Abstract
A robust method was developed to investigate the liposomal behavior of novel enzymatically-synthesized hydroxytyrosol and tyrosol phospholipids. Bilayer characteristic obtained by this method, including bilayer formation stability and adsorption properties, were explored using dynamic light scattering, zeta-potential measurements, and quartz crystal microbalance with dissipation monitoring (QCMD), respectively. Liposome diameters were found to typically increase from pH 5.5 to pH 10. Zeta potentials values, on the other hand, were found to be well below −25 mV at all pH conditions explored, with the lowest values (and thus, the best liposome stability) at pH 5.5 or pH 10. Quartz crystal microbalance with dissipation monitoring measurements demonstrated that 100% 1,2-dioloeoylphosphatidyl-hydroxytyrosol (DOPHT) liposomes adsorbed intact onto silica in buffer conditions at pH 5.5 and with no calcium, or at pH 7.5 with calcium (no adsorption was detected at pH 10). 1,2-Dioleoylphosphatidyl-tyrosol (DOPT) liposomes were shown to adsorb intact under buffer conditions only at pH 5.5 with and without calcium. 1,2-Dioleoylphosphatidyl-2-phenolethanol (DOPPE), in comparison, readily adsorbed intact at pH 7.5 without calcium and just slightly at pH 5.5 with calcium present, but formed a supported bilayer over hours at pH 5.5 in the absence of calcium ions.
Highlights
Hydroxytyrosol and tyrosol are phenolic compounds found abundantly in olive oil
Further research has shown that tyrosol and hydroxytyrosol esters enzymatically synthesized from cuphea oil readily mixed with phospholipids to form liposomes [6,7]
Phospholipids with hydroxytyrosol, tyrosol, and 2-phenyl ethanol (DOPPE—phenyl ring with no hydroxyl group) headgroups were compared to ascertain whether the hydroxyl group was key to liposome adsorption
Summary
Their presence in the traditional Mediterranean diet suggests that hydroxytyrosol and tyrosol may have health benefits [1] These health benefits may arise from their antioxidant properties, making them highly desirable for applications within the food and health industries [2,3]. This work explored the size, zeta potential and silica adhesion properties under salt conditions (100 mM NaCl, ±2 mM CaCl2) of liposomes composed of 100% tyrosol-based phospholipids at pH 10 and pH 5.5. This will allow for determining the influence of pH on the stability and bilayer forming capability of liposomes composed solely of tyrosol-based phospholipids. Phospholipids with hydroxytyrosol (two hydroxyl groups on a phenyl ring), tyrosol (phenyl ring with one hydroxyl group), and 2-phenyl ethanol (DOPPE—phenyl ring with no hydroxyl group) headgroups were compared to ascertain whether the hydroxyl group was key to liposome adsorption
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