In the L29F variant of myoglobin (Mb), the coordination of oxygen (O-2) to the heme Fe atom is stabilized by favorable electrostatic interactions between the polar Fe-O-2 moiety and the multipole of the phenyl ring of the Phe29 side chain (Phe29 interaction), in addition to the well-known hydrogen bond (H-bond) between the Fe-bound O-2 and the 64th residue (distal H-bond; Carver, T. E.; Brantley, R. E., Jr.; Singleton, E. W.; Arduini, R M.; Quillin, M. L.; Phillips, G. N., Jr.; Olson, J. S. J. Biol. Chem. 1992, 267, 14443-14450). The O-2 and carbon monoxide (CO) binding properties and autoxidation of the L29F/H64L and L29F/H64Q variants reconstituted with a series of chemically modified heme cofactors were analyzed and then compared with those of native Mb, and the L29F, H64Q, and H64L variants similarly reconstituted with the chemically modified heme cofactors in order to elucidate the relationship between the Phe29 interaction and the distal H-bond that critically contributes to stabilization of Fe-bound O-2. We found that the Phe29 interaction and distal H-bond act cooperatively to stabilize the Fe-bound O-2 in such a manner that the Phe29 interaction strengthens with increasing strength of the distal H-bond. Comparison of the functional properties between the L29F and H64L variants indicated that the synergistic effect of the two interactions decreases the O-2 dissociation and autoxidation rate constants of the protein by factors of similar to 1/2000 and similar to 1/400, respectively. Although the CO binding properties of the proteins were not greatly affected by the distal polar interactions, their synergistic effects were clearly and sharply manifested in the vibrational frequencies of the Fe-bound C-O stretching of the proteins.