STRUCTURAL RELAXATIONS IN H-2-SUBSTITUTED MYOGLOBIN OBSERVED BY TEMPERATURE-CYCLING HOLE BURNING

摘要

A spectral hole-burning experiment has been carried out on H-2-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 similar 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. (C) 1996 American Institute of Physics. [References: 31]