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Are Methylaluminoxane Activators Sheets?

ABSTRACT: Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me₃ Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]^- ) and [Me₂ Al]⁺ abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me₂ Al]⁺ abstraction, while [Me]^- abstraction from Me₃ Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)_n , suggest that anions such as [(MeAlO)₇ (Me₃ Al)₄ Me]^- =[7,4]^- are especially stable compared to higher homologues, even though their neutral precursors are unstable.

SUBMITTER: Collins S

PROVIDER: S-EPMC8362195 | biostudies-literature | 2021 Jul

REPOSITORIES: biostudies-literature

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Are Methylaluminoxane Activators Sheets?

Collins Scott S Hasan Galib G Joshi Anuj A McIndoe J Scott JS Linnolahti Mikko M

Chemphyschem : a European journal of chemical physics and physical chemistry 20210614 13

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me3 Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]- ) and [Me2 Al]+ abstraction, involving this donor, were studied by polarizable continuum model cal ...[more]

PMID: 33971081

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Project description:The development of peptide beta-hairpins is problematic, because folding depends on the amino acid sequence and changes to the sequence can significantly decrease folding. Robust beta-hairpins that can tolerate such changes are attractive tools for studying interactions involving protein beta-sheets and developing inhibitors of these interactions. This paper introduces a new class of peptide models of protein beta-sheets that addresses the problem of separating folding from the sequence. These model beta-sheets are macrocyclic peptides that fold in water to present a pentapeptide beta-strand along one edge; the other edge contains the tripeptide beta-strand mimic Hao [JACS 2000, 122, 7654] and two additional amino acids. The pentapeptide and Hao-containing peptide strands are connected by two delta-linked ornithine (deltaOrn) turns [JACS 2003, 125, 876]. Each deltaOrn turn contains a free alpha-amino group that permits the linking of individual modules to form divalent beta-sheets. These "cyclic modular beta-sheets" are synthesized by standard solid-phase peptide synthesis of a linear precursor followed by solution-phase cyclization. Eight cyclic modular beta-sheets 1a-1h containing sequences based on beta-amyloid and macrophage inflammatory protein 2 were synthesized and characterized by 1H NMR. Linked cyclic modular beta-sheet 2, which contains two modules of 1b, was also synthesized and characterized. 1H NMR studies show downfield alpha-proton chemical shifts, deltaOrn delta-proton magnetic anisotropy, and NOE cross-peaks that establish all compounds but 1c and 1g to be moderately or well folded into a conformation that resembles a beta-sheet. Pulsed-field gradient NMR diffusion experiments show little or no self-association at low (</=2 mM) concentrations. Changes to the residues in the Hao-containing strands of 1c and 1g improve folding and show that folding of the structures can be enhanced without altering the sequence of the pentapeptide strand. Well-folded cyclic modular beta-sheets 1a, 1b, and 1f each have a phenylalanine directly across from Hao, suggesting that cyclic modular beta-sheets containing aromatic residues across from Hao are better folded.

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Are Methylaluminoxane Activators Sheets?

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Are Methylaluminoxane Activators Sheets?

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