Spectroscopy of proteins at low temperature. Part I: Experiments with molecular ensembles

We discuss aspects of the physics of proteins at low temperature as they are reflected in highly resolved optical spectra of molecular probes. Typical probe molecules are heme-like dyes, aromatic amino acids, but also extended molecular aggregates in light harvesting complexes. We put emphasis on the interactions of the probe with its protein environment, on the range of these interactions, on their specific behavior in external fields, as well as on the characteristic parameters of the protein which can be determined with optical techniques at low temperatures but are not easily accessible otherwise. However, the focus of the review is on spectral diffusion physics of proteins, i.e. on their motion in conformational phase space, and on how this motion is reflected in the optical spectra. These structure changing-processes reflect the non-ergodic nature of low temperature proteins. They are most clearly detected at low temperature where the resolution of the experiment is close to the ultimate limit as given by the natural linewidth and where the dynamics become slow enough to be conveniently measured. In part I we discuss aspects of ensemble experiments, in part II we focus on experiments with single protein complexes. We offer lines of reasoning which may serve as guidelines for an understanding of the phenomena.