Nanosecond Dynamics of the R→T Transition in Hemoglobin: Ultraviolet Raman Studies

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Abstract
Pulse-probe transient Raman spectroscopy, with probe excitation at 230 nanometers, reveals changes in signals arising from tyrosine and tryptophan residues of the hemoglobin molecule as it moves from the relaxed (R) to the tense (T) state after photodeligation. Signals associated with intersubunit contacts in the T state develop in about 10 microseconds but are preceded by quite different signals, which reach maximum amplitude in about 50 nanoseconds. These signals involve the interior tryptophan residues that bridge the A and E helices by means of H bonds between the indole rings and serine or threonine side chains. Alterations of the H bond strengths, as a result of interhelix motions, can account for the signals. A model is proposed here in which loss of the ligand from the heme binding pocket is concerted with inward motion of the adjacent E helix; this motion, along with a complementary motion of the proximal F helix, transmits the energy associated with heme deligation to the subunit interfaces, leading to the T state rearrangement.