Two-Level Systems in Myoglobin Probed by Non-Lorentzian Hole Broadening in a Temperature-Cycling Experiment

We have measured the broadening of persistent spectral holes in ${\mathrm{H}}_2$-protoporphyrin-substituted myoglobin caused by temperature cycling between 4 and 70 K. Non-Lorentzian hole broadening has been observed for excursion temperatures above 40 K. This effect has been explained by assuming that the hole broadening is due to the transition-frequency shift of the chromophore by the random flips of the two-level systems, and that the these systems in the interior and in the exterior of the myoglobin molecule make different contributions to the broadening. From the analysis of the hole spectra, the number and the coupling strength of the TLS's in myoglobin have been determined.