Browsing by Subject "Dimerization"

Now showing 1 - 4 of 4

Open Access
FTIR spectroscopic study on nickel(II)-exchanged sulfated alumina: nature of the active sites in the catalytic oligomerization of ethene
(Springer, 2002) Davydov, A. A.; Kantcheva, M.; Chepotko, M. L.
The nature of the active sites in nickel(II)-exchanged sulfated alumina in the reaction of ethene oligomerization has been studied by means of FTIR spectroscopy of adsorbed CO. It has been established that isolated nickel(I) species are the active sites in this process. These sites are formed by a reduction process, in which protonic centers are involved. The latter are due to the presence of covalently-bonded sulfate ions on the catalyst surface.
Open Access
Hydrogen storage capacity of titanium met-cars
(IOP Publishing Ltd., 2006) Akman, N.; Durgun, Engin; Yildirim, T.; Çıracı, Salim
The adsorption of hydrogen molecules on the titanium metallocarbohedryne (met-car) cluster has been investigated by using the first-principles plane wave method. We have found that, while a single Ti atom at the corner can bind up to three hydrogen molecules, a single Ti atom on the surface of the cluster can bind only one hydrogen molecule. Accordingly, a Ti8C12 met-car can bind up to 16H2 molecules and hence can be considered as a high-capacity hydrogen storage medium. Strong interaction between two met-car clusters leading to the dimer formation can affect H2 storage capacity slightly. Increasing the storage capacity by directly inserting H 2 into the met-car or by functionalizing it with an Na atom have been explored. It is found that the insertion of neither an H2 molecule nor an Na atom could further promote the H2 storage capacity of a Ti8C12 cluster. We have also tested the stability of the H2-adsorbed Ti8C12 met-car with ab initio molecular dynamics calculations which have been carried out at room temperature.
Open Access
Protein folding, misfolding and aggregation: the importance of two-electron stabilizing interactions
(Public Library of Science, 2017) Cieplak, A. S.
Proteins associated with neurodegenerative diseases are highly pleiomorphic and may adopt an all-α-helical fold in one environment, assemble into all-β-sheet or collapse into a coil in another, and rapidly polymerize in yet another one via divergent aggregation pathways that yield broad diversity of aggregates’ morphology. A thorough understanding of this behaviour may be necessary to develop a treatment for Alzheimer’s and related disorders. Unfortunately, our present comprehension of folding and misfolding is limited for want of a physicochemical theory of protein secondary and tertiary structure. Here we demonstrate that electronic configuration and hyperconjugation of the peptide amide bonds ought to be taken into account to advance such a theory. To capture the effect of polarization of peptide linkages on conformational and H-bonding propensity of the polypeptide backbone, we introduce a function of shielding tensors of the Cα atoms. Carrying no information about side chain-side chain interactions, this function nonetheless identifies basic features of the secondary and tertiary structure, establishes sequence correlates of the metamorphic and pH-driven equilibria, relates binding affinities and folding rate constants to secondary structure preferences, and manifests common patterns of backbone density distribution in amyloidogenic regions of Alzheimer’s amyloid β and tau, Parkinson’s α-synuclein and prions. Based on those findings, a split-intein like mechanism of molecular recognition is proposed to underlie dimerization of Aβ, tau, αS and PrPC, and divergent pathways for subsequent association of dimers are outlined; a related mechanism is proposed to underlie formation of PrPSc fibrils. The model does account for: (i) structural features of paranuclei, off-pathway oligomers, non-fibrillar aggregates and fibrils; (ii) effects of incubation conditions, point mutations, isoform lengths, small-molecule assembly modulators and chirality of solid-liquid interface on the rate and morphology of aggregation; (iii) fibril-surface catalysis of secondary nucleation; and (iv) self-propagation of infectious strains of mammalian prions. © 2017 Andrzej Stanisław Cieplak. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Open Access
Transition-metal-ethylene complexes as high-capacity hydrogen-storage media
(American Physical Society, 2006) Durgun, Engin; Çıracı, Salim; Zhou, W.; Yildirim, T.
From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of ∼14wt%. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.