# Structure of Poly($$\gamma$$-Benzyl-L-Glutamate) Monolayers at the Gas–Water Interface: A Brewster Angle Microscopy and X-Ray Scattering Study

 Title: Structure of Poly($$\gamma$$-Benzyl-L-Glutamate) Monolayers at the Gas–Water Interface: A Brewster Angle Microscopy and X-Ray Scattering Study Author: Fukumo, Masafumi; Heilmann, Ralf K.; Pershan, Peter S.; Yu, Seungju M.; Griffiths, Jennifer A.; Tirrell, David A. Note: Order does not necessarily reflect citation order of authors. Citation: Fukumo, Masafumi, Ralf K. Heilmann, Peter S. Pershan, Seungju M. Yu, Jennifer A. Griffiths, and David A. Tirrell. 1999. Structure of poly($$\gamma$$-benzyl-L-glutamate) monolayers at the gas–water interface: A Brewster angle microscopy and x-ray scattering study. Journal of Chemical Physics 111(21): 9761-9777. Full Text & Related Files: Fukuto_Structure_JCP.pdf (596.6Kb; PDF) Abstract: This paper reports Brewster angle microscopy (BAM), x-ray specular reflectivity (XR), grazing incidence diffraction (GID) and off-specular diffuse scattering (XOSDS) measurements of Langmuir monolayers formed on water by both mono- and polydisperse samples of $$\alpha$$-helical poly($$\gamma$$-benzyl L-glutamate) (PBLG) as a function of area/monomer A. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A > $$\sim$$ 21  Å$$^2$$/monomer, surface pressure $$\Pi$$ = 0), BAM images reveal partial surface coverage by solidlike monolayer islands. GID measurements show an interhelix peak corresponding to a local parallel alignment of rodlike PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A < 21  Å$$^2$$/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in $$\Pi$$ that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant $$\Pi$$ in the $$\Pi$$-A isotherm ( $$\sim$$ 11.5 < A < $$\sim$$ 18.5  Å$$^2$$/monomer, $$\Pi$$ $$\sim$$ 9 dyn/cm), which has previously been attributed to a first-order monolayer–bilayer transition. The interfacial electron density profiles determined by the XR measurements on both sides of the coexistence plateau provide direct evidence for this transition. On the basis of x-ray scattering results, the film on the high-density side of the plateau is shown to consist of a newly formed incomplete and incommensurate second layer that sits on top of and has lower average density than a homogeneous first layer. GID measurements indicate that the second layer can be characterized by larger interhelix d-spacing than the first layer, while XOSDS measurements on the bilayer suggest that the second layer is microscopically inhomogeneous. For both mono- and bilayers, the analysis of observed GID peak widths indicates that the extent of lateral positional correlations between parallel PBLG rods ranges from a few to no more than $$\sim$$15 interhelix distances, implying short-range order. Published Version: doi:10.1063/1.480312 Other Sources: http://liquids.seas.harvard.edu/peter/1999.pdf/99_4_PBLG.pdf Terms of Use: This article is made available under the terms and conditions applicable to Other Posted Material, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#LAA Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:10356598 Downloads of this work: