Azophenolate complexes, absorption

In order to find a more reliable indicator of Co(III)-azophenolate coordination in proteins, the circular dichoric properties of the models were examined. Initially an essentially empirical approach was taken. In the visible absorption spectra of the Co(III)-azophenolate complexes, the ligand associated transitions dominate the spectrum since the molar absorptivities of 4000 - 10,000 are at least an order of magnitude more intense than the configurationally forbidden d-d transitions, Figure 5. 

The circular dichroic spectrum of the zinc azoenzyme in solution contains one positive and two negative ellipticity bands at 420, 335, and 510 nm, respectively (Figure 3). On removal of zinc, almost the entire visible CD spectrum is abolished. It is completely restored on the addition of one gram atom of zinc per mole of enzyme. This eflFect of zinc on both the absorption and CD spectra suggests that the red color of the enzyme in solution may be caused by the formation of a zinc-azophenol coordination complex. Closely similar color changes can be observed by adding zinc to a typical azophenol compound, tetrazolyl-N-benzyloxycar-... 

Spectral titrations of zinc azocarboxypeptidase in solution between pH 6.2 and 8.3 generate the absorption band at 510 nm typical of the azophenol-metal complex. Above pH 8.3 this band shifts progressively... 

Based on a series of studies with model azophenol-metal complexes, these absorption- and circular dichroic-pH titration data indicate the formation of an intramolecular coordination complex between arsanilazo-tyrosine-248 and the active site zinc ion where the enzyme is in solution (62, 63). This means that in solution, the phenolic hydroxyl group of tyrosine-248 would have to be much closer to the zinc ion than the 17 A indicated by the crystallographic studies. Consequently, there must be... 

The spectral properties of the complexes are summarized in Figure 5. Unfortunately, absorption spectra analogous to those obtained for the Zn-azophenol enzyme (Figure 4, pH 8.4 spectrum) and models (4,7 ) were observed for the Co(III) complexes. 

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