The quantum mechanically mixed-spin state in a non-symbiotic plant hemoglobin: the effect of distal mutation on AHb1 from Arabidopsis thaliana

Non-symbiotic hemoglobins are hexacoordinated heme proteins found in all plants. To gain insight into the importance of the heme hexacoordination and the coordinated distal histidine in general for the possible physiological functions of these proteins, the distal His(E7) of Arabidopsis thaliana hemoglobin (AHb1) was substituted by a leucine residue. The heme properties of the wild-type and mutant proteins have been characterized by electronic absorption, resonance Raman and electron paramagnetic resonance spectroscopic studies at room and low temperatures. Significant differences between the wild-type and mutant proteins have been detected. The most striking is the formation of an uncommon quantum mechanically mixed-spin heme species in the mutant. This is the first observation of such a spin state in a plant hemoglobin. The proportion of this species, which at room temperature coexists with a minor pentacoordinated high-spin form, increases markedly at low temperature.