LOW-TEMPERATURE OPTICAL SPECTROSCOPY OF COBALT-SUBSTITUTED HEMOCYANINFROM CARCINUS-MAENAS

In this work we report the optical absorption spectra of three cobalt- substituted derivatives of hemocyanin (Hc) from Carcinus maenas, in th e temperature range 300-20 K. The derivatives studied are the mononucl ear (Co2+)-Hc with a single cobalt ion in the ''Cu(A)'' site, the binu clear (Co2+)2-Hc and the binuclear mixed metal (Co2+-Cu1+) Hc. At low temperature three main bands are clearly resolved; the temperature dep endence of their zeroth, first and second moments sheds light on the s tereodynamic properties in the surroundings of the chromophore. Within the limits of the reported analysis, in the binuclear derivatives the motions coupled to the chromophore appear to be ''essentially harmoni c'' in the whole temperature range investigated; moreover the data are consistent with the presence of an exogenous ligand strongly bound to the two metal ions. For the mononuclear derivative an ''essentially h armonic'' behavior is evident only up to 200 K where the data are cons istent with the presence of an exogenous ligand much less strongly bou nd, while at higher temperatures the behavior of the spectra indicates the onset of very large anharmonic contributions to motions, that pla usibly involve the above exogenous ligand and, quite likely, the entir e active site.