Hemocyanin, absorption spectrum

Thiocyanate ion is known to react with oxyhemocyanin to remove oxygen from the coordination sphere of the copper ions (76, 77). We observed that an addition of NCS" to the purple hemocyanin produces a little more reddish and transparent solution, whose absorption spectrum apparently differs from that of the purple hemocyanin, as shown in Figure 5. A drastic decrease of the absorption band at around 340 nm in Figure 5 indicates that the coordinated oxygen ligand is removed with the addition of NCS". The variation in CD spectrum accompanied by the addition of NCS" is so remarkable that the CD band centering at 430-490 nm of the purple hemocyanin almost disappears, as seen in Figure 6. 

Figure 5. Absorption spectrum of the red solution produced by treating the purple hemocyanin with NCS ion...

As an important consequence of resonance stabilization there will be low-lying charge transfer bands in the absorption spectrum. Hemocyanin has a molar extinction coefficient Cmax = 750. Laccase of mammalian plasma has an absorption band at 6050 A with. e,nax = 1200. We may wonder if those bands are really charge transfer bands. On the other hand we must note that the theory of ESR spectra is still in its infancy and that we should be cautious in our interpretation. 

Fig. 25. Absorption spectra (—15 K, pH = 6.3 in 1 1 sucrose glass) and EPR spectra (77 K, v = 9.1 GHz) of the half-met hemocyanin series dotted line for NJ is 7K spectrum (from Ref. 52)...

The oxidative addition model for reversible O2 binding by metal proteins is also reasonable for hemocyanin. Hemocyanin is a copper protein which binds one O2 molecule for every two copper atoms. The deoxy Cu(I) form has no appreciable absorption in the visible region. When oxygenated, the protein is blue and exhibits a rich visible spectrum, wiA bands at 700 (c 75), 570 (c 500), 440 (c 65), and 347 nm (c 8900) (53). The pattern of bands around 570 nm leaves little doubt that oxyhemocyanin contains Cu(II) (53). The enhanced LF band intensities further suggest a dimeric Cu(II) complex. For comparison. 

Optical absorption and CD spectrum of oxy-Hemocyanin and the model complex While the spectrum of oxy-Hc is typical for ii-rf. f Cu peroxo systems with an almost... 

---