Orientational Distributions of the Di-α-helical Synthetic Peptide ZnPPIX-BBC16 in Langmuir Monolayers by X-ray Reflectivity and Polarized Epifluorescence

The vectorial orientation of the peptide ZnPPIX-BBC16 in a Langmuir monolayer was studied by means of X-ray reflectivity and polarized epifluorescence. ZnPPIX-BBC16 consists of two α-helical 31-mers dimerized via a disulfide bridge between two N-terminal cysteines (α-S−S-α), so that two bis-his metalloporphyrin binding sites between positions 10,10‘ and 24,24‘ are formed. Zn(II) protoporphyrin IX was bound to the position 24. To enhance stability at the interface, a palmitoyl (C16) chain was bonded to each N-terminal cysteine. X-ray reflectivity measurements make it possible to infer the orientation of the α-helices by determining the electron density profile of the monolayer. Polarized epifluorescence provides the orientation distribution of the porphyrin with respect to the monolayer plane. Both X-ray and fluorescence measurements show that at low surface pressure (π = 5 mN/m), the peptide α-helices lie in the plane of the water surface with a narrow orientational distribution of the porphyrin. At high ...