PROTON NUCLEAR MAGNETIC-RESONANCE STUDY OF ROTATIONAL POSITION AND OSCILLATORY MOBILITY OF VINYL GROUPS IN ALLOSTERIC MONOMERIC INSECT HEMOGLOBINS

The changes in the heme hyperfine-shifted proton nuclear magnetic resonances arising from the pH-induced t ↔ r transition in the monomeric met-cyanohemoglobins from the insect Chironomus thummi thummi are shown to be localized at one of the vinyl groups. This localized nature of the protein perturbations on the heme is taken as an indication that the structural change detected at the heme involves a pH-modulated change in the orientation of the vinyl group relative to the heme. Detailed analysis of the hyperfine-shifted peaks for the peripheral substituents of model heme compounds, the low-spin, biscyano ferric complexes of protoporphyrin IX and deuteroporphyrin IX, reveals that vinyl groups are sterically restricted from being coplanar with the heme and that the vinyl group yields a characteristic shift pattern which depends on its rotational position. A shift parameter is defined which provides an index of both the average rotational position and the oscillatory mobility of a vinyl group in any heme b containing protein. Application of this parameter to the insect hemoglobins reveals that a vinyl group in the low-pH, t, form of this protein is more coplanar with the heme and possesses less oscillatory mobility than in the high-pH, r, conformation of the protein. This is the first direct observation of any sort of strain in the t conformation of any O2-binding heme protein and suggests that a ”tense” description for the low-pH conformation is appropriate. © 1978, American Chemical Society. All rights reserved.