Abstract
The principles of neutron reflectivity and its application as a tool to provide structural information at the (sub-) molecular unit length scale from models for bacterial membranes are described. The model membranes can take the form of a monolayer for a single leaflet spread at the air/water interface, or bilayers of increasing complexity at the solid/liquid interface. Solid-supported bilayers constrain the bilayer to 2D but can be used to characterize interactions with antimicrobial peptides and benchmark high throughput lab-based techniques. Floating bilayers allow for membrane fluctuations, making the phase behaviour more representative of native membranes. Bilayers of varying levels of compositional accuracy can now be constructed, facilitating studies with aims that range from characterizing the fundamental physical interactions, through to the characterization of accurate mimetics for the inner and outer membranes of Gram-negative bacteria. Studies of the interactions of antimicrobial peptides with monolayer and bilayer models for the inner and outer membranes have revealed information about the molecular control of the outer membrane permeability, and the mode of interaction of antimicrobials with both inner and outer membranes.
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Notes
- 1.
For this reason, it is convenient (and usual) to characterize a material's neutron optical properties by a scattering length density, \(\rho\).
- 2.
The incoherent background is largely due to the presence of hydrogen, which has a very high incoherent cross-section, in the sample/sub-phase, which is unavoidable in biologically relevant samples; in solid/liquid experiments this can be minimized to some extent by using a low sub-phase volume, such as in the laminar flow cell illustrated in the right panel of Fig. 1.
- 3.
At a fundamental level, the average (pseudo)potential experienced by the neutron is \(\bar{V} = \frac{{2\pi \hbar^{2} }}{m}\rho\). Since the momentum change in specular reflection is solely perpendicular to the interface, it is the force on the neutron perpendicular to the interface that is important in determining the reflectivity. Since the force is given by the gradient of the potential, it is clear why it is the gradient of the scattering length density profile that is important in determining the reflectivity. The same conclusion can be drawn in an explicit mathematical form by application of the Born approximation \(R(Q) = \frac{{16\pi^{2} }}{{Q^{4} }}\left| {\tilde{\rho }(Q)} \right|^{2}\), where \(\tilde{\rho }(Q) = \int_{-\infty}^{\infty } {\frac{d\rho (z)}{dz}\exp ( - iQz)dz}\) is the Fourier transform of the derivative of the scattering length density profile perpendicular to the interface.
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Acknowledgments
We acknowledge the STFC for the award of beamtime at ISIS (RB1220350) and the Institut Laue-Langevin (8-02-637/8-02-682/8-02-704), and the EPSRC for a studentship to Laura McKinley (EP/L504956/1).
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Barker, R.D., McKinley, L.E., Titmuss, S. (2016). Neutron Reflectivity as a Tool for Physics-Based Studies of Model Bacterial Membranes. In: Leake, M. (eds) Biophysics of Infection. Advances in Experimental Medicine and Biology, vol 915. Springer, Cham. https://doi.org/10.1007/978-3-319-32189-9_16
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