Novel 2,5-hexanedione analogues. Substituent-induced control of the protein cross-linking potential and oxidation susceptibility of the resulting primary amine-derived pyrroles

The neurotoxic gamma -diketone, 2,5-hexanedione (2,5-HD), induces neurofila mentous swellings at prenodal sites in proximal axons as a consequence of p yrrolation of lysine E-amino groups on neurofilament proteins. However, the re is disagreement as to whether pyrrole formation and the associated alter ation of noncovalent interactions is sufficient to cause neurofilament accu mulation, or whether pyrrole autoxidation and subsequent protein-protein cr oss-linking is an obligatory event. To investigate gamma -diketones that mi ght form pyrroles inert to autoxidative-induced cross-linking, we synthesiz ed 1,1,1-trifluoro-2,5-hexanedione, 3-(trifluoromethyl)-2,5-hexanedione (3- TFMHD), and two 3-(dialkylaminocarbonyl)-2,5-diketones and assessed their r ates of pyrrole formation with amines, the oxidation susceptibility of the resulting pyrroles, and the protein cross-linking potential in vitro, relat ive to those of 3-methyl-2,5-hexanedione. 1,1,1-Trifluoro-2,5-hexanedione d oes not form pyrroles, but the three 2,5-HD analogues with an electron-with drawing 3-substituent all rapidly formed pyrroles that were inert to autoxi dation. Although 3-TMFHD nonetheless still induced cross-linking of ribonuc lease A, by a nonoxidative mechanism independent of the pyrrole, the two 3- (dialkylaninocarbonyl)-2,5-diketones did not exhibit any protein cross-link ing. As these two gamma -diketones possess aqueous-organic partitioning pro perties similar to those of 2,5-HD, they should serve as useful mechanistic probes in further studies.