疏水链长和温度对阳离子单链和Gemini表面活性剂的自组装以及与牛血清蛋白相互作用的影响

Effects of hydrocarbon chain length and temperature on the self-assembly of cationic single-chain and Gemini surfactants and their interactions with bovine serum albumin

利用等温滴定微量热方法研究了疏水链长和温度对六亚甲基-1,6-双(烷基二甲基溴化铵)(C_nC_6C_nBr_2)和烷基三甲基溴化铵(C_nTAB)这两个系列表面活性剂的自组装以及与牛血清蛋白(BSA)相互作用的影响.结果表明,随着疏水链增长,表面活性剂胶束化焓值(DH_(mic))变得更负,胶束化自由能(DG_(mic))的变化主要来自于胶束化熵变(DS_(mic)).温度对表面活性剂的临界胶束浓度(CMC)影响不大,但对DH_(mic)影响较大,在所研究的温度区间内的DH_(mic)都是负的,而TDS_(mic)是正的,并且TDS_(mic)的绝对值明显大于DH_(mic)的绝对值,证明这些表面活性剂的胶束化过程都是熵驱动为主的.随温度升高,焓对胶束化过程的贡献越来越大,而熵贡献越来越小.多数表面活性剂与BSA相互作用的量热曲线呈现两个吸热过程和两个放热过程.疏水链长的变化会显著影响第二个吸热过程和第二个放热过程.温度会显著影响第二个吸热最大值的出现和强度.相比于单链表面活性剂与BSA之间的相互作用,疏水链长和温度对Gemini表面活性剂与BSA之间相互作用的影响更显著.

Protein-surfactant interaction has been a subject of extensive studies over the past few decades due to its importance in manifold applications, such as food industry, drug delivery, skin and body care products, and cosmetics. Mild surfactants are often used to extract membrane proteins while preserving native structure and functional properties of protein, whereas the strong ionic surfactants are known to bind to oppositely charged protein molecules, resulting in denaturation of native protein and complete loss of protein activity. In general, the ionic surfactants bind strongly to protein through electrostatic attraction between surfactant headgroups and oppositely charged amino acids residues, and through hydrophobic interaction between surfactant hydrophobic tails and non-polar amino acids residues. Extensive studies on the interactions between ionic surfactants and protein have been reported. However, most of them focus on single-chain surfactants such as sodium dodecyl sulfate（SDS） or alkyltrimethylammonium bromide（CnTAB）. In recent years, cationic gemini surfactant, a kind of surfactant consisting of two identical hydrophobic chains and two polar headgroups covalently linked by a spacer group, has stimulated the considerable interests due to its lower critical micelle concentration（CMC）, stronger surface activity, better solubility, stronger hydrophobic microdomain, and so on. Thus, gemini surfactant is expected to exhibit quite different interaction behavior with protein from single-chain surfactant. This work has investigated the effect of hydrophobic chain length and temperature on the self-assembly of two series of cationic surfactants, gemini hexamethylene-1,6-bis（alkyldimethylammonium bromide）（CnC6CnBr2, n=10, 12, and 14） and single-chain alkyl trimethylammonium bromide（CnTAB, n=10, 12, 14）, and their interactions with BSA using well-established ITC technique. For the surfactant self-assembly process, the micellization enthalpy change（（35）Hmic） values are more exothermic with the hydrophobic chain increasing. The free energy changes of micellization（（35）Gmic） mainly come from the micellization entropy changes（（35）Smic）. Temperature has a slight effect on the critical micelle concentration（CMC）, but it has a significant effect on the（35）Hmic. In the studied temperature range, the（35）Hmic values are negative, whereas the（35）Smic values are positive, and the absolute values of T（35）Smic are larger than those of（35）Hmic, proving the micellization process is mainly entropy-driven. With the temperature increasing, the contribution of enthalpy change to the micellization becomes larger, whereas the contribution of entropy change to the micellization becomes smaller. For the surfactant and bovine serum albumin（BSA） interaction process, at different surfactant concentration regions, the binding of the surfactants with BSA shows different interaction patterns. For most calorimetric curves of interactions, two endothermic and two exothermic processes are observed, and accompanied by two endothermic peaks. The first endothermic process is the dehydration of BSA upon the binding of the surfactant monomers. The first exothermic process involves the interaction of the surfactant monomers with surfactant molecules already bound on BSA molecule and the resultant formation of the micelle-like hydrophobic aggregates. The protein conformation change and the further dehydration of BSA molecules account for the second endothermic process. The following second exothermic process is the binding of surfactant monomers or micelles to BSA again leads to the formation of more micelle-like aggregates along the unfolded BSA molecules. The hydrophobic chain length remarkably affects the second endothermic process and the second exothermic process. This is because these two processes relate to the formation of the micelle-like aggregates along the BSA chain, whereas the micellelike aggregates are easily formed at relatively high surfactant concentration range. The temperature has a marked effect on the protein/surfactant interaction in the first exothermic process and the second endothermic process. In particular, temperature strongly affects the appearance and amplitude of the second maximum endothermic peak, which can be explained by the effect of temperature on the protein conformation and the enthalpy change of micellization. In addition, gemini surfactants show much stronger binding ability with BSA as compared with single-chain surfactants due to their double hydrophobic chains and double charges.