Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC-CONICET) - Departamento de Química Biológica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA, Córdoba, Argentina.
Brazilian Synchrotron Light Laboratory - LNLS, CNPEM, Rua Giuseppe Maximo Scolfaro, 10000, Campinas, SP 13083-070, Brazil.
Biochim Biophys Acta Biomembr. 2017 May;1859(5):924-930. doi: 10.1016/j.bbamem.2017.02.005. Epub 2017 Feb 16.
Langmuir monolayers at the air/water interface are widely used as biomembrane models and for amphiphilic molecules studies in general. Under controlled intermolecular organization (lateral molecular area), surface pressure, surface potential, reflectivity (R) and other magnitudes can be precisely determined on these planar monomolecular films. However, some physical parameters such as the refractive index of the monolayer (n) still remain elusive. The refractive index is very relevant because (in combination with R) it allows for the determination of the thickness of the film. The uncertainties of n determine important errors that propagate non-linearly into the calculation of monolayers thickness. Here we present an analytical method for the determination of n in monolayers based on refractive index matching. By using a Brewster angle microscopy (BAM) setup and monolayers spread over subphases with variable refractive index (n), a minimum in R is search as a function of n. In these conditions, n equals n. The results shown correspond to monolayers of myelin lipids. The n values remain constant at 1.46 upon compression and equals the obtained value for myelin lipid bilayers in suspension. The values for n and R allow for the determination of thickness. We establish comparisons between these thicknesses for the monolayer and those obtained from two X-ray scattering techniques: 1) GIXOS for monolayers at the air/water interface and 2) SAXS for bilayers in bulk suspension. This allows us to conclude that the thickness that we measure by BAM includes the apolar and polar headgroup regions of the monolayer.
空气/水界面的 Langmuir 单分子层被广泛用作生物膜模型，一般也用于研究两亲性分子。在受控的分子间组织（横向分子面积）下，可以精确地确定这些平面单分子层上的表面压力、表面电势、反射率 (R) 和其他量。然而，一些物理参数，如单分子层的折射率 (n) 仍然难以确定。折射率非常重要，因为（与 R 结合）它允许确定膜的厚度。n 的不确定性确定了重要的误差，这些误差会非线性地传播到单层厚度的计算中。在这里，我们提出了一种基于折射率匹配的确定单分子层中 n 的分析方法。通过使用布儒斯特角显微镜 (BAM) 装置和在具有可变折射率 (n) 的亚相中铺展的单分子层，作为 n 的函数搜索 R 的最小值。在这些条件下，n 等于 n。所示的结果对应于髓磷脂脂质的单分子层。在压缩时，n 值保持在 1.46 不变，与悬浮液中髓磷脂双层的获得值相等。n 和 R 的值允许确定厚度。我们对单层的这些厚度与两种 X 射线散射技术获得的厚度进行了比较：1) 空气/水界面上的单分子层的 GIXOS 和 2) 体悬浮液中的双层的 SAXS。这使我们能够得出结论，我们通过 BAM 测量的厚度包括单分子层的非极性和极性头部区域。
