Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L2,3 edge we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spinstate system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.PACS numbers: 71.28.+d, 71.70.Ch, 78.70.Dm LaCoO 3 shows a gradual non-magnetic to magnetic transition with temperature, which has been interpreted originally four decades ago as a gradual population of high spin (HS, t 4 2g e 2 g , S = 2) excited states starting from a low spin (LS, t 6 2g , S = 0) ground state [1,2,3,4,5,6,7,8]. This interpretation continued to be the starting point for experiments carried out up to roughly the first half of the 1990's [9,10,11,12]. All this changed with the theoretical work in 1996 by Korotin et al., who proposed on the basis of local density approximation + Hubbard U (LDA+U) band structure calculations, that the excited states are of the intermediate spin (IS, t 5 2g e 1 g , S = 1) type [13]. Since then many more studies have been carried out on LaCoO 3 with the majority of them [14,15,16,17,18,19,20,21,22,23,24,25,26,27] claiming to have proven the presence of this IS mechanism. In fact, this LDA+U work is so influential [28] that it forms the basis of most explanations for the fascinating properties of the recently synthesized layered cobaltate materials, which show giant magneto resistance as well as metal-insulator and ferroferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [29,30].In this paper we critically re-examine the spin state issue in LaCoO 3 . There has been several attempts made since 1996 in order to revive the LS-HS scenario [31,32,33,34,35], but these were overwhelmed by the above mentioned flurry of studies claiming the IS mechanism [14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Moreover, a new investigation using inelastic neutron scattering (INS) has recently appeared in Phys. Rev. Lett.[36] making again the claim that the spin state transition involves the IS states. Here we used soft xray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the Co-L 2,3 edge and we revealed that the spin state transition in LaCoO 3 can be well described by a LS ground state and a triply degenerate HS excited state, and that an inhomogeneous mixed-spinstate system is formed. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility and electron spin resonance (ESR) data, and provide support for an alternative interpretation of the INS [37]. C...

The x-ray structure of carbon monoxide (CO)-ligated myoglobin illuminated during data collection by a laser diode at the wavelength A = 690 nm has been determined to a resolution of 1.7 A at T 36 K For comparison, we also measured data sets of deoxymyoglobin and CO-ligated myoglobin. In the photon-induced structure the electron density associated with the CO ligand can be described by a tube extending from the iron into the heme pocket over more than 4 A. This density can be interpreted by two discrete positions of the CO molecule. One is close to the heme iron and can be identified to be bound CO. In the second, the CO is dissociated from the heme iron and lies on top of pyrrole ring C. At our experimental conditions the overall structure of myoglobin in the metastable state is close to the structure of a CO-ligated molecule. However, the iron has essentially relaxed into the position of deoxymyoglobin. We compare our results with those of Schlichting et al.

The crystal structure ofsperm whale metmyoglobin has been determined at 80 K to a resolution of2 A. The overall structure at 80 K is similar to that at 300 K except that the volume is smaller. Refinement of the structure by the method of restrained least squares (current R = 0.175) permits the assignment ofisotropic atomic mean-square displacements to all nonhydrogen atoms. Comparison with the values obtained earlier at 250-300 K indicates that the protein at 80 K is more rigid. The average experimentally determined Debye-Waller factor, B, for the protein is 14 A at 300 K and 5 A2 at 80 K. Plots of backbone meansquare displacement vs. temperature show a discontinuity of slope for at least one-third ofall residues. This behavior is in good agreement with the temperature dependence of the mean-square displacement of the heme iron as measured by Mossbauer absorption. The magnitudes of the smallest mean-square displacements observed at 80 K indicate that intramolecular motions can be frozen out to a surprisingly large degree. Even at 80 K, however, some atoms in myoglobin still have mean-square displacements greater than 0.1 A2, thus providing evidence for conformational substates.The view of protein molecules as systems that fluctuate over a large number of conformational substates is now accepted (1-3). Conformational fluctuations are important for biological function, and detailed studies of their properties are therefore desirable. Myoglobin (Mb), "the hydrogen atom of biology," is a good choice for such studies. Mb is presumed to have a simple function, storage and transport of oxygen in muscles (4). Some of its properties can be understood in terms of its static threedimensional structure, determined by single-crystal x-ray diffraction at 300 K (5, 6). However, dynamic features, especially the access of oxygen to the heme and the kinetics of binding of carbon monoxide to the iron (7,8), cannot be explained by a static picture. X-ray crystallography is a powerful tool for mapping average displacements of atoms in a protein (9-11). Here, we present the determination of the structure of metMb at 80 K to a resolution of 2 A and compare this structure and atomic displacements with earlier results at 250-300 K and with the results of Mossbauer absorption studies at 4.2-300 K (12)(13)(14).Atomic displacements are involved in the interconversion of different local configurations (conformational substates) of the same overall protein structure (7)(8)(9)(10)(11)(12)(13)(14) K. If, moreover, the bottoms of the two conformational positions differ by 1 kJ/mol or less, both substates will be appreciably populated even at 80 K.Information about the spatial distribution of conformational substates can be expressed in terms ofindividual atomic meansquare displacements, (x2). By measuring these atomic displacements as a function of temperature, the shape of the effective conformational potential well in which the atom moves can be determined. In earlier crystallographic work, meansquare displacements have been measured...

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The thermal expansion of a protein, metmyoglobin, was investigated by analysis of the refined X-ray crystal structures at 80 and 255-300 K. On heating from 80 to 300 K, the volume occupied by myoglobin increases by approximately 3%. The linear thermal expansion coefficient is estimated to be 115 X 10(-6) K-1. This value is more than twice as large as that of liquid water but less than that of benzene. As the temperature is raised, the internal volume change does not come from the large, atom-sized internal cavities in the structure but from an increase in the small, subatomic free volumes between atoms. The largest expansion occurs in the region of the CD and GH corners; both these regions move away from the center of the protein. The remainder of the expansion results from the lengthening of contacts between segments of secondary structure.