A fluorescent electrophile for CLIPS: self indicating TrkB binders.

Combination of cysteine-containing peptides with electrophiles provides efficient access to cyclo -organopeptides. However, there are no routes to intrinsically fluorescent cyclo -organopeptides containing robust, brilliant fluorophores emitting at wavelengths longer than cellular autofluorescence. We show such fluorescent cyclo -organopeptides can be made via S N Ar reactions of cysteine-containing peptides with a BODIPY system. Seven compounds of this type were prepared to test as probes; six contained peptide sequences corresponding to loop regions in brain-derived neurotrophic factor and neurotrophic factor 4 (BDNF and NT-4) which bind tropomyocin receptor kinase B (TrkB). Cellular assays in serum-free media indicated two of the six key compounds induced survival of HEK293 cells stably transfected with TrkB whereas a control did not. The two compounds inducing cell survival bound TrkB on those cells ( K d ∼40 and 47 nM), illustrating how intrinsically fluorescent cyclo -organopeptides can be assayed for quantifiable binding to surface receptors in cell membrane environments.