Die Fe:sup:2:/sup::img:=plus.gif:/img:-His(F8) Resonanz-Ramanbande: Der Nachweis für taxonomische Konformationssubzustände in der proximalen Bindung

Abstract

The band shape of the Raman line attributed to the Fe:sup:2:/sup::img:=plus.gif:/img:-N:sub::img:=epsilon.gif:/img::/sub:(His-F8) stretching mode in heme proteins contains significant information of the Fe-His proximal linkage. In the case of myoglobin Raman lines at the low and the high frequency wing of this profile obscure its true band shape. Measurements with different pH-values, different solvents and different polarizations showed that these lines are not caused by the Fe-His-linkage.:br:In order to separate these accompanying lines from the Fe-His-band the isotopic shift has been used by substituting the natural iron-isotope :sup:56:/sup:Fe of the heme with the :sup:54:/sup:Fe-isotope.This enables us to isolate the true line shape. Detailed analysis reveals that the band is composed of five distinct lines attributable to taxonomic conformational substates of the Fe-His linkage. The temperature dependence of the band is caused by intensity variations of these five different sublines. The fits reveal a glass transition temperature of T:sub:f:/sub:=155±15K with a relatively broad transition region of :img:=delta1.gif:/img:T :img:=approx.gif:/img:80K for myoglobin. The enthalpic differences between conformational substates yield values between 0.7kJ/mol and 1.5kJ/mol for myoglobin. Our data show, that the :img:=nue.gif:/img:(Fe-His) vibrational mode is governed by at least one coordinate which determines its frequency. This may be the tilt angle :img:=Theta.gif:/img:, or the displacement :img:=delta.gif:/img: of the central iron, or a combination of both. The sublines are therefore interpreted as resulting from different conformational substates of the Fe:sup:2:/sup::img:=plus.gif:/img:-N:sub::img:=epsilon.gif:/img::/sub:(His-F8) complex with respect to these comformational coordinates.