Antonijevic, Milan

Chowdhry, Babur

Leharne, Stephen

Owusu-Ware, Samuel

Greenwich Academic Literature Archive

Phospholipids (PLs) are importantbiological compounds that haveapplication in various industries. It iswell known that the acyl chain length ofPLs influences the crystal-to-liquidtransition temperature (Tm) [1], whichis an important property that influencesuses of PLs.Understanding the molecular dynamicsof anhydrous PLs below the Tm canpotentially shed light on their intrinsicbehaviour in living systems and inindustrial applications. TSDC is animportant technique that can be used toobtain such information, by means ofmolecular relaxation processes, attemperatures below the Tm.Thermally Stimulated Depolarisation Current (TSDC) as a Novel Analytical Approach for the Study of Anhydrous PhospholipidsSamuel K Owusu-Ware, Babur Z. Chowdhry, Stephen A. Leharne and Milan D. Antonijevic*School of Science, Faculty of Engineering and Science, University of Greenwich,Chatham Maritime, Kent ME4 4TB, UK                                                                                   The application of TSDC has provided novel information pertaining to anhydrous PCs below their Tm. This studyhas demonstrated that significant molecular mobility exist below the Tm and the temperature at which mobilityoccurs increases with carbon chain length. Relaxation rate is also more sluggish for PCs with longer carbon chainlength. These offer some explanations as to why increasing carbon chain length of PCs increasing their physicalstability. TSDC therefore enhances capabilities in understand physical stability of materials by analysingrelaxation processes in materials that occur at temperatures well below any structural change.Fig. 2. Graphs showing (a) overlay of the global TSDC signal obtained for the PCs polarized at 90°C and (b) plot of relaxation current Tmax against diacyl chain length.INTRODUCTION MEASUREMENT PRINCIPLEMATERIALS and METHODSThe phosphatidylcholines (PCs) 1,2-dilauryl-sn-glycoro-3-phosphocholine (DLPC), 1,2,-dimyristol-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) of >99% were kindly provided by Lipoid (Germany), and dehydrated in-situ (Dehydration wasconfirmed by thermogravimetric analysis (TGA). Samples are placed between two parallel electrodes and polarised at 90°C for 2 minutes using 350 V/mm in the TSDCexperiments covering the temperature -25 to 100°C. For TW experiments conditions were Ep = 400 V/mm at Tp of -20 to 60°C in increments of 3°C (Tw = 3°C, tw = 2 min).The global TSDC curves (Fig.2 (a)) contain information of the overall molecular relaxation process which is indicative of molecular mobility. The results demonstrate, for thefirst time, that anhydrous PCs undergo significant molecular mobility at temperatures below their Tm. The temperature of relaxation current maxima was found to increase withincreasing carbon chain length (Fig. 2 (b)). This shows that increasing carbon chain length requires an increase in heat energy to initiate molecular motions in PCs. Both,molecular mobility below Tm and Tm for series of PCs show correlation between measured values (Tmax in TSDC and TM in DSC) implying that carbon chain length playsimportant role in structural arrangements, flexibility and mobility of PCs molecules (Table 1).CONCLUSIONSFig.1. Schematic of the response of molecules during a  typical TSDC experiment.  RESULTS and DISCUSSIONFig. 3.  Graphs of TW result showing discrete relaxation modes under the global TSDC signals for each PC (a-d) with their respective distribution of relaxation times (e-h).Table 1. Values obtained for Tm by DSC, Tmax, relaxation times (τ) and activation energy (Ea) obtained from TSDC and TW experiments(n=3).REFERNCESTSDC measures movement of molecular dipoles as a function oftemperature, in response to the application of a static electrical field(Fig.1). The sample is placed between two parallel electrodes andpolarised (1) using static electrical field at a temperature (Tp). This isfollowed by quench cooling to a temperature well below the polarisationtemperature (T0) at which point oriented dipoles are “frozen in” (2). Theelectrical field is short circuited and the sample is heated to a finaltemperature (Tf ) >Tp. Dipoles move back during linear heating generatinga small current that is measured using a sensitive ammeter, which containsinformation about molecular mobility. The complex dipole relaxationcurrent in TSDC output can be deconvoluted into discrete elementaryrelaxation modes by the application of thermal windowing (TW)experiments. For TW step (2) is modified so that the sample is initiallycooled just below Tp and held isothermal with the electrical field switchedoff, before cooling to T0. From TW results, the distribution of relaxationtimes (τ) is obtained and kinetic information e.g. the Activation energy(Ea) which is used to characterise molecular relaxation behaviour [2].The TW results (Fig. 3a-d) shows several discrete relaxation processes within the global TSDC curve. Therespective distribution of relaxation times for the isolated processes (Fig. 3e-h) are generally straight and werefitted with the Arrhenius equation. The relaxation time (τ) and Ea of the isolated relaxation process with thehighest current generated is used to compare the molecular mobility characteristics of the PCs (Table 1).Relaxation time for the PCs in general increases with increasing carbon chain, which implies a more sluggishmolecular mobility as you increase carbon chain length, that is due to increased hydrophobic interaction andviscosity. On the other hand, activation energy of molecular relaxation decreases with increasing carbon chainlength. This observation is attributed to difference in the degree of ordering in the materials. PCs in the solid-stateare semi-crystalline (confirmed by XRPD) and it is likely that PCs with longer carbon chain length will havegreater state of mobility of their chains involved in formation of an amorphous fraction.[1] Chapman D, Byrne P, Shiply GG. The physical properties of phospholipids. I. Solid state and mesomorphic properties of some 2,3-diacyl-DL-phosphatidylethanolamines. Proceedings of the Royal Society A. 1966;290(1420):115-142.[2] Correia NT, Alvarez C, Ramos JJM, Descamps M. Molecular motions in molecular glasses as studied by thermally stimulated depolarization currents (TSDC). Chemical Physics. 2000;252:151-163.

Thermally Stimulated Depolarisation Current (TSDC) as a Novel Analytical Approach for the Study of Anhydrous Phospholipids

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Thermally Stimulated Depolarisation Current (TSDC) as a Novel Analytical Approach for the Study of Anhydrous Phospholipids

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