The homonuclear Overhauser effect in H2O solution of low-spin hemeproteins. Assignment of protons in the heme cavity of sperm whale myoglobin.

Proton-proton Overhauser effects were observed in 1H2O solutions of sperm whale metcyano myoglobin. Dipolar connectivities involving hyperfine-shifted exchangeable protons such as the proximal and distal histidine ring NH's allowed us to categorize signals as arising from residues located on one side of the heme plane or on the other. With these connectivities, as well as spin-lattice relaxation times, spectral assignments were reached that were used to derive structural and dynamic information about the heme environment. Thus, it was shown that the distal histidine residue does not titrate down to pH approximately 4.1 and that the beta CH2 of the proximal histidine side chain tumbles with the same correlation time as the protein. Some other applications and limitations are presented.