Perceptual simulation in conceptual tasks

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XANES/EXAFS STUDY OF THE COPPER ACTIVE SITE IN METHANOL SYNTHESIS CATALYST

We have used X-ray absorption spectroscopy to determine the electronic and geometric structure of the copper active site in methanol synthesis catalysts. Compared to the edge and FT of the EXAFS data of CuO, the data suggest that most of the Cu(II) in the calcined form does not have the same ligation as in the CuO lattice and more importantly that some of the Cu is dissolved in the ZnO lattice. For the reduced sample, the edge shifts to lower energy after reduction, indicating the presence of either Cu(I) or Cu(0). The EXAFS data analyses of reduced catalysts show the presence of copper clusters. Reaction with H2S is shown to decompose the Cu clusters in the reduced catalyst and the level of Cu cluster decomposition increases with further reaction at higher temperature and longer time period

QUANTITATIVE Cu X-RAY ABSORPTION EDGE STUDIES : OXIDATION STATE AND SITE STRUCTURE DETERMINATION

X-ray absorption edge studies of Cu(I) complexes with different coordination number and covalency reveal that the 8983-8984 eV feature (assigned as the 1s→4p transition) can be correlated with ligation and site geometry. These Cu(I) features have been qualitatively interpreted using a Ligand Field model, and this has been applied to analyze the polarized single crystal, pH dependent edge spectra of reduced plastocyanin. In addition, normalized difference X-ray absorption edge analysis has been used to quantitatively determine the percent of Cu(I) in several derivatives of the multicopper oxidase, laccase

A Cognitive Model of Multi-Objective Multi-Concept Formation

Abstract. The majority of previous computational models of high-order human cognition incorporate gradient descent algorithms for their learning mechanisms and strict error minimization as the sole objective of learning. Recently, however, the validity of gradient descent as a descriptive model of real human cognitive processes has been criticized. In the present paper, we introduce a new framework for descriptive models of human learning that offers qualitatively plausible interpretations of cognitive behaviors. Specifically, we apply a simple multi-objective evolutionary algorithm as a learning method for modeling human category learning, where the definition of the learning objective is not based solely on the accuracy of knowledge, but also on the subjectively and contextually determined utility of knowledge being acquired. In addition, unlike gradient descent, our model assumes that humans entertain multiple hypotheses and learn not only by modifying a single existing hypothesis but also by combining a set of hypotheses. This learning-by-combination has been empirically supported, but largely overlooked in computational modeling research. Simulation studies show that our new modeling framework successfully replicated observed phenomena.

Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin π->π* transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P->Fe σ and π-donation and Fe->P π back-bonding. We find that π-donation to Fe(III) is much larger than back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry

Investigation of the anomalous spectroscopic features of the copper sites in chicken ceruloplasmin: comparison to human ceruloplasmin.

Chicken ceruloplasmin has been previously reported to display a number of key differences relative to human ceruloplasmin: a lower copper content and a lack of a type 2 copper signal by electron paramagnetic resonance (EPR) spectroscopy. We have studied the copper sites of chicken ceruloplasmin in order to probe the origin of these differences, focusing on two forms of the enzyme: "resting" (as isolated by a fast, one-step procedure) and "peroxide-oxidized''. From X-ray absorption, EPR, and UV/visible absorption spectroscopies, we have shown that all of the copper sites are oxidized in peroxide-oxidized chicken ceruloplasmin and that none of the type 1 copper sites display the EPR features typical for type 1 copper sites that lack an axial methionine. In the resting form, the type 2 copper center is reduced. Upon oxidation, it does not appear in the EPR spectrum at 77 K, but it can be observed by using magnetic susceptibility, EPR at similar to 8 K, and magnetic circular dichroism spectroscopy. It displays unusually fast relaxation, indicative of coupling with the adjacent type 3 copper pair of the trinuclear copper cluster. From reductive titrations, we have found that the reduction potential of the type 2 center is higher than those of the other copper sites, thus explaining why it is reduced in the resting form. These results provide new insight into the nature of the additional type 1 copper sites and the redox distribution among copper sites in the different ceruloplasmins relative to other multicopper oxidases

Fe L-Edge X-ray Absorption Spectroscopy of Low-Spin Heme Relative to Non-heme Fe Complexes: Delocalization of Fe d-Electrons into the Porphyrin Ligand

Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin π->π* transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P->Fe σ and π-donation and Fe->P π back-bonding. We find that π-donation to Fe(III) is much larger than back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry