Membrane Molecular Interactions and Induced Structures of CPPs.

Cell penetrating peptides (CPPs) are generally defined as short positively charged peptides, containing 5-30 amino acids. Based on their physicochemical properties, they are classified as three main groups, namely hydrophobic, amphipathic, and hydrophilic. They are capable of interacting with the cell membrane without inducing serious toxicity, and they can carry cargo molecules across the membrane. Cargo molecules could be different therapeutics which makes CPPs valuable in the field of drug delivery into living cells. Nowadays, CPPs are considered as potential parts of therapeutics against several diseases.Despite similarities in their primary structure, the interactions of CPPs with a cell membrane may vary a lot. This is even more complicated when the CPP is bound to the cargo molecule. The mechanism(s) of their cellular uptake and endosomal escape have not been completely resolved. Understanding the mechanism of membrane interaction will help us designing a CPP with enhanced, selective cargo delivery, hopefully resulting in better disease treatments. So far energy independent direct membrane penetration and energy-dependent endocytosis have been suggested as two main mechanisms of cellular entry for CPPs, and both may be applicable for the same CPP-complex, depending on the conditions.In order to understand which mechanism is associated with a particular CPP 's cellular uptake in a particular cell (sometimes including endosomal escape), different biological and biophysical methods and strategies have been applied. In this chapter, we will address several biophysical methods, such as fluorescence spectroscopy, circular dichroism (CD) spectroscopy, dynamic light scattering, and NMR .We also review different membrane model systems which are suitable for the biophysical studies. These include large unilamellar phospholipid vesicles (LUVs ), which are the most commonly used in the lipid-peptide interaction studies. Detergent micelles and mixed micelles (bicelles) are also suitable membrane model systems, particularly in high-resolution NMR studies.