The interaction of Ca2+, with dipalmitoyl phosphatidylcholine (DPPC) model membranes was studied in the presence and absence of vitamin D-2 by using Fourier transform infrared spectroscopy. Addition of vitamin D-2 and/or Ca2+ into pure DPPC liposomes shifts the phase transition to higher temperature, orders and decreases the dynamics of the acyl chains in both phases and does not induce hydrogen bond formation in the interfacial region. Moreover, the dynamics of the head group of the phospholipid decreases in both phases. The addition of vitamin D-2 into DPPC liposomes containing Ca2+, decreases the effect of Ca2+ at all the functional groups under investigation. Similarly, the effect of vitamin D-2 also decreases in the presence of Ca2+. This behavior is dominant at high Ca2+ concentrations. Our results show how simultaneous presence of vitamin D-2 and Ca2+ alter the behavior of each other, which is reflected as a decrease in the interactions between the ions and vitamin D-2 within the membrane. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.