POLYPEPTIDES - STRUCTURES AND INTERACTIONS IN MONOLAYERS AND AQUEOUS SOLUTIONS

Sammanfattning: The aim of this thesis work is to understand and be able to control polypeptide conformation and interactions at interfaces and in solution by changing the solution conditions. For this purpose simple model polypeptides, so-called polyamino acids, have been investigated at the air-liquid interface, as well as in aqueous solutions. The influence of subphase characteristics, spreading solvent and polypeptide molecular weight on the film morphology and molecular conformation have been investigated for polypeptide monolayers of a number of different polypeptides. The studies have been conducted by surface film balance experiments combined with circular (CD) and linear (LD) dichroism spectroscopy and atomic force microscopy (AFM) on monolayers deposited with the Langmuir-Blodgett (LB) technique. Polypeptides spread from water soluble spreading solvents have been shown to aggregate at the air-water interface. This type of aggregation easily lead to misinterpretations of experimental data and probably accounts for inconsistencies of previously published reports on the conformation of polypeptides at interfaces.A method to determine the polypeptide secondary structure in films comprising laterally orientated polypeptide strands has been developed. Lateral orientation, as observed in CD spectroscopy, has been used as a tool to study the surface arrangement of all-helical polypeptide rods. Even for long polypeptides, consisting only of aliphatic amino acid residues, subphase characteristics (e.g. pH and salt) was shown to affect the surface lateral polypeptide arrangement. This amply demonstrates a surprisingly large influence of end-group charges on the inter-molecular ordering in polypeptide films. The pH induced conformational transition from alpha-helix to random coil of titrating polypeptides has been studied both at the air-liquid interface and in solution. The results show that the transition at the air-water interface is shifted around two pH units towards lower pH, as compared with the transition in solution. In the investigations of the helix-coil transition in solution, both experimental studies (by CD spectroscopy) and theoretical calculations have been performed. The helix-coil transition in two different lysine-containing polypeptides displayed an unexpected salt dependence in the transition region, where an increase of the ionic strength was found to lead to a decreased fraction of polypeptide in alpha-helix conformation.In addition, the interactions between polypeptides and non-ionic surfactants have been investigated, mainly utilising CD spectroscopy, NMR and surfactant binding isotherms. It was concluded that the mutual interaction between charged polypeptides and non-ionic surfactants can be used to tune the conformation of the polypeptide in solution: 1) A certain degree of polypeptide hydrophobicity was shown to be essential in order to obtain a stabilisation of the alpha-helical conformation upon surfactant addition. 2) Surfactants with small head groups and a propensity to form spherical or nearly spherical micelles (i.e. surfactants with low values of the critical packing parameter) yielded the most efficient stabilisation of alpha-helix. The polypeptide-surfactant complex can best be described by a necklace model, with surfactant micelles formed on the polypeptide chain with the hydrophobic polypeptide side chains interacting with the hydrophobic micellar core and the inner part of the surfactant head group.