Comparison of the helix–coil transition of a titrating polypeptide in aqueous solutions and at the air–water interface Original Research Article

The transition from α-helix to random coil of the titrating polyamino acid co-poly-l-(lysine, phenylalanine), (p-(Lys,Phe)), has been investigated as a function of pH and ionic strength in aqueous solution and at the air–water interface by means of circular dichroism (CD) spectroscopy and the Langmuir surface film balance technique. The results strongly suggest that the helix–coil transition for peptides at the air–water interface can be determined by using the two-dimensional Flory exponent, ν, to express the pH dependent peptide surface conformation. The helix–coil titration curve of p-(Lys,Phe) shifts approximately 2.5 pH units towards lower pH at the air–water interface, as compared with the bulk solution. This finding is of relevance for the understanding of conformation and conformational changes of membrane-transporting and membrane penetrating peptides as well as for the use of peptides in molecular devices.