The highly stereoselective cleavage of hemin in myoglobin by coupled oxidation has been attributed to steric barriers that leave more space near the alpha- than the other meso-positions. The steric barriers near meso positions in myoglobin have been investigated by establishing the thermodynamics and dynamics of possible seatings in the pocket of horse myoglobin of a four-fold symmetric etioheme I modified with a bulky nitro group at a single meso position. The cyanomet complex of this reconstituted myoglobin exhibits three sets of H-1 NMR resonances that are linked dynamically and occur in approximate populations ratios of 0.82:0.10:0.08. Two dimensional H-1 NMR has been used to assign the hemin and heme pocket resonances in the major isomer in solution and to determine that the hemin is oriented with the nitro group at the canonical gamma -meso position of native hemin. The dominance of this isomer is attributed to the solvent exposure of this portion of the hemin which stabilizes the highly polar nitro group. Using a combination of magnetization transfer among methyl groups of the three isomers due to "hopping" of the hemin about it, normal, the assigned resonances of an isoelectronic, bis-cyano complex of meso-nitro-etioheme I, and the known essentially constant rhombic perturbation of heme pocket sites on the hyper-fine shifts of heme methyl (Kolczak, U.; Hauksson, J. B.; Davis, N. L.; Pande, U.; de Ropp, J. S.; Langry, K. C.; Smith, K. M.; LaMar, G. N. J. Ain. Chem. Soc. 1999, 121, 835-843); the two minor isomers are shown to place their bulky nitro group at the canonical delta -meso (8%) and alpha -meso positions (10%). The comparable population of the isomers with nitro groups at the hydrophobic alpha- and delta -meso positions dictates that, while the static crystal structure finds more room near the alpha -meso position, the deformation at minimal energetic expense near the alpha- and delta -meso positions is comparable. These results argue that factors other than simple steric influences control the selectivity of the ring cleavage in myoglobin.