Showing papers by "University of Groningen published in 1993"

Essential dynamics of proteins

Abstract: Analysis of extended molecular dynamics (MD) simulations of lysozyme in vacuo and in aqueous solution reveals that it is possible to separate the configurational space into two subspaces: (1) an "essential" subspace containing only a few degrees of freedom in which anharmonic motion occurs that comprises most of the positional fluctuations; and (2) the remaining space in which the motion has a narrow Gaussian distribution and which can be considered as "physically constrained." If overall translation and rotation are eliminated, the two spaces can be constructed by a simple linear transformation in Cartesian coordinate space, which remains valid over several hundred picoseconds. The transformation follows from the covariance matrix of the positional deviations. The essential degrees of freedom seem to describe motions which are relevant for the function of the protein, while the physically constrained subspace merely describes irrelevant local fluctuations. The near-constraint behavior of the latter subspace allows the separation of equations of motion and promises the possibility of investigating independently the essential space and performing dynamic simulations only in this reduced space.

Density-functional theory and NiO photoemission spectra

Abstract: The generalization of the local-density-approximation method for the systems with strong Coulomb correlations is proposed, which restores the discontinuity in the one-electron potential as in the exact density functional. The method is based on the model-Hamiltonian approach and allows us to take into account the nonsphericity of the Coulomb and exchange interactions. The calculation scheme could be regarded as a first-principle method due to the absence of adjustable parameters. The method was applied to the calculation of the photoemission (x-ray photoemission spectroscopy) and bremsstrahlung isochromat spectra of NiO.

X-ray circular-dichroism and local magnetic-fields

Abstract: Sum rules are derived for the circular dichroic response of a core line (CMXD). They relate the intensity of the CMXD signal to the ground-state expectation value of the magnetic field operators (orbital, spin, and magnetic dipole) of the valence electrons. The results obtained are discussed and tested for transition metals and rare earths.

Hydrophobic effects - opinions and facts

Abstract: The term hydrophobic interactions denotes the tendency of relatively apolar molecules to stick together in aqueous solution. These interactions are of importance in many chemical disciplines, including the chemistry of in vivo processes. Enzyme-substrate interactions, the assembly of lipids in biomembranes, surfactant aggregation, and kinetic solvent effects in water-rich solutions are all predominantly governed by hydrophobic interactions. Despite extensive research efforts, the hydration of apolar molecules and the noncovalent interactions between these molecules in water are still poorly understood. In fact, the question as to what the driving force for hydrophobic intractions is shifts the study into a quest for a detailed understanding of the remarkable properties of liquid water. This review highlights some of the novel insights that have been obtained in the past decade. The emphasis is on both hydrophobic hydration and hydrophobic interactions since both phenomena are intimately connected. Several traditional views have been found to be deeply unsatisfactory, and courageous attempts have been made to conceptualize the driving force behind pairwise and bulk hydrophobic interactions. The review presents an admittedly personal selection of the recent experimental and theoretical developments, and when necessary, reference is made to relevant studies of earlier date.

Treatment of single brain metastasis: Radiotherapy alone or combined with neurosurgery

Abstract: Most patients treated for single or multiple brain metastases die from progression of extracranial tumor activity. This makes it uncertain whether the combination of neurosurgery and radiotherapy for treatment of single brain metastasis will lead to better results than less invasive treatment with radiotherapy alone. The effect of neurosurgical excision plus radiotherapy was compared with radiotherapy alone in a prospectively randomized trial with 63 evaluable patients with systemic cancer and a radiological diagnosis of single brain metastasis. Radiotherapy was given to the whole brain by a novel scheme of 2 fractions per day of each 2 Gy for a total of 40 Gy. Before randomization, patients were stratified by site (lung cancer vs nonlung cancer) and status of extracranial disease (progressive vs stable). Survival as such and functionally independent survival (FIS; defined as World Health Organization performance status < or = 1 and neurological function < or = 1) were compared between both treatment arms. The combined treatment compared with radiotherapy alone led to a longer survival (p = 0.04) and a longer FIS (p = 0.06). This was most pronounced in patients with stable extracranial disease (median survival, 12 vs 7 mo; median FIS, 9 vs 4 mo). Patients with progressive extracranial cancer had a median overall survival of 5 months and a FIS of 2.5 months irrespective of given treatment. Improvement in functional status occurred more rapidly and for longer periods of time after neurosurgical excision and radiotherapy than after radiotherapy alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Interspecific variation in the growth-response of plants to an elevated ambient co2 concentration

Abstract: The effect of a doubling in the atmospheric CO2 concentration on the growth of vegetative whole plants was investigated. In a compilation of literature sources, the growth stimulation of 156 plant species was found to be on average 37%. This enhancement is small compared to what could be expected on the basis of CO2-response curves of photosynthesis. The causes for this stimulation being so modest were investigated, partly on the basis of an experiment with 10 wild plant species. Both the source-sink relationship and size constraints on growth can cause the growth-stimulating effect to be transient.

Commitment Processes in Close Relationships: An Interdependence Analysis

Abstract: This article employs interdependence theory as a means of understanding how and why some relationships survive difficult times whereas other promising relationships end. Interdependence theory makes important distinctions between satisfaction and dependence. These distinctions are extended in the investment model, a theory of the process by which individuals become dependent on and committed to their relationships. The investment model suggests that dependence increases not only as a consequence of increasing satisfaction, but also because available alternatives are perceived to be poor and numerous important resources are invested in a relationship. Subjective commitment summarizes the nature of an individual's dependence on a partner, and represents broad, long-term orientation toward a relationship. Strong commitment not only makes individuals more likely to remain with their partners, but also promotes a variety of relationship maintenance behaviors such as adaptive social comparison and perceived rel...

Regularity properties and pathologies of position-space renormalization-group transformations: Scope and limitations of Gibbsian theory

Abstract: We reconsider the conceptual foundations of the renormalization-group (RG) formalism, and prove some rigorous theorems on the regularity properties and possible pathologies of the RG map. Our main results apply to local (in position space) RG maps acting on systems of bounded spins (compact single-spin space). Regarding regularity, we show that the RG map, defined on a suitable space of interactions (=formal Hamiltonians), is always single-valued and Lipschitz continuous on its domain of definition. This rules out a recently proposed scenario for the RG description of first-order phase transitions. On the pathological side, we make rigorous some arguments of Griffiths, Pearce, and Israel, and prove in several cases that the renormalized measure is not a Gibbs measure for any reasonable interaction. This means that the RG map is ill-defined, and that the conventional RG description of first-order phase transitions is not universally valid. For decimation or Kadanoff transformations applied to the Ising model in dimension d⩾3, these pathologies occur in a full neighborhood {β>β0, ¦h¦

A hierarchical set of models for species response analysis

Abstract: Variation in the abundance of species in space and/ or time can be caused by a wide range of underlying processes. Before such causes can be analysed we need simple math- ematical models which can describe the observed response patterns. For this purpose a hierarchical set of models is presented. These models are applicable to positive data with an upper bound, like relative frequencies and percentages. The models are fitted to the observations by means of logistic and non-linear regression techniques. Working with models of increasing complexity allows us to choose for the simplest possible model which sufficiently explains the observed pat- tern. The models are particularly suited for description of responses in time or over major environmental gradients. Deviations from these temporal or spatial trends may be statistically ascribed to, for example, climatic fluctuations or small-scale spatial heterogeneity. The applicability of this approach is illustrated by examples from recent research. A combination of simple, descriptive models like those pre- sented in this paper and causal models as developed by several others, is advocated as a powerful tool towards a fuller under- standing of the dynamics and patterns of vegetational change.

Viability and Isolation of Marine Bacteria by Dilution Culture: Theory, Procedures, and Initial Results

Abstract: Dilution culture, a method for growing the typical small bacteria from natural aquatic assemblages, has been developed. Each of 11 experimental trials of the technique was successful. Populations are measured, diluted to a small and known number of cells, inoculated into unamended sterilized seawater, and examined three times for the presence of 104 or more cells per ml over a 9-week interval. Mean viability for assemblage members is obtained from the frequency of growth, and many of the cultures produced are pure. Statistical formulations for determining viability and the frequency of pure culture production are derived. Formulations for associated errors are derived as well. Computer simulations of experiments agreed with computed values within the expected error, which verified the formulations. These led to strategies for optimizing viability determinations and pure culture production. Viabilities were usually between 2 and 60% and decreased with >5 mg of amino acids per liter as carbon. In view of difficulties in growing marine oligobacteria, these high values are noteworthy. Significant differences in population characteristics during growth, observed by high-resolution flow cytometry, suggested substantial population diversity. Growth of total populations as well as of cytometry-resolved subpopulations sometimes were truncated at levels of near 104 cells per ml, showing that viable cells could escape detection. Viability is therefore defined as the ability to grow to that population; true viabilities could be even higher. Doubling times, based on whole populations as well as individual subpopulations, were in the 1-day to 1-week range. Data were examined for changes in viability with dilution suggesting cell-cell interactions, but none could be confirmed. The frequency of pure culture production can be adjusted by inoculum size if the viability is known. These apparently pure cultures produced retained the size and apparent DNA-content characteristic of the bulk of the organisms in the parent seawater. Three cultures are now available, two of which have been carried for 3 years. The method is thus seen as a useful step for improving our understanding of typical aquatic organisms.

An in vivo Study of the Influence of the Surface Roughness of Implants on the Microbiology of Supra- and Subgingival Plaque

Abstract: In nine patients with fixed prostheses supported by endosseous titanium implants, 2 titanium abutments (transmucosal part of the implant) were replaced by either an unused standard abutment or a roughened titanium abutment. After 3 months of habitual oral hygiene, plaque samples were taken for differential phase-contrast microscopy, DNA probe analysis, and culturing. Supragingivally, rough abutments harbored significantly fewer coccoid micro-organisms (64 vs. 81%), which is indicative of a more mature plaque. Subgingivally, the observations depended on the sampling procedure. For plaque collected with paper points, only minor qualitative and quantitative differences between both substrata could be registered. However, when the microbiota adhering to the abutment were considered, rough surfaces harbored 25 times more bacteria, with a slightly lower density of coccoid organisms. The presence and density of periodontal pathogens subgingivally were, however, more related to the patient's dental status than to the surface characteristics of the abutments. These results justify the search for optimal surface smoothness for all intra-oral and intra-sulcular hard surfaces for reduction of bacterial colonization and of periodontal pathogens.

The Frenchay Activities Index. Assessment of functional status in stroke patients.

Abstract: Assessment of functional status in stroke patients is of major importance in both clinical practice and outcome studies The Frenchay Activities Index has been developed specifically for measuring disability and handicap in stroke patients The purpose of the study was to evaluate the metric properties of this instrument and to obtain normal values in a group of unselected elderly subjectsThe Frenchay Activities Index was tested in a group of stroke patients and a group of unselected subjects aged 65 or older The functional status of the stroke patients was measured 26 weeks after stroke Their prestroke status was registered retrospectively Reliability and validity of the instrument were assessedThe mean scores in the prestroke, poststroke, and control group demonstrated differences in functional status The reliability of unweighted scores (range of Cronbach's alpha-coefficients, 078 to 087) was sufficient The construct validity was supported by meaningful correlations between the Frenchay Activi

Dormancy in non-sporulating bacteria

Abstract: \"All stood amazed, until an old woman, tottering out from among the crowd, put her hand to her brow, and peering under it in his face for a moment exclaimed , 'Sure enough! it is Rip Van Winkle-it is himself. Welcome home again, old neighbour-Why, where have you been these twenty long years?\"' lrving, W. An analogical \"field\" construct in cellular biophysics: history and present status. \"These germs-these bacilli-are transparent bodies. Like glass. Like water. To make them visible you must stain them. Well, my dear Paddy, do what you will, some of them won't stain; they won't take cochineal, they won't take any methylene blue, they won't take gentian violet, they won't take any colouring matter. Consequently, though we know as scientific men that they exist, we cannot see them.\" Sir Ralph Bloomfield-Bonington. The Doctor's Dilemma.

Sequence and analysis of the genetic locus responsible for surfactin synthesis in Bacillus subtilis.

Abstract: The chromosomal region of Bacillus subtilis comprising the entire srfA operon, sfp and about four kilobases in between have been completely sequenced and functionally characterized. The srfA gene codes for three large subunits of surfactin synthetase, 402, 401 and 144 kDa, respectively, arranged in a series of seven amino acid activating domains which, as shown in the accompanying communication, recognize and bind the seven amino acids of the surfactin peptide. The srfA amino acid activating domains share homologies with similar domains of other peptide synthetases; in particular, regions can be identified which are more homologous in domains activating the same amino acid. A fourth gene in srfA encodes a polypeptide homologous to grsT. Four genes are positioned between srfA and sfp, the disruption of which does not affect surfactin biosynthesis.

Interfacial Self-Assembly of a Fungal Hydrophobin into a Hydrophobic Rodlet Layer.

Abstract: The Sc3p hydrophobin of the basidiomycete Schizophyllum commune is a small hydrophobic protein (100 to 101 amino acids) containing eight cysteine residues. Large amounts of the protein are excreted into the culture medium as monomers, but in the walls of aerial hyphae, the protein is present as an SDS-insoluble complex. In this study, we show that the Sc3p hydrophobin spontaneously assembles into an SDS-insoluble protein membrane on the surface of gas bubbles or when dried down on a hydrophilic surface. Electron microscopy of the assembled hydrophobin shows a surface consisting of rodlets spaced 10 nm apart, which is similar to those rodlets seen on the surface of aerial hyphae. When the purified Sc3p hydrophobin assembles on a hydrophilic surface, a surface is exposed with high hydrophobicity, similar to that of aerial hyphae. The rodlet layer, assembled in vivo and in vitro, can be disassembled by dissolution in trifluoroacetic acid and, after removal of the acid, reassembled into a rodlet layer. We propose, therefore, that the hydrophobic rodlet layer on aerial hyphae arises by interfacial self-assembly of Sc3p hydrophobin monomers, involving noncovalent interactions only. Submerged hyphae merely excrete monomers because these hyphae are not exposed to a water-air interface. The generally observed rodlet layers on fungal spores may arise in a similar way.

Tansley Review No. 45 Wall growth, protein excretion and morphogenesis in fungi

Abstract: SUMMARY With the exception of the unicellular yeasts, fungi typically grow by means of hyphae that extend only at their apices and ramify into a mycelium. This mode of growth provides fungi with a certain mobility and the ability to invade dead and living organic substrata. They are thus the main decomposers of plant residues but they also have established intricate symbiotic relationships with plants, both mutualistic and parasitic. The process of apical growth of a hyphae requires the controlled expansion of the apical wall which must be transformed subsequently into a wall that resists turgor pressure and maintains the tubular shape of the hyphae. Although the driving force for hyphal extension is probably the turgor pressure, a subtle interplay between wall extension and cytoplasmic activity is necessary because only a precise gradient of wall-synthetic activity can maintain uniform wall thickness during expansion. Possibly, the presence in the plasma membrane of mechanico-sensitive proteins plays a role in conjunction with the cytoskeleton at the apex, particularly action. Although the major structural wall polysaccharides are probably manufactured directly on the expanding apical plasma membrane, proteins (and probably some wall components) are delivered to the growing surface by a continuous stream of exocytotic vesicles that fuse with the plasma membrane, at the same time extending its surface. Our analyses of the chemistry of the fungal wall and its biosynthesis and assemblage have disclosed a simple mechanism (though complex in detail) that may explain the transition from a newly formed expandable wall at the apex to a more rigid wall at the base of the hyphal extension zone. Two individual wall polymers, chitin and β-glucan, extruded at the apex are modified within the domain of the wall. Among the modifications observed are the formation of covalent crosslinks between these two polymers and hydrogen bonds between the homologous polymer chains, leading to the formation of chitin microfibrils crosslinked to a glucan matrix. This process is thought to convert an initially plastic wall into a rigid wall as the polymers fall behind the advancing tip. We have called this the steady-state growth theory for apical wall extension because a steady-state amount of plastic wall is always maintained at the growing apex. Excretion of lytic enzymes is a vital process in filamentous fungi because, in nature, they thrive on organic polymers which must be degraded extracellularly. Such enzymes are also necessary for infection processes. Cytological data suggest that such enzymes are extruded by the vesicles that continuously fuse with the plasma membrane at the growing apex. We have shown that a large portion of the excreted enzymes indeed leaves the hypha at the growing apex but another portion may be retained by the wall and is slowly released into the medium. In relation to the steady-state growth theory we hypothesize that enzymes can pass the wall at the apex by bulk flow, that is, by being carried by the flow of plastic wall material, making pores in the wall less important than previously thought. Proteins excreted by filamentous fungi not only serve dissimilatory purposes but are also important for a variety of other activities of the whole mycelium, including morphogenesis. By cloning genes abundantly expressed during formation of aerial hyphae and fruit bodies, we have discovered a class of proteins, named hydrophobins, which are only produced when the mycelium has reached a certain stage of maturity. Whilst excreted by submerged hyphae as monomers into the medium, they self-assemble as insoluble complexes in the walls of emergent hyphae. In aerial hyphae a particular hydrophobin takes the form of rodlets which probably coat the hyphae with an impermeable layer. During fruit-body formation other hydrophobins are produced which may function in the aggregation of hyphae to form a multicellular tissue. Apart from such specific morphogenetic functions, the hydrophobins may play a general role in insulating hyphae from the environment, converting the differentiating structures into sinks for translocation of water and nutrients from the assimilating mycelium.

Nonlocal Screening Effects in 2p X-Ray Photoemission Spectroscopy Core-Level Line Shapes of Transition Metal Compounds

Abstract: We show that the 2p core-level line shapes of transition metal (TM) compounds are strongly influenced by the presence of more TM ions. Using calculations of clusters, involving more than one TM ion we explain the double peaked main line of the Ni 2p spectrum of NiO as well as the difference in line shape compared with Ni impurities in MgO and show the sensitivity of the line shape in NiO to surrounding defects. This intersite charge transfer screening also explains the wide asymmetric shape of the main line of the Cu 2p spectrum of CuO and the high-${\mathit{T}}_{\mathit{c}}$ compounds.

Crystal structure of deoxygenated Limulus polyphemus subunit II hemocyanin at 2.18 A resolution: clues for a mechanism for allosteric regulation.

Abstract: The crystal structure of Limulus polyphemus subunit type II hemocyanin in the deoxygenated state has been determined to a resolution of 2.18 A. Phase information for this first structure of a cheliceratan hemocyanin was obtained by molecular replacement using the crustacean hemocyanin structure of Panulirus interruptus. The most striking observation in the Limulus structure is the unexpectedly large distance of 4.6 A between both copper ions in the oxygen-binding site. Each copper has approximate trigonal planar coordination by three histidine N epsilon atoms. No bridging ligand between the copper ions could be detected. Other important new discoveries are (1) the presence of a cis-peptide bond between Glu 309 and Ser 310, with the carbonyl oxygen of the peptide plane hydrogen bonded to the N delta atom of the copper B ligand His 324; (2) localization of a chloride-binding site in the interface between the first and second domain; (3) localization of a putative calcium-binding site in the third domain. Furthermore, comparison of Limulus versus Panulirus hemocyanin revealed considerable tertiary and quaternary rigid body movements, although the overall folds are similar. Within the subunit, the first domain is rotated by about 7.5 degrees with respect to the other two domains, whereas within the hexamer the major movement is a 3.1 degrees rotation of the trimers with respect to each other. The rigid body rotation of the first domain suggests a structural mechanism for the allosteric regulation by chloride ions and probably causes the cooperative transition of the hexamer between low and high oxygen affinity states. In this postulated mechanism, the fully conserved Phe49 is the key residue that couples conformational changes of the dinuclear copper site into movements of the first domain.

Cytotoxicity of Artemisinin-Related Endoperoxides to Ehrlich Ascites Tumor Cells

Abstract: A series of artemisinin-related endoperoxides was tested for cytotoxicity to Ehrlich ascites tumor (EAT) cells using the microculture tetrazolium (MTT) assay. Artemisinin [1] had an IC50 value of 29.8 microM. Derivatives of dihydroartemisinin [2], being developed as antimalarial drugs (artemether [3], arteether [4], sodium artesunate [5], artelinic acid [6], and sodium artelinate [7]), exhibited a somewhat more potent cytotoxicity. Their IC50 values ranged from 12.2 to 19.9 microM. The presence of an exocyclic methylene fused to the lactone ring, as for artemisitene [9], led to higher cytotoxicity than 1. From the two epimeric 11-hydroxyartemisinin derivatives, the R form 12 showed a considerably higher cytotoxicity than the S form 13. Opening of the lactone ring of 1 dramatically reduced the cytotoxicity. The ether dimer 8 of 2 was the most potent cytotoxic agent, its IC50 being 1.4 microM. The variations in cytotoxicity between the structurally related compounds mostly correlated well with the theoretical capacity of radical formation and stabilization. In some cases lipophilicity or the presence of an electrophilic moiety seemed to have a determinant influence on cytotoxicity. The artemisinin-related endoperoxides showed cytotoxicity to EAT cells at higher concentrations than those needed for in vitro antimalarial activity, as reported in the literature.

Depression, anxiety, and social disability show synchrony of change in primary care patients.

Abstract: Objectives. The purposes of this study were to (1) characterize the social disability associated with the common psychiatric illnesses of primary care patients in terms of role dysfunction (self-care, family role, social role, occupational role) and (2) establish whether severity of psychiatric illness and disability level show synchrony of change. Methods. A two-stage sample design was employed. In the first stage, 1994 consecutive attenders of 25 general practitioners were screened on psychiatric illness by their physicians and with the General Health Questionnaire. A stratified random sample (n = 285) with differing probabilities was selected for a second-stage interview. Patients with psychiatric symptoms were reinterviewed 1 and 3.5 years later (n = 143). Results. (1) Disability level among patients was increased (moderately for depression, mildly for anxiety) and was associated with severity of psychiatric illness. (2) Most disability was found in occupational and social roles. (3) Change in severity of psychiatric illness was concordant with change in level of disability and was largely invariant across diagnosis (depression, anxiety, mixed anxiety/depression). At follow-up, disability among improved patients had returned to normal levels. Conclusions, Psychiatric illness in primary care patients is associated with mild to moderate disability, and severity of psychiatric illness and disability show synchrony of change.

Pediocin PA-1, a Bacteriocin from Pediococcus acidilactici PAC1.0, Forms Hydrophilic Pores in the Cytoplasmic Membrane of Target Cells

Abstract: Pediocin PA-1 is a bacteriocin which is produced by Pediococcus acidilactici PAC1.0. We demonstrate that pediocin PA-1 kills sensitive Pediococcus cells and acts on the cytoplasmic membrane. In contrast to its lack of impact on immune cells, pediocin PA-1 dissipates the transmembrane electrical potential and inhibits amino acid transport in sensitive cells. Pediocin interferes with the uptake of amino acids by cytoplasmic membrane vesicles derived from sensitive cells, while it is less effective with membranes derived from immune cells. In liposomes fused with membrane vesicles derived from both sensitive and immune cells, pediocin PA-1 elicits an efflux of small ions and, at higher concentrations, an efflux of molecules having molecular weights of up to 9,400. Our data suggest that pediocin PA-1 functions in a voltage-independent manner but requires a specific protein in the target membrane.

Atomic structure of the cubic core of the pyruvate dehydrogenase multienzyme complex.

Abstract: The highly symmetric pyruvate dehydrogenase multienzyme complexes have molecular masses ranging from 5 to 10 million daltons. They consist of numerous copies of three different enzymes: pyruvate dehydrogenase, dihydrolipoyl transacetylase, and lipoamide dehydrogenase. The three-dimensional crystal structure of the catalytic domain of Azotobacter vinelandii dihydrolipoyl transacetylase has been determined at 2.6 angstrom (A) resolution. Eight trimers assemble as a hollow truncated cube with an edge of 125 A, forming the core of the multienzyme complex. Coenzyme A must enter the 29 A long active site channel from the inside of the cube, and lipoamide must enter from the outside. The trimer of the catalytic domain of dihydrolipoyl transacetylase has a topology identical to chloramphenicol acetyl transferase. The atomic structure of the 24-subunit cube core provides a framework for understanding all pyruvate dehydrogenase and related multienzyme complexes.

Electronic-structure and spin-state transition of lacoo3

Abstract: We present soft-x-ray-absorption spectra (XAS) of ${\mathrm{LaCoO}}_{3}$ taken at different temperatures (80--630 K). The shape of the multiplets in the Co 2p XAS spectra conveys information on the symmetry and spin of the ground state. The O 1s XAS spectra are related to unoccupied metal bands through covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highly covalent low-spin state. The main contribution to the ground state in this case is given by ${\mathit{t}}_{2\mathit{g}}^{6}$${(}^{1}$${\mathit{A}}_{1}$) with an occupancy of 0.56. At higher temperature, 550 and 630 K, the results indicate a gradual transition to a mixed-spin state. The main contribution to the high-spin part of the mixture is given by ${\mathit{t}}_{2\mathit{g}}^{4}$${\mathit{e}}_{\mathit{g}}^{2}$${(}^{5}$${\mathit{T}}_{2}$) with an occupancy of 0.71. There is no evidence of charge disproportionation in the temperature range 80--630 K. The O 1s XAS spectra reflect important changes in the unoccupied Co 3d bands across the spin-state transition.