Colloids and Surfaces B: Biointerfaces 53 (2006) 72–77 Effects of newcastle disease virus glycoproteins on the structural and thermal behaviour of 1,2-dihexadecyl-sn-glycero-3-phosphatidylcholine lipid membranes under osmotic stress conditions Rita Pravchanska a , Petrana Borissova b , Lyubka Doumanova c , Vassil Neitchev a,∗ , Peter Laggner d a Institute of Geology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria b Institute of Biophysics, 1113 Sofia, Bulgaria c Institute of Microbiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria d Institute of Biophysics and X-ray Structure, Schmeidlstrasse 6 A-8010, Graz, Austria Received 3 April 2006; received in revised form 26 July 2006; accepted 26 July 2006 Available online 7 August 2006 Abstract The interaction of hem agglutininneuraminidase (HN) and fusion (F) glycoproteins with swollen vesicles of 1,2-dihexadecyl-sn-glycero-3- phosphatidylcholine (DHPC) was investigated under transition from gel to fluid phase. X-ray studies of the structure of lipid/HN-F mixtures in normal and swollen vesicles have shown that the lamellar bilayer structure predominate in the gel and liquid crystalline phases. A swollen lipid phase, in which the mean repeat distance of lipid bilayers is larger than in the other phases was found. The nature of this phase is similar to the anomalous bilayer swelling reported in literature. The presence of HN and F in the vesicles led to the coexistence of structures with low and high lamellar order, showing larger repeat distance in comparison with the pure lipid. This finding was attributed to the increase in the lipid bilayer thickness due to the HN-F included in the free water layer. The thermal behaviour of the system was not affected by the vesicle swelling. The data showed the existence of gel and liquid crystalline lamellar phases and changes in lipid/HN-F specific heats, mainly due to the concentration effect of the HN-F and its location in the free water layer. © 2006 Elsevier B.V. All rights reserved. Keywords: Membranes; Vesicles; Viral glycoproteins; Phase transition; X-ray; DSC 1. Introduction It has been found that the viral enveloped glycoproteins in the paramyxoviruses can interact with lipids to change the spontaneous membrane curvature, as well as the lipid hydra- tion and surface tension, with minimum osmotic work required for the transition from gel to fluid lipid phase [1]. Besides, the said glycoproteins alter their water/membrane solubility in response to environmental changes associated with the pro- cesses of glycoprotein–membrane interaction. As shown by previous results, the hemagglutinin-neuraminidase glycoprotein (HN) and the fusion glycoprotein (F) isolated from NDV virions react preferentially with zwitteronic lipids in neutral pH and, at ∗ Corresponding author. Tel.: +359 2 9793125; fax: +359 2 9712493. E-mail address: vas@polymer.bas.bg (V. Neitchev). high lipid/glycoprotein molar ratios, they cause a decrease in the enthalpy of lipid phase transitions [2]. We have previously found that both HN and F, when associated with vesicles, affect the structure [3] and permeability of lipid bilayers under positive (higher inside) osmotic gradients across the membrane [4] and [5]. Additionally, there is data in literature indicating a possi- ble effect of osmotic stress on membrane fusion [6–11]. These findings have encouraged us to start investigations on the inter- action of the HN-F complex with swollen lipid vesicles. In the present contribution, we investigate the effect of HN-F on the structure and thermal behaviour of such vesicles under transi- tion from lamellar gel to fluid state. We have chosen to use the ether lipid DHPC, since it has been found that DHPC vesicles exhibit different osmotic permeability in comparison with vesi- cles from dipalmitoyllecithin (DPPC) [4,12,13]. In spite of the overall structural similarity of DHPC and DPPC, there are some notable differences in their physical–chemical properties. The 0927-7765/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2006.07.021