Endogenous Ligand Bridge

Chemical and spectral studies have indicated that in addition to bridging exogenous ligands, there is an endogenous protein bridge at the coupled binuclear copper site. Evidence for this is seen with the half-met derivative and the binuclear cupric derivatives met and dimer. 

A charge transfer transition is found at 425 nm in both met and oxy (vide infra) and is characteristic of the coupled binuclear copper site. Elimination of sulfur ligation leaves the most reasonable assignment of this feature as a phenolate-to-copper(II) CT transition (see Table 4). This indicates that tyrosine is bound to the binuclear cupric site and the most likely candidate for the endogenous bridge. 

Optical features and exchange coupling for structurally defined binuclear copper(II) complexes 

M--0- phenyl Cu2LBr2(CI04)Me0H square pyramidal (tetra amine, //-phen- and distorted trioxide ligand with /(-Br gonal bipyramidal axial Br and MeOH) 139 425-450R 650 R 84 h 

Optical features of structurally defined mononuclear copper(II) phenolate complexes Complex Cu+2 Geometry CT(nm)a d-d(Alna,)(nm)a Ref. 

---

There are fewer peptides that are selective for x-receptors. Dynorphin A, however, is a 17 amino acid peptide that has been proposed as the endogenous ligand for x-receptors. Only the first 11 amino acids of the peptide are necessary for x-selectivity, and it is possible to reduce the conformational freedom of the peptide further with a disulfide bridge between cysteines in positions 5 and 11. An energy minimization and molecular dynamics study of model compounds for the latter was performed in an effort to determine the conformational features associated with the model compounds and to design novel compounds based on these features. However, as the authors acknowledge, these studies are preliminary given the flexibility possible in a peptide of this size. 

At a resolution of 3.2 A, the limit for these crystals, there is no evidence in the electron density map for an endogenous bridging ligand required according to spectroscopic analysis (Solomon, 1988), but it is not likely... 

Fenton, D. E. Kitchen, S. J. Spencer, . M. Tamburini, S. Vigato, P. A. Complexes of ligands providing endogenous bridges. Part 5. Solution studies on a novel 3 + 3 hexamine Schiff-base macrocyclic complex of lanthanum. J. Chem. Soc., Dalton Trans. 1988, 685-690. 

---