Fifth ligand

FIGURE 15.25 The six liganding positions of an iron ion. Four ligands lie in the same plane the remaining two are, respectively, above and below this plane. In myoglobin. His F8 is the fifth ligand in oxymyoglobin, O9 becomes the sixth. 

Raman spectra were performed by Wang s group100. They found the heme to have a 5-coordinate high spin configuration. The fifth ligand was an axial bond to the thiolate, which was confirmed by the Fe—C—O bending mode at 562 cm-1. 

The nickel proteins and the native iron globins show the same behavior in regard to fifth-ligand photodissociation but for totally different reasons. Heme proteins and other metal-reconstituted heme proteins have been investigated by transient absorption spectroscopy (49-53) and transient Raman spectroscopy (16,54-62). In none of the Fe proteins is loss of the histidine ligand observed even on a picosecond timescale. Soret excitation of carbonmonoxy and oxy Hb and Mb photolyzes CO or O2, but not the histidine fifth ligand. 

Although neither the Ni nor the Fe globins show evidence of photodissociation of the histidine ligand, the model compound data would, nevertheless, lead us to expect ligand ejection for Ni but not Fe. Time-resolved absorption (45-47) and transient Raman measurements (10,11) for 6-coordinate Ni-porphyrin complexes clearly predict that photodissociation of the fifth ligand will occur, since... 

Copper-reconstituted cytochrome c ( cyt-c) has also been investigated with transient absorption methods (49). No evidence of ghotoinduced ejection of the fifth ligand is observed in either cyt-c or the model Cu-porphyrin 5-coordinate complexes (60-62). This is consistent with the likely transient state being a non-dissociative w w or d 2 2 charge-transfer state. 

Figure 14.5 DR-UV spectra of (a) Ti-silsesquioxanes complexes, with the assigned major species for each maximum, and (b) the Ti(OPr )4 grafted on MCM-41 and deconvolution. The fifth ligand in the pentacoordinated species is represented as an adjacent surface siloxane bridge, but water or surface silanols are also possible (see text). (From Reference [64].)...

The inhibitors are generally monoanions or neutral molecules capable of deprotonation to yield anionic species. These neutral inhibitors (mostly sulfonamides) (189) bind to the active site Zn-H O. Monovalent anions like I-, CN-, SCN-, N3, NCO-, SH- etc. (190,191), inhibit the catalyzed reaction of CA enzyme by binding directly to the metal ion either by displacing H2O to yield a tetra-coordinate metal ion or by adding to the coordination sphere to yield a penta-coordinate metal ion with H2O as the fifth ligand (see Table V). In some cases an equilibrium between these two coordination geometries is also reported. 

If the action of the enzyme is to produce R—CH2—As(0)(0H)—OR (Section IV,B,2,a), the product will spontaneously hydrolyze, because water can approach the arsenic atom as a fifth ligand in a way it cannot do with the corresponding phosphorus compound. If R OH is a carboxylic or phosphoric acid rather than an alcohol, the hydrolysis is rapid, with a half-life of seconds or less, rather than of about half an... 

---