The Interaction Of Surfactants With Ovalbumin

The interactions of OVA with one typical anionic surfactant, sodium dodecyl sulfate(SDS), and two cationic surfactants, dodecyl trimethylammonium bromide (DTAB) and cetyl trimethylammonium bromide (CTAB), and the effect of surfactants and mixed micelles on the interaction of OVA and Vc have been studied through fluorescence and UV-vis spectroscopy, negative-TEM (transmission electronic microscopy), NMR technology, combined with the measurements of conductivity, pH, surface tension and phase diagram.1. The interactions of OVA with surfactants.The addition of 0.05wt%OVA to SDS solution increases the critical micelle concentration (cmc) of SDS micelles, which shows that OVA goes against the formation of SDS micelles by the methods of conductivity and the fluorescent probe technique. The interaction between SDS monomers and OVA is greater than that between SDS micelles and OVA. The results from synchronous fluorescence show that the fluorescence of OVA is mainly offered by the Tryptophane (Trp) residues, and the binding sites of SDS and OVA are near or at the Trp residues. Different aggregation states of SDS molecules have different effect on the fluorescence of Trp and Tyrosine (Tyr) residues. OVA has little effect on the cmc of oppositely charged DTAB because of the combination effect of electrostatic attraction and the hydrophobic interaction of OVA and DTAB, DTAB micelles have little effect on the conformation of OVA. The addition of OVA increases the cmc of CTAB and CTAB micelles have little effect on the conformation of OVA. It seems that the micropolarity of Tyr residues is more sensitive to the addition of CTAB than that of Trp residues. The difference of electrostatic interaction of OVA and surfactants is the main reason for the change of the conformation of OVA and the micropolarity of amino acid residues on the surface of OVA, and it seems that the electrostatic repulsion has more effect than the electrostatic attraction if the two different charged surfactants have the same carbon chain.2. The influence of surfactants on the interaction of OVA and Vc.The charges of OVA can be regulated by Vc for the acidity of Vc, and Vc can lead OVA to self-assembling. Vc can decrease the fluorescence intensity of OVA by combining with some ammo acid residues on the surface of OVA through electrostatic interaction and hydrogen bonds and it is a static quenching. Because the unfolding of OVA caused by small amount of surfactants induces more amino acid to expose on the surface of OVA, the addition of small amount of surfactants is helpful to the combination of Vc and OVA. The decreasing order of the influence of different surfactants on the combination of Vc and OVA is SDS monomer, followed by DTAB and then CTAB monomer. However, the formation of surfactant micelles has little effect on the the combination of Vc and OVA.3. The interactions of OVA and mixed-micelles formed by anionic and cationic surfactants.The ratio of cationic and anionic surfactants determines the charges of the mixed micelles, which then determine the changes of spectrum of OVA. The formation of SDS/DTAB micelles tempers the denaruration of SDS on OVA. The results from 1HNMR show that in the OVA/SDS/DTAB system, the electrostatic interaction and hydrophobic interaction of SDS and DTAB are the main interactions which are stronger than the interaction of OVA with SDS or DTAB. While in the OVA/SDS/CTAB system, the electrostatic interaction of SDS and CTAB is the main interaction which conceals the interaction of OVA with SDS or CTAB. The formations of negative-charged SDS/DTAB and SDS/CTAB mixed-micelles are helpful to the combination of Vc and OVA. Because the cooperative effect of mixed-micelles formed by the same carbon chain surfactants SDS and DTAB is smaller than that formed by the different carbon chain surfactants SDS and CTAB, the binding constant Ka and the binding number n of Vc and OVA are both smaller in SDS/DTAB system than those in SDS/CTAB system.