School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India.
Soft Matter. 2018 Aug 8;14(31):6463-6475. doi: 10.1039/c8sm00809d.
We report on the competitive phenomenon of complex coacervation versus bicontinuous gelation between pectin (P, a polyanionic carbohydrate, [P] = 0.01-2% (w/v)) and zein nanoparticles (Z, a hydrophobic protein and a weak polyampholyte, [Z] = 0.1 and 0.5% (w/v), in an ethanolic solution of effective concentration 4 and 27% (v/v)), which was studied below (pH ≈ 4), and above (pH ≈ 7.4) the pI (≈ 6.2) of zein at room temperature, 25 °C. The uniqueness of this study arises from the interaction protocol used, where the pectin used was in the extended polyelectrolyte (persistence length ≈ 10 nm) conformation while zein was used as a charged globular nanoparticle (size ≈ 80-120 nm) that was formed in situ. Their mixing ratio, r = [P] : [Z] (w/w), was varied from 0.02 to 4.0 (for [Z] = 0.5% (w/v)), and from 0.1 to 7.5 (for [Z] = 0.1% (w/v)) in the ionic strength range 10-4 to 10-2 M NaCl. Zeta potential data revealed that at pH ≈ 4, the complementary binding condition, r = 1 : 1 (equivalent to 1 : 5 molecule/nanoparticle) demarcated the coacervate from the gel region. The measured rigidity (G0, low frequency storage modulus) of these materials revealed the following: for r < 1, (low pectin content samples, coacervate region) the material had lower values of Gcoac0, whereas for r > 1, an excess of pectin facilitated gelation with Ggel0 ≫ Gcoac0. Above pI, surface patch binding caused associative interactions and complex coacervation though both biopolymers had similar net charge. The network density was used as a descriptor to distinguish between the coacervate and gel samples. Their microstructures were probed by small angle neutron scattering (SANS), and viscoelastic properties by rheology. Simple modeling shows that formation of the interpolymer complex was favored in higher protein containing samples. Mixing ratio dependent selective coacervation (a kinetic process) and bicontinuous gelation (a thermodynamic process) are rarely seen to coexist in biopolymer interactions.
我们报告了果胶（P，一种带负电荷的碳水化合物，[P] = 0.01-2%（w/v））和玉米醇溶蛋白纳米颗粒（Z，一种疏水性蛋白质和弱两性电解质，[Z] = 0.1 和 0.5%（w/v））之间复杂凝聚与双连续凝胶化之间的竞争现象，在室温 25°C 下，在玉米醇溶蛋白等电点（约 6.2）以下（pH ≈ 4）和以上（pH ≈ 7.4）进行了研究。这项研究的独特之处在于所使用的相互作用方案，其中使用的果胶处于伸展聚电解质（持久长度≈10nm）构象，而玉米醇溶蛋白则作为带电荷的球形纳米颗粒（尺寸≈80-120nm）原位形成。它们的混合比 r = [P]：[Z]（w/w）在离子强度范围 10-4 到 10-2 M NaCl 中从 0.02 变化到 4.0（对于[Z] = 0.5%（w/v）），从 0.1 变化到 7.5（对于[Z] = 0.1%（w/v））。Zeta 电位数据表明，在 pH ≈ 4 时，互补结合条件 r = 1：1（相当于 1：5 个分子/纳米颗粒）将凝聚物与凝胶区分开。这些材料的测量刚性（G0，低频储能模量）显示出以下结果：对于 r < 1（低果胶含量样品，凝聚物区域），材料的 Gcoac0 值较低，而对于 r > 1，果胶过量有利于凝胶化，Ggel0 ≫ Gcoac0。在等电点以上，表面斑块结合引起缔合相互作用和复杂凝聚，尽管两种生物聚合物具有相似的净电荷。网络密度被用作区分凝聚物和凝胶样品的描述符。它们的微观结构通过小角中子散射（SANS）进行探测，粘弹性特性通过流变学进行探测。简单的建模表明，在含有较高蛋白质的样品中，形成聚合物间复合物更有利。在生物聚合物相互作用中，很少看到混合比依赖性选择性凝聚（动力学过程）和双连续凝胶化（热力学过程）共存。
