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Aminotroponiminates: Impact of the NO₂ Functional Group on Coordination, Isomerisation, and Backbone Substitution.

ABSTRACT: Aminotroponiminate (ATI) ligands are a versatile class of redox-active and potentially cooperative ligands with a rich coordination chemistry that have consequently found a wide range of applications in synthesis and catalysis. While backbone substitution of these ligands has been investigated in some detail, the impact of electron-withdrawing groups on the coordination chemistry and reactivity of ATIs has been little investigated. We report here Li, Na, and K salts of an ATI ligand with a nitro-substituent in the backbone. It is demonstrated that the NO₂ group actively contributes to the coordination chemistry of these complexes, effectively competing with the N,N-binding pocket as a coordination site. This results in an unprecedented E/Z isomerisation of an ATI imino group and culminates in the isolation of the first "naked" (i. e., without directional bonding to a metal atom) ATI anion. Reactions of sodium ATIs with silver(I) and tritylium salts gave the first N,N-coordinated silver ATI complexes and unprecedented backbone substitution reactions. Analytical techniques applied in this work include multinuclear (VT-)NMR spectroscopy, single-crystal X-ray diffraction analysis, and DFT calculations.

SUBMITTER: Hanft A

PROVIDER: S-EPMC8597084 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BACKGROUND: The amyloid-? (A?) peptide is the primary component of the extracellular senile plaques characteristic of Alzheimer's disease (AD). The metals hypothesis implicates redox-active copper ions in the pathogenesis of AD and the Cu(2+) coordination of various A? peptides has been widely studied. A number of disease-associated modifications involving the first 3 residues are known, including isomerisation, mutation, truncation and cyclisation, but are yet to be characterised in detail. In particular, A? in plaques contain a significant amount of truncated pyroglutamate species, which appear to correlate with disease progression. METHODOLOGY/PRINCIPAL FINDINGS: We previously characterised three Cu(2+)/A?1-16 coordination modes in the physiological pH range that involve the first two residues. Based upon our finding that the carbonyl of Ala2 is a Cu(2+) ligand, here we speculate on a hypothetical Cu(2+)-mediated intramolecular cleavage mechanism as a source of truncations beginning at residue 3. Using EPR spectroscopy and site-specific isotopic labelling, we have also examined four A? peptides with biologically relevant N-terminal modifications, A?1[isoAsp]-16, A?1-16(A2V), A?3-16 and A?3[pE]-16. The recessive A2V mutation preserved the first coordination sphere of Cu(2+)/A?, but altered the outer coordination sphere. Isomerisation of Asp1 produced a single dominant species involving a stable 5-membered Cu(2+) chelate at the amino terminus. The A?3-16 and A?3[pE]-16 peptides both exhibited an equilibrium between two Cu(2+) coordination modes between pH 6-9 with nominally the same first coordination sphere, but with a dramatically different pH dependence arising from differences in H-bonding interactions at the N-terminus. CONCLUSIONS/SIGNIFICANCE: N-terminal modifications significantly influence the Cu(2+) coordination of A?, which may be critical for alterations in aggregation propensity, redox-activity, resistance to degradation and the generation of the A?3-× (×?=?40/42) precursor of disease-associated A?3[pE]-x species.

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Aminotroponiminates: Impact of the NO₂ Functional Group on Coordination, Isomerisation, and Backbone Substitution.

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Aminotroponiminates: Impact of the NO₂ Functional Group on Coordination, Isomerisation, and Backbone Substitution.