Far-UV circular dichroism signatures indicate fluorophore labeling induced conformational changes of penetratin.

Fluorescent labeling is a broadly utilized approach to assess in vitro and in vivo behavior of biologically active, especially cell-penetrating and antimicrobial peptides. In this communication, far-UV circular dichroism (CD) spectra of penetratin (PEN) fluorophore conjugates reported previously have been re-evaluated. Compared to the intrinsically disordered native peptide, rhodamine B and carboxyfluorescein derivatives of free and membrane-bound PEN exhibit extrinsic CD features. Potential sources of these signals displayed above 220 nm are discussed suggesting the contributions of both intra- and intermolecular chiral exciton coupling mechanisms. Careful evaluation of the CD spectra of fluorophore-labeled peptides is a valuable tool for early detection of labeling-provoked structural alterations which in turn may modify the membrane binding and cellular uptake compared to the unconjugated form.