Specular neutron reflectivity and the structure of artificial protein maquettes vectorially oriented at interfaces

Abstract

Artificial peptides can be designed to possess a variety of functionalities. If these peptides can be ordered in an ensemble, the functionality can impart macroscopic material properties to the ensemble. Neutron reflectivity is shown to be an effective probe of the intramolecular structures of such peptides vectorially oriented at an interface, key to ensuring that the designed molecular structures translate into the desired material properties of the interface. A model-independent method is utilized to analyze the neutron reflectivity from an alkylated, di- alpha -helical peptide, containing perdeuterated leucine residues at one or two pre-selected positions, in mixed Langmuir monolayers with a phospholipid. The results presented here are more definitive than prior work employing x-ray reflectivity. They show explicitly that the di-helical peptide retains its designed alpha -helical secondary structure at the interface, when oriented perpendicular to the interface at high surface pressure, with the helices projecting into the aqueous subphase without penetrating the layer of phospholipid headgroups.

The interfacial structure and Young's modulus of peptide films having switchable mechanical properties

“…This result, coupled with the measured interfacial modulus of 120 mN m K1 , allows a Young modulus of EZ80 MPa to be calculated. This quantitative result is consistent with estimates for other biological materials such as peptide fibres (1-20 MPa; Leon et al 1998), collagen fibres in buffer (200-500 MPa; van der Rijt et al 2006) and the wall of the yeast cell (100-130 MPa; Smith et al 2000a,b). To the best of our knowledge, this result represents the first in situ experimental determination of the Young modulus of a biological material self-assembled at a soft interface and has been performed on a molecularly thin film.…”

“…This high modulus in the populated 'on' state is stronger than that of the AM1 film (80 MPa), most probably as a consequence of sequence differences and the higher peptide concentration at the interface (70% volume fraction for Lac21E compared with 55% for AM1). The Lac21E modulus is comparable to that reported for collagen fibres (200-500 MPa; van der Rijt et al 2006), which comprise a dense nanostructure of interacting helices. These results suggest an upper limit for the Young modulus of hydrated biomaterials comprising interacting peptide or protein helices and a significant dependence of modulus on peptide concentration.…”

“…In the present study, we carried out neutron reflectivity studies of the AM1 interfacial film to directly determine film thickness and also to investigate how the individual peptide molecules are organized at the interface. Neutron reflectometry is a direct method for probing the amount of material adsorbed at a soft interface (Lu et al 1996, while selective deuteration allows the reflectivity profile of a nanostructure to be altered without changing its chemical composition (Fragneto et al 1995;Strzalka et al 2004). We confirmed that the AM1 surface coverage is approximately the same in both highand low-elasticity states and showed that in both cases the film comprises a thin layer of ordered peptides.…”

“…Figure 2 shows neutron reflectometry results for peptide AM1 self-assembled at the air-water interface, under conditions shown previously to create a mechanically strong film (interfacial elasticity, E!t of 120 mN m K1 and maximum interfacial tensile stress of 6.9 mN m K1 ) which is able to stabilize an emulsion or foam (Dexter et al 2006). All valine and leucine residues in the 21-residue AM1 sequence were deuterated, allowing contrast matching to reveal different molecular features ( Fragneto et al 1995;Strzalka et al 2004). Data were described using a two-sub-layer hydrophilic-hydrophobic Gaussian model, with an allowance for roughness of the water surface Penfold & Thomas 2002).…”

Optical Response Near the Soft X-Ray Absorption Edges and Structural Studies of Low Optical Contrast System Using Soft X-Ray Resonant Reflectivity

“…If there is change in composition of material, density of materials and slight variation in effective electron density, the near edge optical response would vary drastically. The result is analogous to using deuteration as marker in neutron reflectivity, however, without requiring any special sample preparation [74,75]. Therefore, tuning incident photon energy near absorption edge would make possible the analysis of composition of buried interfaces, formation of native oxide and depth profiling of variation of atomic density in thin film using soft X-ray resonant reflectivity.…”