Interactions Between Cationic Gemini Surfactant And Protein

Proteins/surfactants mixtures not only have wide applications in cosmetic, pharmaceutics, and food products, etc. but also can simulate biological systems, which expedites the penetration of biological technology into chemical industry and medicine field. Thus, the interest in interaction between protein and surfactant is very high for many years.To our knowledge, the interaction between protein and surfactant is mainly focus on the traditional surfactant such as CTAB, SDS and Triton X-100, less attention has been paid to protein/double-chains surfactant. Compared with single-chain surfactant, Gemini surfactant, a new class of amphiphilic molecules, has many unique properties. Thus, Gemini surfactant is expected to exhibit quite different interaction behavior with protein from single-chain surfactant. In this thesis, the interaction between symmetric Gemini surfactant (12-2-12) and BSA is studied. For comparison, parallel measurements have also been made on the interaction between dissymmetric Gemini surfactant (12-2-8), single-chain surfactant (CTAB, DTAB) and protein. This thesis is divided into six parts.In the first section, the importance of study on Gemini surfactant and protein and the study progress on the interaction between surfactant and protein are summarized.In the second section, the interaction between surfactants (12-2-12, CTAB and DTAB) and protein is studied by surface tension. The effects of temperature and NaBr on their interaction are also investigated. The results show that the efficiency and the effectiveness of Gemini surfactants to decrease the surface tension of water are higher than DTAB. The surface tension curve of gelatin/12-2-12 system exhibits two break points, but that of BSA/12-2-12 system does not, indicating the complexes of gelatin/12-2-12 form easily via electrostatic and hydrophobic forces. There are also no two transition points in the surface tension curves of BSA/DTAB and gelatin/DTAB systems. All surfactant/protein mixtures show the lowering of surface tension at low surfactant concentration, but the effects of gelatin/12-2-12 and gelatin/DTAB systems are more obvious than those of BSA/12-2-12 and BSA/DTAB systems, respectively. At high surfactant concentration, their surface tensions agree with those of protein-free surfactant solutions, suggesting that gelatin or BSA molecules at interface are displaced by surfactant molecules eventually.In the third section, the dilational rheological properties of surfactants (12-2-12,CTAB and DTAB), gelatin, and surfactant/gelatin system at air/water interface are investigated at low frequency (0.005～0.1 Hz). The effects of temperature and NaBr on 12-2-12 solution are also investigated. For surfactant solution, high dilational frequency, low temperature and the addition of NaBr conduce to the formation of the stable film. The dilational modulus passes through a maximum value with surfactant concentration increasing. For surfactant/gelatin solution, the dependences of the dilational modulus and tanθon dilational frequency have the similar tendency with those of surfactant concentration. The order of the concentration corresponding to the maximum value of the dilational modulus is as follows: gelatin/12-2-12 > gelatin/CTAB > gelatin/DTAB; At high surfactant concentration, the dilational modulus of surfactant/gelatin system becomes close to that corresponding to pure surfactant, suggesting gelatin at interface is replaced by surfactant. This replacement is also observed by surface tension measurement.In the fourth section, steady-state fluorescence has been used to investigate the interaction between surfactants (12-2-12, 12-2-8 and DTAB) and proteins including bovine serum albumin (BSA) and gelatin. It can be seen from synchronous spectra that surfactant mainly interacts with trp residues compared to tyr residues and the conformation of BSA induced by surfactant changes. When surfactant is added to BSA solution, the fluorescence intensity decreases and a blue shift is observed in the maximum emission peak; then, the fluorescence intensity increases gradually when surfactant concentration is above cmc. Unlike BSA, at low surfactant concentration, the fluorescence spectra of gelatin change a little; Subsequent increase of surfactant concentration leads to the remarkable increase in the fluorescence intensity. But there is no change in peak position of gelatin. The experiments from fluorescence polarization and I₁/I₃ value of pyrene show addition of BSA or gelatin to surfactant solution can provide a higher viscosity and lower polarity of microenvironment. But the polarity of the microenvironment in surfactant/gelatin solution is higher than that in surfactant/BSA solution at low surfactant concentration.In the fifth section, the fluorescence lifetimes of pyrene in surfactants (12-2-12 and 12-2-8) and surfactant/protein systems are investigated by time-resolved fluorescence method. The lifetime of pyrene changes a little and then drops with surfactant concentration increasing. Because the polarity of the microenvironment decreases as surfactant tail length increasing, the fluorescence lifetime of pyrene in 12-2-12 solution is longer than that in 12-2-8 solution. For surfactant/protein solution, the lifetime is longer than in the absence of protein at low surfactant concentration. And the lifetime of pyrene is longer in the presence of BSA than in the presence of gelatin, but is the longest in 12-2-12/BSA systems. The lifetime in surfactant/protein system decreases to the value observed for the free surfactant micelle at high surfactant concentration.In the sixth section, circular dichroism (CD) method has been used to investigate the conformation of protein induced by surfactants (12-2-12, 12-2-8 and DTAB). It can be seen from far-UV CD spectra that theα-helical content of BSA remains constant and then decreases with surfactant concentration increasing, at the same time theβ-sheet content increases, suggesting that surfactant at high concentration disrupts the secondary structure and leads to the unfolding of BSA. At the same concentration of surfactants, the decrease of theα-helical content caused by surfactants is as follows: 12-2-12 > 12-2-8 > DTAB. Addition of surfactant to gelatin solution does not alter the far-UV CD spectra significantly when its concentration is low. But the content of the random coil increases at high concentration. The results from near-UV CD spectra show that the binding of surfactants induces the changes of the microenvironment around aromatic amino acid residues and disulfide bonds of BSA at high surfactant concentration. But comparing 12-2-12 with 12-2-8, the effect of 12-2-12 on near-UV spectra is more obvious at high surfactant concentration.The chief characteristics of this thesis are as follows:1. The interaction between dissymmetric Gemini surfactant (12-2-8) and BSA is firstly studied. For comparison, both the interaction between 12-2-12 and BSA and the interaction between 12-2-8 and gelatin are also investigated. The effects of temperature and salt on their interaction are discussed. This research can not only enrich the systems about the interaction between biological macromolecules and small molecules, but also provide valuable data of simulating biological systems.2. The aggregation behavior of protein with Gemini surfactant at air/water interface and the interaction mechanism in bulk are firstly and systematically studied by dilational rheology and fluorescence polarization. Thus, methods for studying the interaction between biological macromolecules and double-chains surfactants are enlarged.3. The reason why the interaction between BSA and Gemini surfactant is different from the interaction between gelatin and Gemini surfactant is explained. And the possible interaction mechanism between protein and Gemini surfactant is proposed via many experimental methods.