The Energy Landscape of Myoglobin: An Optical Study

In this paper we demonstrate how the potential energy surface of a protein, which determines its conformational degrees of freedom, can be constructed from a series of advanced nonlinear optical experiments. The energy landscape of myoglobin was probed by studying its low-temperature structural dynamics, using several spectral hole burning and photon echo techniques. The spectral diffusion of the heme group of the protein was studied on a time scale ranging from nanoseconds to several days while covering a temperature range from 100 mK to 23 K. The spectral line broadening, as measured in three-pulse stimulated photon echo experiments, occurs in a stepwise fashion, while the exact time dependence of the line width is critically dependent on temperature. From these results we obtained the energy barriers between the conformational states of the protein. Aging time dependent hole-burning experiments show that, at 100 mK, it takes several days for the protein to reach thermal equilibrium. When, after this period a spectral hole is burned, the line broadening induced by well-defined temperature cycles is partly reversed over a period of several hours. From this we conclude that a rough structure is superimposed on the overall shape of the potential energy surface of the protein. By combining the hole burning and photon echo results, we construct a detailed image of this energy landscape, supporting the general concept of a structural hierarchy. More specifically, we show that the number of conformational substates in the lower hierarchical tiers is much lower than was previously anticipated and, in fact, is comparable to the number of taxonomic substates.

Epitaxial strain effects in the spinel ferrites CoFe2O4 and NiFe2O4 from first principles

The inverse spinels CoFe2O4 and NiFe2O4, which have been of particular interest over the past few years as building blocks of artificial multiferroic heterostructures and as possible spin-filter materials, are investigated by means of density functional theory calculations. We address the effect of epitaxial strain on the magneto-crystalline anisotropy and show that, in agreement with experimental observations, tensile strain favors perpendicular anisotropy, whereas compressive strain favors in-plane orientation of the magnetization. Our calculated magnetostriction constants $\lambda_{100}$ of about -220 ppm for CoFe2O4 and -45 ppm for NiFe2O4 agree well with available experimental data. We analyze the effect of different cation arrangements used to represent the inverse spinel structure and show that both LSDA+U and GGA+U allow for a good quantitative description of these materials. Our results open the way for further computational investigations of spinel ferrites