Influence of charge ratio on physical properties. All the physical properties discussed in the previous sections are strongly dependent on the +/- charge ratio. Both pDNA and siRNA complexes have been reported to follow a three zone model of colloidal stability; zone A which represents a region of colloidal stability for complexes with excess negative charge ratio (excess nucleic acid) and uncondensed nucleic acid surrounding lipids; one B which represents colloidal instability where complexes are uncharged and tend to form large aggregates; and zone C where colloidally stable complexes of excess positive charge and highly complexed nucleic acid are observed (Tranchant, I. et al., 2004, Desigaux, L. et al., 2007). Although pDNA and siRNA follow the same three-zone model, the charge ratio at which these zones occur varies between the two nucleic acids (Xxxxx, Y. et al., 2010). In addition the pH and salt concentration influence these physical properties as at high ionic strength complexes are unstable and tend to form large aggregates (Xxxxx, F. et al., 2002, Xxxxxx, X. et al., 2004, Xxxxxxxx, X.X. xx al., 2004, Xxxxx, X. et al., 2006).
