Application of turbidity technique on peptide-lipid and drug-lipid interactions Fatma Eker a , H. Okan Durmus b , Bu ¨lent G. Akinoglu b , Feride Severcan a, * a Middle East Technical University, Department of Biological Sciences, Ankara 06531, Turkey b Middle East Technical University, Department of Physics, Ankara 06531, Turkey Abstract Turbidity technique was employed to study the mutual interaction of melittin and vitamin D 2 with 1,2-dipalmitoyl phos- phatidylcholine (DPPC) bilayers. In addition, the data obtained from the phase transition curve is used to calculate the main phase transition temperature, conformational stability and activation energy values by using two state transition thermodynamic approach. The results indicate that melittin decreases the main phase transition temperature and also dramatically decreases the stability. Melittin on its own has disordering effect on phospholipid membrane structure. At low concentrations (3, 6 mol %) vitamin D 2 does not alter the main phase transition temperature, whilst the main phase transition temperature significantly decreases upon addition of a high concentration of vitamin D 2 (12 mol %). Stability values of mutual interaction of vitamin D 2 with DPPC indicates that vitamin D 2 has an ordering effect. Moreover, the activation energy increases as vitamin D 2 concen- tration increases.  1999 Elsevier Science B.V. All rights reserved. Keywords: Melittin; Turbidity; Vitamin D 2 ; Activation energy; Stability 1. Introduction Melittin is one of the most studied cationic, amphi- pathic peptide (26 amino acids) which can lyse various cells by permeablising their phospholipid membrane [1–3]. Despite intensive studies on melittin-phospho- lipid systems [1–5] a detailed knowledge of the mechanism of lysis, or morphological perturbations of biomembranes are still scarce and controversial whilst vitamin D-membrane interactions have not been inves- tigated in detail yet. A very limited study is available. Recently vitamin D 2 is known to display antioxidant and membrane stabilising properties [6,7]. In the present article we aimed to investigate melittin-lipid, vitamin D 2 -lipid interactions by a turbidity technique which has some advantages: The instrument is cheap, the tech- nique is rapid and data evaluation is easy. We showed that turbidity technique at visible range is one of the spectroscopic technique which provides valuable infor- mation on membranes. In addition we used the data obtained from the turbidity study to calculate thermo- dynamic parameters where a least square analysis method assuming a two state transition is used. Although this approach is widely used for protein denaturation curve, in this work it was used for the first time for lipid phase transition curve. 2. Experimental Vitamin D 2 (ergocalciferol) and dipalmitoyl L- phosphatidylcholine (DPPC) were purchased from Journal of Molecular Structure 482–483 (1999) 693–697 0022-2860/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-2860(98)00690-5 * Corresponding author. Tel: + 90-312-210-51-66; fax: + 90- 312-210-12-89. E-mail address: feride@rorqual.cc.metu.edu.tr (F. Severcan)