Proteins and Glasses: A Relaxation Study in the Millikelvin Range

Abstract

Comparative spectral diffusion studies in the millikelvin range between a protein and a glass point to a strong shielding of the chromophore from the strain and/or electric fields of the host and to quite specific features of the energy landscape.

Spectral diffusion and the energy landscape of a protein

“…We note that the very different time evolutions of hole A for the protein and glass systems provide clear evidence that the protein experiments are measuring protein rather than glass dynamics. This conclusion is also supported by previous thermal cycling experiments (13).…”

“…It was suggested over a decade ago that the energy landscape of a protein may be organized in a hierarchical fashion (15). Recently, we found evidence for such self-similar features, although only in a qualitative sense, based on spectral diffusion experiments in the millikelvin range (13). The results showed that features in the energy landscape that were found at temperatures where the molecules become physiologically active (16) were also found in experiments below 1 K. These features are the roughness of the energy surface and the so-called deep minimum states.…”

“…Hole burning is probably the best technique for spectral diffusion experiments. In the past decade it has mainly been exploited in experiments on glasses (23,24), but several experiments on proteins have appeared as well (6,12,13). Two types of spectral diffusion experiments are common: (i) time-dependent experiments where the width of a hole is monitored as a function of waiting time after burning (25) and (ii) temperature-cycling experiments where the width of a hole immediately after the cycle is monitored as a function of cyclic temperature changes (26,27).…”

“…This landscape is very complex and, hence, very difficult to measure (1), even qualitatively. What is well known, so far, is that despite the high level of structural organization (2), there is randomness in the energy landscape (3), which is reflected, for instance, in a variation of Debye-Waller factors in x-ray diffraction (4, 5), in inhomogeneous line broadening (6-8), in dispersive kinetics (9-11), and in spectral diffusion (6,8,(12)(13)(14). Much less is known about whether the randomness itself shows features of organization.…”

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

“…It is obvious that temperature cycling spectral diffusion experiments yield information on the roughness of the EL, on the depth of basins, on threshold barriers, etc. It can even be combined with dynamical features from which information on relaxation and aging behavior may be obtained [12,13].…”