Hole burning experiments with proteins: Relaxations, fluctuations and glass-like features

We present a series of different spectral hole burning experiments, such as aging and waiting time experiments and thermal cycling experiments to gain information on features of the EL of proteins and the associated physics. We focus on heme proteins. Spectral diffusion dynamics in proteins follow a power law in waiting time. If the protein is unfolded the dynamics change to a logarithmic time law as widely observed in glasses. Thermal cycling experiments with proteins show, in addition to random features, also discrete features. There are severe solvent effects demonstrating that the EL is strongly influenced by the solvent. We demonstrate that the hole recovery reaction may strongly depend on protein relaxation, and finally, we link features of the EL to characteristic properties of the folding–unfolding transition.