Elucidating the backbone conformation of photoswitchable foldamers using vibrational circular dichroism

Abstract

The backbone conformation of amphiphilic oligo(azobenzene) foldamers is investigated using vibrational circular dichroism (VCD) spectroscopy on a mode involving the stretching of the NN bonds in the backbone. From denaturation experiments, we find that the VCD response in the helical conformation arises mainly from through-space interaction between the NN-stretch transition-dipole moments, so that the coupled-oscillator model can be used to predict the VCD spectrum associated with a particular conformation. Using this approach, we elucidate the origin of the VCD signals in the folded conformation, and can assign the observed partial loss of VCD signals upon photo-induced unfolding to specific conformational changes. Our results show that the NN-stretch VCD response provides an excellent probe of the helical conformation of the NN bonds in this type of switchable molecular system.