J. Phys. II France
Volume 4, Numéro 7, July 1994
Page(s) 1107 - 1116
DOI: 10.1051/jp2:1994190
J. Phys. II France 4 (1994) 1107-1116

Effect of high electrolyte concentration on the cooperativity of the main phase-transition of DPPC

P. Sapia and L. Sportelli

Dipartimento di Fisica, Laboratorio di Biofisica Molecolare, Università della Calabria and Unita'INFM, 87036 Arcavacata di Rende (CS), Italy

(Received 3 December 1993, revised 28 March 1994, accepted 8 April 1994)

We have investigated the effect on the cooperativity of the $\rm P_{\beta'}\rightarrow L_{\alpha}$ main phase transition of multilamellae of dipalmitoylphosphatidylcholine (DPPC) of a 1 : 1 electrolyte up to the concentration of 3 M in the dispersion medium. By using the ESR spectroscopy with the spin label partition technique, the mean size of the cooperative unit, $\nu$, at the centre of the transition has been calculated according to the Zimm and Bragg (J. Chem. Phys. 31 (1959) 526-535) theory of cooperative transitions. By increasing the electrolyte concentration in the dispersion medium the $\nu$-value increases suggesting that the lipid molecules in the bilayers become more tigthly packed. Screening of the electrostatic interactions between the dipoles on the polar head of the DPPC molecules with increase of the hydrophobic interactions as well as modification of the structure of the water layer around the phosphocholine fragment of DPPCs can explain the results obtained.

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