Looking into frozen proteins with hole burning

We show how the diversity of hole burning experiments, for instance in combination with external fields or in probing spectral diffusion at extremely low temperatures, can give deep insights into specific features of the solid state physics of proteins. Particularly, we demonstrate that the chromophore is insensitive to the host glass, and hence, is really a probe for the protein, that proteins have a structural randomness which compares well with glasses, and that the density of conformational states becomes rather coarse-grained if the temperatures are lowered into the mK range.