β-ketoenole dyes: synthesis and study as fluorescent sensors for protein amyloid aggregates

The series of the new β-ketoenole dyes ((2E,5Z,7E,9E)-6-hydroxy-2-(alkylamino)-10-phenyldeca-2,5,7,9-tetraen-4-ones) with variation of alkylamino tail groups was synthesized and studied as potential probes for the sensing of protein aggregates amyloid fibrils. The dyes are low fluorescent when free but able to increase their emission intensity in dozens of times in the presence of fibrillar insulin. The fluorescent response of the dye on fibrillar insulin strongly depends on the nature of the alkylamino tail group. For compounds with propylamino (dye 13) and 2-hydroxyethylamino (dye 14) fragments the fluorescence intensity in the presence of fibrillar insulin exceeds that for the native one in 22 and 66 times correspondingly. However dyes demonstrate from low or moderate exceed of the fluorescence intensity in the presence of aggregated lysozyme compared to native one (up to 8.7 times for the dye 53 bearing methyl ester tail group), due to their pronounced sensitivity to native lysozyme. The dyes in complexes with insulin have rather height quantum yield up to 0.15, the large Stokes shifts values (about 100 nm and more), their binding constant values are about 105 M-1. The dye 14 allows fluorescent detection of the insulin amyloid fibrils in the concentration range 1-50 μg/ml. This causes an interest in the future study of the β-ketoenole as prospective fluorescent amyloid-sensitive molecules.