304
is also due to SUV fusion to large unilamellar vesicles (LUVs)
upon heating, resulting in size inhomogeneity.
5. The grade of hydration can play a crucial role at the calorimetric results. Most frequently, the lipidic bilayers are fully
hydrated [4, 5, 8, 18–20] that corresponds to 50 wt% hydration medium or 0.28 mmol of water and 7 × 10
−3
mmol of
phospholipid or water:phospholipid 40:1 molar ratio [22]. In
the case of using the lipid bilayers as dry material, being sealed
into the crucibles without hydration medium, but also with no
precaution taken of preventing hydration, then this sample can
be called “partially hydrated sample” (this sample is found to
absorb 2.1% w/w water) and presents significant differences at
the obtained DSC data [22]. For example, in the case of
DPPC bilayers, the partially hydrated sample presented
absence of pretransition, due to the different organization of
the polar head groups in DPPC molecules after heating and
cooling cycles, while the fully hydrated sample presented the
reversible pretransition before its main transition [22].
6. In the case of hydrophilic guests, these type of molecules can
also be dissolved into the aqueous hydration medium, rather
than be incorporated into the membrane at the stage of bilayer
preparation. Furthermore, in the case of studying the perturbation ability of a guest molecule, it is preferred to carry on DSC
measurements on two types of bilayers, prepared by two different preparation protocols (the drug is provided outside during
the hydration step or included inside during the preparation of
the bilayer [17]), in order to evaluate the impact of the method
of incorporation inside the membranes. For example, Liossi
et al. [17] prepared bilayers with incorporated irbesartan drug
in pure form or complexed with HP-β-CD cyclodextrin (as
described in Subheading 2.1) on the one hand and on the other
hand the irbesartan–HP-β-CD complex was also added to the
aqueous phase of readily formed lipidic MLV dispersions.
7. The scanning temperature range should certainly include the
transition phenomena (main transition and pretransition) of
the used lipid. More specifically, it should be used at a temperature starting about 20 °C prior to the transition temperature of
the phospholipid employed and ending at about 20 °C above
the transition temperature of the phospholipid employed [23].
8. The choice of scan rate is a critical parameter that should be
carefully investigated before each experimental procedure.
Sometimes, trying different scanning rates is required in order
to select the most appropriate one that is able to reveal all the
secondary transitions that are taking place. After all, the
presence of foreign molecules is supposed to alter the calorimetric profile of the lipid at an unknown manner. A scanning
rate of 2.5–5.0 °C min
−1
is usually used in the cases of lipoMaria Chountoulesi et al.
is also due to SUV fusion to large unilamellar vesicles (LUVs)
upon heating, resulting in size inhomogeneity.
5. The grade of hydration can play a crucial role at the calorimetric results. Most frequently, the lipidic bilayers are fully
hydrated [4, 5, 8, 18–20] that corresponds to 50 wt% hydration medium or 0.28 mmol of water and 7 × 10
−3
mmol of
phospholipid or water:phospholipid 40:1 molar ratio [22]. In
the case of using the lipid bilayers as dry material, being sealed
into the crucibles without hydration medium, but also with no
precaution taken of preventing hydration, then this sample can
be called “partially hydrated sample” (this sample is found to
absorb 2.1% w/w water) and presents significant differences at
the obtained DSC data [22]. For example, in the case of
DPPC bilayers, the partially hydrated sample presented
absence of pretransition, due to the different organization of
the polar head groups in DPPC molecules after heating and
cooling cycles, while the fully hydrated sample presented the
reversible pretransition before its main transition [22].
6. In the case of hydrophilic guests, these type of molecules can
also be dissolved into the aqueous hydration medium, rather
than be incorporated into the membrane at the stage of bilayer
preparation. Furthermore, in the case of studying the perturbation ability of a guest molecule, it is preferred to carry on DSC
measurements on two types of bilayers, prepared by two different preparation protocols (the drug is provided outside during
the hydration step or included inside during the preparation of
the bilayer [17]), in order to evaluate the impact of the method
of incorporation inside the membranes. For example, Liossi
et al. [17] prepared bilayers with incorporated irbesartan drug
in pure form or complexed with HP-β-CD cyclodextrin (as
described in Subheading 2.1) on the one hand and on the other
hand the irbesartan–HP-β-CD complex was also added to the
aqueous phase of readily formed lipidic MLV dispersions.
7. The scanning temperature range should certainly include the
transition phenomena (main transition and pretransition) of
the used lipid. More specifically, it should be used at a temperature starting about 20 °C prior to the transition temperature of
the phospholipid employed and ending at about 20 °C above
the transition temperature of the phospholipid employed [23].
8. The choice of scan rate is a critical parameter that should be
carefully investigated before each experimental procedure.
Sometimes, trying different scanning rates is required in order
to select the most appropriate one that is able to reveal all the
secondary transitions that are taking place. After all, the
presence of foreign molecules is supposed to alter the calorimetric profile of the lipid at an unknown manner. A scanning
rate of 2.5–5.0 °C min
−1
is usually used in the cases of lipoMaria Chountoulesi et al.