Debnath Sisir, Dasgupta Antara, Mitra Rajendra Narayan, Das Prasanta Kumar
Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India.
Langmuir. 2006 Oct 10;22(21):8732-40. doi: 10.1021/la061487d.
This paper delineates how the different counterions affect the physicochemical properties of the aqueous aggregates and thereby the lipase activities at the interface of cationic water-in-oil microemulsions. To this end, we have synthesized a series of cetyltrimethylammonium-based surfactants, 1-14, having aliphatic, aliphatic with aromatic substitution at the alpha position, and aromatic carboxylate anion as the counterion. The physicochemical characterizations of these aqueous aggregates were done by conductometric, tensiometric, fluorometric techniques to determine counterion binding (beta), critical micelle concentration (cmc), and micropolarity at the microenvironment. It has been found that the activity of lipase mainly increases with hydrophobicity (which is directly proportional to the counterion binding (beta) of the surfactant) of the counterion and reaches a maximum when the beta value is around 0.5. Increase in hydrophobicity as well as beta leads to the attachment of more counterions at interface resulting in enhancement of interfacial area. Consequently, the enzyme may attain flexible secondary conformation at the augmented surface area and also allow larger population of substrates and enzyme molecules at the interface leading to the enhancement in lipase activity. After an optimum value of beta, further increase probably produces a steric crowding at the interface, hindering the smooth occupancy of enzyme and the substrate in this region leading to decrease of enzyme activity, while molecular surface area of the counterion did not show any virtual influence on the lipase activity. Thus, the variation in the counterion structure and hydrophobicity plays a crucial role in modulating the lipase activity.
本文阐述了不同抗衡离子如何影响水性聚集体的物理化学性质，进而影响阳离子油包水微乳液界面处的脂肪酶活性。为此，我们合成了一系列基于十六烷基三甲基铵的表面活性剂1-14，其抗衡离子分别为脂肪族、α位带有芳族取代基的脂肪族以及芳族羧酸盐阴离子。通过电导法、张力测定法、荧光测定技术对这些水性聚集体进行物理化学表征，以确定抗衡离子结合（β）、临界胶束浓度（cmc）以及微环境中的微极性。研究发现，脂肪酶的活性主要随着抗衡离子的疏水性（与表面活性剂的抗衡离子结合（β）成正比）增加而提高，当β值约为0.5时达到最大值。疏水性以及β的增加会导致更多抗衡离子附着在界面上，从而增加界面面积。因此，酶可能在增大的表面积上获得灵活的二级构象，并且在界面处允许更多的底物和酶分子存在，导致脂肪酶活性增强。在β达到最佳值后，进一步增加可能会在界面处产生空间拥挤，阻碍酶和底物在该区域的顺利占据，导致酶活性降低，而抗衡离子的分子表面积对脂肪酶活性没有任何实际影响。因此，抗衡离子结构和疏水性的变化在调节脂肪酶活性方面起着至关重要的作用。
