Sph

Sponsored by: CEA (Co mmissariat 11 I'Energie Atomique)/ DSM

DGA (Ministere de la Défense)

Elf-Atochem

IFP (Institut Franr,;ai s du Pétrole)

Unilever UK

CNRS (Centre National de la Recherche Scientifique)

s

XVth Moriond Workshop Villars sur Ollon , Swilzerland - January 21-28 , 1995

Short and long chains at interfaces Series: Moriond Condensed Maller Phys ics

ISBN 2-863 32- 177-3 © Copyright 1995 by Editions Frontieres AII righlS reserved. Tlris book, or pans Ilrereof, ma)' nOI be reproduccd in any form or by any mean.r, eleClronic or meclrlll1 ica/, including pholOcopying. recording Or any informnlion slorage relrivia/ syslem now known or 10 be invented. wilhoul wrillell pem¡jssion from Ihe Pub/isher.

EDITlONS FRONTlERES B.P.3 3 91192 Gif-sur-Yvette Cedex - France

Printcd in Singaporc

189

MOLECULAR REARRANGEMENTS IN LIPID MONOLAYERS:

A STUDY ON PHASE TRANSmONS

j. Sánchez-González, M.A. Cabrerizo-Vílchez, and MJ. Gálvez-Ruiz'ir

Departamento de Física Aplicada, Universidad de Granada

18071 Granada, ESPAÑA

ABSTRACT

The behavior of three different long chain lipids, at the air-water interface, has been studied as a function of temperature. In order to analyze the influence of the molecular structure on this behavior, the lipids distearoylphosphatidyltholine (DSPC), distearoylphosphatidylethanolamine (DSPE) and Sph1ngomyelin (Sph) have been chosen, since DSPC and Sph molecules have the same polar group and DSPC and ' DSPE molecules show identical hydrocarbon part. Experimental isotherms surface pressure (n)-molecular area (A), obtained by compressing simple and mixed monomolecular layers of these lipids have been performed The monolayer properties of these compounds are influenced by the temperature bringlng about important changes in both interaction characteristics and the stability of the monolayers. These changes are mainly attributed to a decrease in the van der Waals type attractive forces between the hydrophobic regions, in case of the DSPE lipid, also to an increase of the dehydration effect. Moreover, the occurrence of the liquid extended (lE)-liquid condensed (Le) phase transition during the compression of these systems depends strongly on temperature. Only the films formed by DSPC, Sph or a mixture on both of them undergo this phase transition. The transition surface pressure values corresponding to the beginning and the end of the phase have been calculated by analyzing the successive derivatives of the experimental isotherms.

A performance of the phase transition has been carried out by a thermodynamic study of the experimental results. * To whom correspondence should be sent

/

/

190

INTRODUCTION

The study of mixed monolayers underdifferent experimental conditions, is of great interest because it enables us to gain a much better understanding of the interactions between the monolayer compounds. In addition, superfidal processes are strongly dependent on those interactions. In the present work we have focused. our attention on the thermodynamic study of the liquid expanded UE)-liqUid condensed (LC) phase transition for monolayers formed by the lipids distearoyl ­ phosphatidylcholine (DSPC) and Sphyngomyelin (Sph). The surface pressure values corresponding to the beginning and the end of the transition have been calculated as it is explained next. Following a suitable thermodynamic formula tion 1.21, the apparent molar entropy, enthalpy and internal energy changes during the lE-LC phase transition have been evaluated MA TERIALS AND METHODS

In all the experiments the subphase used was water (Milli-Q), the conductivity and pH were ::; 10-4 n I cm'1 and 5.5-6.0, respectively. The three lipids forming the monolayers, Distearoyl-L-a.-Phosphatidylcholine (DSPC), Distearoyl-L-a-Phosphatidylethanolamine (DSPE) and Sphyngomyelin (Sph) were supplied by SIGMA The solvents used for spreading the films were a 4:1 (v/v) mixture of n-hexane and ethanol for the DSPC and Sph lipids and a 5:1 (v/v) mixture of chloroform and methanol for the DSPE lipid The experimental device employed to perform the surface pressure-molecular area (n-A) isotherms has been previously described3.5), as well as experimental condition. In order to carry out a thermodynamic study of the lE-LC phase transition, the surface-pressure values corresponding to the beginning and the end of this transition have been calculated as in previous studies4.6 1, based on other works 7.8l.The followed method is shown in fig. 1. In that, a computer simulated isotherm and its successive derivatives are represented We can observe that the searched for points (the start and the end of the transition) corresponding with an inflection point in the first derivative and therefore, with a minimum and a maximum in the second derivative, respectively, consequently with a third vanishing derivative.

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191

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,

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70

Molecular Area (x 10.20

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90

100

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Figure 1. Model of isothenn and its sucessive derivatives.

RESULTS AND DISCUSSION

We have studied the n-A expeIimental isotherms corresponding to the cornpression of simple and mixed rnonolayers formed by the lipids DSPC, DSPE or Spb, or by a mixture of two of them, under different temperatures (298.2, 303.2, 308.2 and 3l3.2 K). As rnain result we reveal that out of all the studied rnonolayers, only films cornposed by Sph andjor DSPC undergo the lE-LC phase transition under sorne conditions of temperature. These cases are represented in Figs. 2-4. The rest of the experimental isotherms do not show this phase transition. In sorne cases the rnonolayers are at the lE state and others, Le. the DSPE films, are at the LC state. In Fig. 2 the n-A isotherms, corresponding to the cornpression of Sph films that

undergo the lE-LC phase transition, are represented. As expected the transition surface pressure increases with temperature. At higher temperatures, the simple rnonolayers remain in lE state.

192

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Sph

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DSPC at 313.2 K

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Molecular Area (x 10. 20 m 2 /molecule) Figure 2. Compression isothenns for monolayers of Sph and DSPC.

The only film formed by DSPC that suffers the LE-LC phase transition is the film compres sed at 313.2 K and shown in Fig. 2. At different temperatures, the DSPC monolayers are in the LC state, except for the higher temperature (313.2 K), which provokes an increase of the hydrocarbon chains mobility, showing this transition. The mixed monolayers of DSPC and Sph arise a somewhat intermediate behavior compared with that shown by pure components. The characteristic of the isotherms is a progressive condensation as the condensed structure of DSPC is approached

Therefore, when the proportion of DSPC is high in the mixed monolayers, the behavior is similar to that shown by the pure DSPC monolayers. As we can observe in fig. 3, only few mixed monolayers suffer the LE-LC phase transition. Comparing the behavior with that show in the three dimensions for different systems, are more plausibly explained by conformational changes or molecular reorientation than by phase changes, properly speaking. In the table 1, the surface pressure and molecular area corresponding to the

beginning and the end of the LE-LC phase transition that undergo our films are represented, included the experimental conditions of temperature. Comparing these values, we observe that for monolayers containing DSPC, the LE-LC phase transition

193

appears at lower surface pressure values. In these cases, there is a less important molecular reorientation compared with the monolayers formed by Sph. As expected, apparent molar entropy, enthalpy and internal energy changes

- durtng the transition, have negative values. Tbis means that during the lE-Le phase transitions, the molecules in monolayer become more ordered and the transition is exothermic. The lipids with high molecular asymmetry suffer a higher molecular reorganization when they are compressed as monolayers, for this reason, the transition is observed, mainly in monolayers containing Sph. 60

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