Interaction between Ascorbyl Palmitate and water

ERICA P. SCHULZ, LUCIANO BENEDINI, PAULA V. MESSINA, PABLO C. SCHULZ, DANIEL A. ALLEMANDI, SANTIAGO D. PALMA

Cordoba

Congreso; 1ª Reunión Internacional de Ciencias Farmacéuticas; 2010

Universidad Nacional de Cordoba

Introduction The knowledge of the interaction between water and drugs having pharmaceutical applications is of fundamental importance for the design of applications. The interplay of experimental, structural and molecular dynamics simulation gives a complete picture of this interaction. We have performed differential scanning calorimetry (DSC) experiments and have interpreted the results on the basis of a  geometric model of  hydrated crystals and lamellar liquid crystals of  ascorbyl palmitate (Asc16) and molecular dynamics simulation (MDS). Materials and methods Ascorbyl Palmitate (AP) was purchased from (Flukka-Italia). Redistilled water by Allchemistry (Buenos Aires, Argentina) was used in all experiments. Samples were prepared having 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 95 % weight/weight (w/w). Calorimetric measurements were performed with a Q20 Differential Scanning Calorimeter (TA Instruments). Samples were prepared using closed hermetic aluminium pans which have been weighed with a fifth cipher balance Sartorius (Germany). All runs were performed at the rate 5 ºC.min-1. The samples were treated cooling to -20ºC during 5 minutes. Then, they were heated to 150ºC at a rate of 5 ºC.min-1 . Eventually, they were kept at this temperature for a minute.   Optical microscopy was performed with a Nikon Eclipse E-200 POL polarizing (Tokyo, Japan) microscope. The scatters were heated until the temperature that DSC thermograms have been shown a phase change. The simulation of a solvated ascorbyl-6-O-dodecanoate? monomer was performed by molecular dynamic (MDS) using the AMBER101 molecular simulation suite. After a minimization to adjust the angles and bond lengths, the monomer was solvated with 7524 TIP3 model waters. First the system was equilibrated at 300 K (Langevin thermostat) and 1 bar. Afterwards, a dynamic trajectory (canonical ensemble NPT) of 10 ps was performed, saving the configurations every 0.1 ps. Results DSC thermograms show that there are tree kinds of water. We have identified one of them as hydration (i.e, water strongly attached to the polar headgroups), corresponding to a hydration number of 11.5 ± 1.3 water molecules per Asc16 molecule. Other kind of water was identified as water associated to the polar surface of crystals, which amounts 54 ± 4 water molecules per surfactant one in very dilute samples, but diminishes with increasing concentration and disappears at  C » 0.48 wt %. All water exceeding the above amounts is free (bulk) water.  The fact that the Nwater/Nsurf of water associated to the surface diminishes with increasing C indicates that this water is loosely related to the surface and an increasing crowding of the polar groups will free some of these water molecules. These findings agree with literature ones2. MDS shows that the Asc16 headgroup has a hydrophobic side and there is a folding of the chain producing a hydrophobic bonding between the hydrophobic face of the polar head group and the chain. There is a first hydration layer, extended up to 3 Å which are directly attached via hydrogen bonds to the oxygen and hydroxyl groups of the polar headgroups, having 11.47 ± 0.95 water molecules per surfactant molecule and coincides with the number of water molecules which are undetectable by DSC, i.e., those appertaining to the first hydration layer (11.5 ± 1.3). A second hydration layer is extended up to 9 Å from the polar headgroup, formed by water molecules associated with that of the first hydration layer trough hydrogen-bondend strings of molecules. This second hydration layer has 59 ± 17 water molecules per surfactant one (excluding those in the first hydration layer). Finally, a physical steric model of a bilayer of surfactant molecules with the polar heads pointing to the central plane and having water between the two surfactant layers was made using molecular dimensions. This model showed that when the distance between the two polar surfaces is 6 Å (i.e., two first hydration layers) the content of water per surfactant molecule corresponds to about 11, and when the distance is augmented to 18 Å, the increase in water is about 50-60 water molecules. Then, the findings obtained by DSC, MDS and the model agree perfectly. Conclusions DSC, MDS and the physical model results agree in that the polar headgroup of Asc!6 has a first hydration layer which is not detectable by DSC, having about 11 water molecules per surfactant one, strongly attached to the oxygen and hydroxyl groups of the polar head of Asc16 and extended up to 3 Å. A second hydration layer is formed by strings of hydrogen-bonded water molecules associated to those of the first hydration layer. That layer is extended up to 9 Å from the polar headgroup surface, and contains about 60 water molecules in diluted samples, but the number is reduced when the concentration augments. All other water present in the system is “free” or bulk. Moreover, the polar headgroup has a hydrophobic side and in dilute (molecular) solution the molecule folds forming an hydrophobic bond between that face and the chain. Acknowledgments This work was supported by a grant of the Universidad Nacional del Sur. PVM, EPS, DAA and SDP, are researchers of Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET). LB has a fellowship of CONICET. 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