Nuclear magnetic resonance studies of xenon-129 with myoglobin and hemoglobin

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Abstract
NMR studies of 129Xe are consistent with 1 kinetically distinguishable binding environment in [human] methemoglobin and 2 in [sperm whale] metmyoglobin. The Xe binding site in methemoglobin is assigned to a cavity formed by the A-B and G-H corners of the globin chain. The small differences between .alpha.-Hb and .beta.-Hb are not resolved by the NMR experiments. The Xe association rate constant at 18.degree. C with methemoglobin is greater than 6 .times. 107 M-1 s-1 with an activation barrier of .apprx. 13 kcal/mol. One of the binding sites in metmyoglobin is associated with a cavity on the proximal side of the porphyrin ring, opposite the O2 binding site. An estimate of the association rate constant of Xe at 18.degree. C is 1 .times. 107 M-1 s-1 with an activation barrier of .apprx. 16 kcal/mol. The second metmyoglobin binding site has similar NMR and kinetic properties to those for methemoglobin.