Unusual spin state equilibrium of azide metmyoglobin induced by ferric corrphycene

Myoglobin was reconstituted with the ferric complex of corrphycene, a novel porphyrin isomer with a rearranged tetrapyrrole array, to investigate the influence of porphyrin deformation on the equilibrium between high-spin (S = 5/2) and low-spin (S = 1/2) states in the azide derivative. The azide aff inity, 2.5 x 10(4) M-1, was 1 order of magnitude lower than the correspondi ng values of a reference myoglobin containing an electron-deficient diformy lheme similar to the corrphycene. Analysis of the visible absorption spectr um over a range of 0-40 degreesC reveals that the population of high-spin i ron is 76-82% at room temperature for azide metmyoglobin complexed with fer ric corrphycene. The unusual predominance of the high-spin state was verifi ed from the infrared spectrum of coordinating azide, where the high-spin pe ak at 2046 cm(-1) is 4-fold larger in intensity than the 2023 cm(-1) low-sp in band. Electron paramagnetic resonance at 15 K further indicated that the iron-histidine bond is cleaved to form a five-coordinate derivative in som e fraction of the myoglobin. The remarkable high-spin bias of the spin equi librium at room temperature and cleavage of the iron-histidine bond at 15 K could be explained in terms of the contracted and trapezoidal metallo core that weakens the iron-histidine bond of azide metmyoglobin bearing corrphy cene.