DSC STUDY OF PHASE-TRANSITION OF ANHYDROUS PHOSPHOLIPID DHPC AND INFLUENCE OF WATER-CONTENT

The phase transition behavior of 1,2-di-n-heptadecanoyl phosphatidylch oline (DHPC) with and without water has been studied by use of differe ntial scanning calorimetry. It was found by experiment that the glass transition occurred at first during the first heating of a sample of D HPC without water and then the sample underwent melting as an ordinary crystal. Therefore the sample of DHPC without water was a glassy crys tal. However, the DHPC sample crystallizing from melt was an ordinary crystal. From the relationship between the total melting enthalpy Q(f) of freezable water and the water content h, it was concluded that the water contained in the DHPC samples might exist in three states recog nizable thermodynamically. The water in the first state was an unfreez able water. It was the water bound directly with the head groups of th e phospholipid, i.e. the primary hydration water. Every head group mig ht bind seven such molecules of water. The water in the second state w as the secondary hydration water, its melting point was close to that of pure water and its specific melting enthalpy dQ(f)/dh was lower tha n that of pure water. Every molecule of DHPC might possess at most abo ut six such molecules of water. With the further increasing of the wat er content, the bulk water appeared in the system, its melting point a nd specific melting enthalpy were close to those of pure water. The ex periments demonstrated that the melting peak of DHPC shifted toward th e side of lower temperature with the increasing of water content. At h > 16.2 (percent by weight), the temperature T-tr and the enthalpy Del ta H-tr of the gel-liquid crystal phase transition of the lipid were u nchanged substantially with the increasing of water content. The exper imental values of the quantities, T-tr and Delta H-tr, and phase trans ition entropy Delta S-tr were in good agreement with those calculated by the set of Huang equations (1981).