Investigating Membrane Interactions and Structures of CPPs

Despite many studies made on cell-penetrating peptides (CPPs), the mechanism of their cellular uptake and endosomal escape has not been completely resolved. This is even more unclear when the CPP is bound either covalently or non-covalently to the cargo molecules. To answer remaining questions, we require a combination of different methods, model systems, and experiments since there is no single method which could give a complete answer to all questions. Biophysical investigations of CPPs have a significant impact on CPP research considering their molecular mechanisms of action. In this chapter, we present different membrane model systems suitable for biophysical studies as well as the basic practical aspects underlying several common biophysical methods and experiments. The methods include fluorescence spectroscopy, circular dichroism spectroscopy, and dynamic light scattering and concern peptide-membrane interactions and vesicle model membrane leakage. We have also described the potential and limitations of biophysical studies on the CPP-membrane interactions and their impact on our understanding of how CPPs mediate the transport of cargoes into living cells.