Thomas and Luisi

Figure 10.18 Dynamic light scattering (DLS) of vesicle mixtures, (a) P-index phase diagram and (b) size distributions (from DLS) for DDAB-oleate mixtures, total concentration 1 mM in 0.2 M borate buffer at pH 8.5, 25.0 °C, scattering angle 90°. (From Thomas and Luisi, 2004.)...

Now, when these two species are added to each other, in a given relative concentration, a new species appears with a much narrower size distribution. This is shown in Figure 10.18, where the P-index (a measure of the polydispersity) is plotted against the molar fraction of DDAB. The P-index drops from the initial value of 0.20 (a very broad distribution) to 0.04, a very narrow distribution (stable for months), at a relative percent of 0.4 DDAB to 0.6 oleate (Thomas and Luisi, 2004). Between DDAB molar fractions of 0.41 and 0.60, flocculation occurs, which indicates a thermodynamic instability, in agreement with other cationic systems (Kaler etal., 1989 Marques etal., 1998 Kondo etal, 1995). 

The effect is not limited to oleate, as DDAB also when added to itself or to POPC hposomes is characterized hy a strong matrix effect (Thomas and Luisi, 2004). The matrix effect was re-investigated by Szostak s group, who confirmed the basic findings (Hanczyc et ah, 2003). 

Figure 10.22 Vesicle size selection by RNA added to a mixture of small ( 80 nm diameter) and large vesicles ( 160 nm) from 0.5 mM POPC-3.5% CTAB in 20 mM sodium phosphate buffer (pH 7.0). (a) The initial size distribution of the 1 1 mixture (b) the size distribution 4 min after RNA addition, during the selection process (c) the stable final size distribution after 15 min, with one peak for small diameters ( 90 nm) and a second peak indicating aggregates of large vesicles (> 1000 nm). As mentioned in the text, this process is reversible upon addition of RNAase (Thomas and Luisi, 2004.)...

---