Leghemoglobin. Low temperature optical spectra of acid and alkaline forms of leghemoglobin(IV). Configuration of the heme.

Abstract Leghemoglobin(IV), the derivative of leghemoglobin at the formal oxidation state IV, when cooled to liquid nitrogen temperature exhibits radically different spectra at acid and alkaline pH. The acid and alkaline forms are freely interconvertible. The optical spectrum of the acid form is closely similar to optical spectra of the red higher oxidation states of horseradish and cytochrome c peroxidases, showing that the configuration of the heme iron is the same throughout this family of compounds. That configuration is believed to be Fe(IV) in a porphyrin environment. The optical spectrum of the alkaline form of leghemoglobin(IV) recalls that of alkaline low spin ferric leghemoglobin. Near infrared spectra of leghemoglobin(IV), myoglobin(IV), and the higher oxidation states of the peroxidases are featureless to 1300 nm, suggesting a common structural feature. The acid form of leghemoglobin(IV), seen in fluid buffer as a transient species at pH 5 or less, is conveniently generated by cooling a solution of the more stable alkaline form in borate buffer to liquid nitrogen temperature. At this temperature borate buffers become acid.