Novel 2,5-Hexanedione Analogues. Substituent-Induced Control of the Protein Cross-Linking Potential and Oxidation Susceptibility of the Resulting Primary Amine-Derived Pyrroles

Abstract

The neurotoxic gamma-diketone, 2,5-hexanedione (2,5-HD), induces neurofilamentous swellings at prenodal sites in proximal axons as a consequence of pyrrolation of lysine epsilon-amino groups on neurofilament proteins. However, there is disagreement as to whether pyrrole formation and the associated alteration of noncovalent interactions is sufficient to cause neurofilament accumulation, or whether pyrrole autoxidation and subsequent protein-protein cross-linking is an obligatory event. To investigate gamma-diketones that might form pyrroles inert to autoxidative-induced cross-linking, we synthesized 1,1,1-trifluoro-2,5-hexanedione, 3-(trifluoromethyl)-2,5-hexanedione (3-TFMHD), and two 3-(dialkylaminocarbonyl)-2,5-diketones and assessed their rates of pyrrole formation with amines, the oxidation susceptibility of the resulting pyrroles, and the protein cross-linking potential in vitro, relative to those of 3-methyl-2,5-hexanedione. 1,1,1-Trifluoro-2,5-hexanedione does not form pyrroles, but the three 2,5-HD analogues with an electron-withdrawing 3-substituent all rapidly formed pyrroles that were inert to autoxidation. Although 3-TMFHD nonetheless still induced cross-linking of ribonuclease A, by a nonoxidative mechanism independent of the pyrrole, the two 3-(dialkylaminocarbonyl)-2,5-diketones did not exhibit any protein cross-linking. As these two gamma-diketones possess aqueous-organic partitioning properties similar to those of 2,5-HD, they should serve as useful mechanistic probes in further studies.

Chromogenic and Neurotoxic Effects of an Aliphatic γ-Diketone: Computational Insights into the Molecular Structures and Mechanism

“…The molecules in Chart are models of the possible pyrrole−protein/amino acid adduct monomers formed from 2,5-HD. Xu et al observed that 2,5-HD-derived pyrroles underwent autoxidation at pH 7.4 and that the methyl group (CH 3 ) on the pyrrole ring could easily be oxidized to become CHO. As shown in Chart , our TD-DFT results suggest that linking one or two CHO (or COO - ) groups to the pyrrole ring slightly increases the wavelength of the first absorption, but not by enough to shift the absorption to the visible region.…”

“…A number of experimental studies have provided insights into the molecular mechanisms of the chromogenic effects of aromatic and aliphatic γ-diketones. Recent experimental studies suggest that aromatic and aliphatic γ-diketones have common neural targets. , Other reported experimental results and structure−activity considerations led to hypotheses for the possible molecular mechanisms of the chromogenic effects. ,, However, it is difficult to identify the chromophores by experiment alone because the reaction systems are complicated mixtures and the reaction products have not yet been separated. Therefore, reliable predictions of possible chromophores based on good calculations of their electronic spectra are needed to complement ongoing experimental investigations to uncover the molecular mechanisms of the chromogenic effects.…”

“…However, it is well-established that pyrrole itself ( 2 in Chart ) and some substituted pyrroles do not absorb in the visible region. , However, the pyrrole−protein adduct can exhibit color by either subsequent autoxidation 16 or dimerization 5 (Scheme ). On the basis of the hypotheses described in the literature, the autoxidation of the dimethyl pyrrole−protein adduct formed from 2,5-HD should lead to an adduct structure in which the two methyl groups are all converted into CHO groups, whereas the dimerization of the pyrrole−protein adducts should lead to structure 10 as shown in Scheme 1

2

…”

Electronic Excitations in Pyrrole: A Test Case for Determination of Chromophores in the Chromogenic Effects of Neurotoxic Hydrocarbons by Time-Dependent Density Functional Theory and Single-Excitation Configuration Interaction Methods

“…For example, pyrrole as a liquid turns brown in air (6) and 3-methylpyrrole is readily autoxidized, as evidenced by a rapid color change (7). The structures of the chromophores are unknown, although recently reported experiments on the autoxidation of the neopentylamine-derived from 3-methylpyrrole suggest that the chromophore could be pyrrolecarboxyaldehydes, pyrrole-pyrrole dehydrodimers, or other adducts (7). Reliable prediction of electronic excitations in pyrrole-involving Supplementary data for this article may be found on the journal home page.…”

Neurotoxicity: The Broad Framework of Electron Transfer, Oxidative Stress and Protection by Antioxidants

“…Little attention has been paid to an alternate route for crosslinking which is independent of pyrrole [41]. We speculate that facile oxidation of CH 2 CH 2 may occur.…”