We have studied the tertiary structure of the heme pockets of human adult carbon monoxide hemoglobin and oxyhemoglobin by investigating the ring-current shifted proton resonances in the 250-MHz nmr spectra. We have found that the conformation of the E11 valine residues in a and β chains relative to the heme plane is quite dependent on the nature of the anions and the pD of the solution as well as on the nature of the ligands. The Ell valines are located on the distal histidine side of the heme plane and are believed to play a vital role in the cooperative oxygenation of hemoglobin. We have attempted to correlate the structural transitions manifested by the differences in the ring-current shifts with the known functional effects produced by the different buffer systems. We have suggested a possible relationship between the conformation of the Ell valine methyl groups and the ligand affinity and have proposed a structural mechanism for the effects of anion binding on tertiary structure of the heme pockets in hemoglobin. © 1973, American Chemical Society. All rights reserved.