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 affinity, 2.5 x 10(4) M-1, was 1 order of magnitude lower than the corresponding values of a reference myoglobin containing an electron-deficient diformylheme similar to the corrphycene. Analysis of the visible absorption spectrum over a range of 0-40 degreesC reveals that the population of high-spin iron is 76-82% at room temperature for azide metmyoglobin complexed with ferric corrphycene. The unusual predominance of the high-spin state was verified from the infrared spectrum of coordinating azide, where the high-spin peak at 2046 cm(-1) is 4-fold larger in intensity than the 2023 cm(-1) low-spin band. Electron paramagnetic resonance at 15 K further indicated that the iron-histidine bond is cleaved to form a five-coordinate derivative in some fraction of the myoglobin. The remarkable high-spin bias of the spin equilibrium 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 corrphycene.