Cryo EM study on the effect of the additional amphiphilic molecules on size of cylindrical micelle

Abstract

The self assembled structures in aqueous solution vary their size and shape in response to thermodynamic variables, such as concentration and temperature. However, there are difficulties to measure the real size and shape of self-assembled structures which are needed to understand the actual systems by the model dependent scattering techniques. Cryo-TEM is advantageous in measuring the size and shape directly. In this study, the size and shape changes of the cylindrical micelle due to the additional amphiphilic molecules were measured with the Cryo-TEM method. Laser light scattering measurements were also performed to acquire hydrodynamic radius for comparison. The nonionic cylindrical micelles were made of $C_{12}E_5$ (penta-ethyleneglycol mono n-dodecyl ether). DTAB(Dodecyl trimethyl ammonium bromide), LysoPC(1- Tridecanoyl-2-Hydroxy-sn-Glycero-3-Phosphocholine) and DMPC (DL-$\alpha$-phosphatidylcholine dimyristol) were used as the additive amphiphilic molecules. They have the same chain length with $C_{12}E_5$ but different packing shapes and charge. DTAB, which prefers spherical micelle and has positive charge, causes the shortening of the micelle length as the total concentration of the mixture and the mole fraction of DTAB increases. The micellar length shortening of a nonionic/ionic mixture is contrary to the previous theory. In the case of nonionic additives micellar length was strongly affected by the spontaneous curvature of the molecule. When LysoPC, which prefers spherical micelle was added, the mixture micellar length was smaller than that of the cylindrical micelle made of only $C_{12}E_5$ On the other hands, when DMPC, which prefers bilayer, was added, it was larger. In both mixtures, the micelle length increases as the total concentration of the mixture increases.