Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 00 Prague 2, Czech Republic; Department of Material Science and Physical Chemistry & Institute of Theoretical and Computational Chemistry (IQTC), University of Barcelona, C/ Martí i Franquès, 1, 08028 Barcelona, Catalonia, Spain.
Universidad Tecnología Nacional & Grupo Bionanotecnología y Sistemas Complejos. (UTN-CONICET), Facultad Regional San Rafael, Av. General Urquiza 314C.P.:5600, San Rafael, Mendoza, Argentina.
Colloids Surf B Biointerfaces. 2022 Sep;217:112617. doi: 10.1016/j.colsurfb.2022.112617. Epub 2022 Jun 10.
We analyze the conditions of the adsorption of a flexible peptide onto a charged substrate in the 'wrong side' of the isoelectric point (WSIP), i.e. when surface and peptide charges have the same sign. As a model system, we focus on the casein macropeptide (CMP), both in the aglycosylated (aCMP) and fully glycosydated (gCMP) forms. We model the substrate as a uniformly charged plane while CMP is treated as a bead-and-spring model including electrostatic interactions, excluded volume effects and acid/base equilibria. Adsorption coverage, aminoacid charges and concentration profiles are computed by means of Monte Carlo simulations at fixed pH and salt concentration. We conclude that for different reasons the CMP can be adsorbed to both positively and negatively charged surfaces in the WSIP. For negatively charged surfaces, WSIP adsorption is due to the patchy distribution of charges: the peptide is attached to the surface by the positively charged end of the chain, while the repulsion of the surface for the negatively charged tail is screened by the small ions of the added salt. This effect increases with salt concentration. Conversely, a positively charged substrate induces strong charge regulation of the peptide: the acidic groups are deprotonated, and the peptide becomes negatively charged. This effect is stronger at low salt concentrations and it is more intense for gCMP than for aCMP, due to the presence of the additional sialic groups in gCMP.
我们分析了在等电点（isoelectric point，简称“IEP”）的“错误侧”（wrong side），即表面电荷和肽电荷具有相同符号的情况下，柔性肽在带电荷的底物上吸附的条件。作为模型体系，我们专注于酪蛋白巨肽（casein macropeptide，简称“CMP”），包括糖基化（glycosylated，简称“gCMP”）和去糖基化（aglycosylated，简称“aCMP”）两种形式。我们将底物建模为均匀带电的平面，而 CMP 则被视为包含静电相互作用、排除体积效应和酸碱平衡的珠-弹簧模型。通过在固定 pH 值和盐浓度下进行蒙特卡罗模拟，我们计算了吸附覆盖率、氨基酸电荷和浓度分布。我们得出结论，由于不同的原因，CMP 可以在 IEP 中吸附到带正电荷和负电荷的表面上。对于带负电荷的表面，IEP 吸附是由于电荷的斑片状分布：肽通过链的带正电荷端附着在表面上，而表面对带负电荷的尾部的排斥被添加盐的小离子屏蔽。这种效应随着盐浓度的增加而增加。相反，带正电荷的底物会导致肽的强烈电荷调节：酸性基团去质子化，肽带负电荷。这种效应在低盐浓度下更强，并且 gCMP 比 aCMP 更强，这是由于 gCMP 中存在额外的唾液酸基团。
