Structural relaxations in H₂-substituted myoglobin observed by temperature-cycling hole burning

A spectral hole-burning experiment has been carried out on H₂-protoporphyrin-substituted myoglobin by cycling temperature from 4 K up to 70 K. When the excursion temperature T_c is below 30 K, the spectral diffusion kernel (SDK) has been found to show a Lorentzian shape. A steep increase in the hole width has been observed around 20 K, which is attributed to a narrow distribution of the barrier height of two-level systems (TLS's) in myoglobin. When T_c is raised to ∼50 K, on the other hand, the major broadening has occurred in the tail of the hole, and the SDK has deviated significantly from a Lorentzian line shape. The hole profiles after the temperature cycling have been analyzed by the stochastic model which assumes that the spectral diffusion is induced by random flips of TLS's. Both the non-Lorentzian SDK for T_c around 50 K and the Lorentzian SDK around 20 K have been found to be reproduced well by this model, if the finite size of the protein is taken into account. The details of the fitting procedure and the determined values of the number and the coupling constant of TLS's in myoglobin are presented.