphere, distorted inner sphere, adjacent pyrophosphate and second sphere complexes from crystallographic and model building studies. Original references are given in (14—16). Corresponding distances in Ca2+ complexes would be 0.2 A greater and in Mg2+ complexes 0.1 A smaller...

The geometry of a kinetically active complex of pyruvate kinase, Mn2+, K+, phosphate, and pyruvate which catalyzes the enolization of pyruvate has been elucidated by 13C and 31P NMR ((45) Fig. 4B)). The distances from Mn to the carboxyl and carbonyl carbon atoms of pyruvate (7.3 0.1 A) establish the substrate to be in the second coordination sphere, while the lower limit Mn2+ to phosphorus distance (4.5 0.4 A) suggests either a distorted inner sphere complex or the rapid averaging of 6% inner sphere complex with 94% second sphere complex. Mechanisms for pyruvate kinase, consistent with the geometries are given in Fig. 4 (15, 43, 45). 

The presence of a metal on a phosphoryl transferring-enzyme provides no assurance that the metal is directly involved in phosphoryl transfer. Thus with alkaline phosphatase, no direct interactions of Cl- with enzyme bound Zn2+ (69) or water with enzyme-bound Mn2+ (70) were detected by nuclear relaxation. Similarly no direct interaction of phosphate with enzyme bound Co2+ (71) or Mn2+ (71, 72) was detected by 31P nuclear relaxation. A Mn2+ to phosphate distance of 7.3 A was calculated from NMR data on the inactive Mn2+-enzyme (73) indicative of a second sphere complex. These results are in accord with crystallographic data on the enzyme which at 7.7 A resolution indicate that substrates cannot easily gain direct access to the metal site (74). More recent proton relaxation studies with the Cu2+ enzyme, which retains 5% of the activity, indicate the presence of a rapidly exchanging axial hydroxyl ligand on Cu2+ suggesting that the active metals may promote the nucleophilicity of the water molecule which is to attack the phosphorus (75). 

This static model for second sphere complex formation is not fully valid and the above values of AH do not have full thermodynamic significance. Thus, if one assumes a static model for the second sphere complex. Equation I enables the relative shifts of the ortho, meta, para, and NH2 protons to be calculated for any position of the aniline. Such calculations give qualitative but not quantitative agreement with experiment. The nature of the disagreement is illustrated by the data in Table I. 

The dependence is closer to I/f than I/r and the closest fit is obtained with a Co-N separation of around 7 or 8A, corresponding nicely to the Van der Waals radii of the molecules. It seems that the correct interpretation of these results is that we should not regard a second sphere complex as a rigid well-defined entity. Rather, there is a prefer-... 

The general trend is maintained but there is substantial variation in the ortho-to-metal ratio, indicating that the fiexibility of the second sphere complexes is varying. 

In a series of papers,Cmmbliss illustrated that the siderophore ferrioxamine-B can be selectively recognized through second-sphere complexation of the protonated amine side chain by different ionophore host molecules. 

Second sphere complexes are sometimes obtained from organometallics and crown ethers. In these cases the crown ethers do not interact directly with the metal center of the organometallic compound, but rather with the ligands coordinated to the metal. The interaction usually involves hydrogen-bonding. 

In addition, formation of ground state complexes between chromophores and non-chromophoric species in solution, so called second sphere complexation, has also been shown to influence the... 

FIGURE 32. View of the [Cu (H20)4(dibenzo-24-crown-8] cation, showing direct Cu-crown ether bonding and the interactivity hydrogen bonding as second sphere complexation Color code. Cu, blue O, red C, green H, purple. Hydrogen bonds are shown as dashed purple lines. (Reprinted with permission from ref 70.)... 

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