Azophenol spectra

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-... 

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. 

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. 

For CD transitions the selection rules are not as clearly defined. However, more complex CD spectra were anticipated for the Co(III) azophenols than for the Zn(II) azophenols due to the overlap of d-d and ligand transitions. Since Zn(II) has no d-d transitions a relatively simple CD spectrum is expected in the visible region as is observed for the zinc complex in AA-CPA-Zn, Figure 4 (pH 8.3 spectrum). 

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