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Structural relationships between lamellar, cubic and hexagonal phases in monoglyceride-water systems. possibility of cubic structures in biological systems
Chemistry and Physics of Lipids, 27 (1980) 321-328 © Elsevier/North-Holland Scientific Publishers Ltd.
STRUCTURAL RELATIONSHIPS BETWEEN LAMELLAR, CUBIC AND HEXAGONAL PHASES IN MONOGLYCERIDE-WATER SYSTEMS. POSSIBILITY OF CUBIC STRUCTURES IN BIOLOGICAL SYSTEMS
K. LARSSON, K. FONTELL and N. KROG Department of Food Technology, University of Lurid, Box 740, S-220 07 Lund (Sweden) and A/S Grindstedvaerket, DK.8220, Brabrand (Denmark) Received March 24th, 1980
accepted June 26th, 1980
The transitions lamellar ~ cubic ~ hexagonal in the aqueous system of sunflower oll monoglycerides are analysed. X-Ray diffraction data show linear relationships between the lattices of the three phases, which are discussed on the basis of structures formed by lipid bilayer units. The cubic structure is related to 'Schwarz's primitive cubic minimal surface' and consists of a three-dimensional continuous bilayer system separating two separate water channel systems. It is also pointed out that the three-dimensional membrane system in plant plastids, the prolarnellar body, which is involved in the formation of thylakoid membranes of chloroplasts, has a structure which is closely related to or identical with that of the cubic phase of monoglyeeride-water systems. Introduction Three different liquid-crystalline phases have been observed in binary monoglyceride-water systems [1,2]. They have lamellar, cubic and reversed hexagonal structures and their X-ray diffraction data reveal geometric relationships between them. These relationships and their structural aspects will be considered in the present paper, which concerns the system sunflower oil monoglycerides-water. This aqueous system was chosen for two reasons: it exhibits all these phases and the fatty acid pattern is rather similar to that o f chloroplast membrane. The cubic phase is o f interest with regard to biomembranes. Arguments will be given below that the ordered membrane system involved in the formation o f chloroplast lameUae in plastids o f plants, the so-called prolamellar body, has the same type o f structure as the cubic phase occurring in monoglyceride-water systems. Furthermore recent findings suggest that this cubic phase is an intermediate physical state in fat digestion [3].
Experimental The sunflower oil monoglyceride was prepared by molecular distillation and
321
322
K. Larsson et al., Structural phases in monoglyceride.watersystems
consists of about 90% of the 1-monoglyceride and about 10% of the 2-isomer. Its fatty acid pattern is: C 12:0 0.1%, C 14:0 0.2%, C 16:0 7.2%, C 17:0 0.1%, C 18:0 3.9% C 20:0 0.1%, C 22:0 0.5%, C 18:l 26.4%, C 20:1 0.1%, C 22:1 0.1%, C 18:2 60.8%, C 18:3 0.4% [4]. The sample preparation and X-ray technique were the same as in previous work [5]. The X-ray diffractograms were recorded by a DPT-camera [6] and by a Guinier-camera according to Luzzati [7].
Results and discussion The phase diagram
The formation of other hquid-crystalline phases than the lamellar one in monoglyceride-water systems is favoured when the monoglyceride has a long chain and also by cis-double bonds in the chain. A characteristics phase diagram for such systems is that of the sunflower oil monoglyceride-water system (Fig. 1). With increasing temperature the lamellar phase is transformed into a cubic phase, which in turn is transformed into a hexagonal phase. The hexagonal phases of the monoglyceride-water systems have been shown to be of the reversed type (a structure with water channels in a hydrocarbon chain continuous matrix) [2]. The cubic phase
Arguments have recently been presented for a structure of the cubic monoglyceride-water phase consisting of lamellar bilayer units [5]. In this structure hexagon-shaped bilayer discs are connected into a three-dimensional network. Relations between the cubic and lamellar phases, shown by X-ray diffraction data and by values of the translational diffusion coefficients of the lipid, were taken as evidence for the proposed structure.
80 o
t
c 60
40
20
40
60
80
-' % (w/w) H20 Fig. 1. Phase.diagram for the binary system sunfloweroil monoglyceridesand water (cf. Ref. 4).
K. Larsson et aL, Structural phases in monoglyceride-water systems
323
The body-centered cubic structure of the monoglyceride system has some features which merit further consideration: (1) The structure is in fact bicontinuous with regard to water. There are two separate three-dimensional water channel systems and the two sets of polyhedra which form the channels in the three axial directions correspond to the positions (0,0,0) and (1/2, 1/2, 1/2)of the cubic cell. (2) There is a geometric relation between the lattices of the lamdlar and the cubic phases as demonstrated in the previous paper [5], and we have also found a similar relation between the cubic and hexagonal lattices. The geometry of the lamellar to cubic transition will first be considered: The DPT-diagrams reported in ref. [5 ] showed that the lamellar d (00 l)-spacing connects with the cubic (222)-spacing. In order to analyse this relation, let us for simplicity consider only the methyl-end group planes of the lipid bilayer hexagons which form the cubic phase (Fig. 2). The connection implies that the interplanar spacing of the methyl-end group planes in the lamellar phase is equal (or nearly equal) to the interplanar spacing of the (222)-planes of the cubic phase. The crosssection area of the (222)-planes within the cubic unit cell is 2a2V~. Compared with the total area of the hexagons forming the methyl-end group planes of the cubic structure, 3/2a:x/~ per unit cell, this implies a reduction by 25% in the molecular cross-section area at the methyl-end group plane. A further reduction in the molecular cross-section area will ultimately result in a hexagonal structure (see below).
Fig. 2. Schematic illustration of the body-centered cubic structure in monoglyceride-water systems. The cubic unit cell axes are dotted. The structure unit is a hexagon-shaped lipid monolayer, there are in a cell sixteen equivalents, which degenerate to eight equivalent methyl contact planes. The planes have the positions +_ [(~1 P), (I I I), (IF I), (t IT)].
324
K. Larsson et al., Structural phases in monoglyceride-water systems
In this connection it may be mentioned that there exists another group of cubic phases whose structures also are body-centered. These phases have the rod-structure proposed by Luzzati and coworkers; the structure is built up of short rod-like aggregates, which are joined three and three at each end and form two interwoven but otherwise independent three-dimensional networks [8]. There is a difference between these two structure proposals in that NMR diffusion studies both of lipid and water dearly makes a distinction between a structure based upon rodaggregates and upon lamellar disc-aggregates [5]. The now discussed cubic structure is as mentioned bicontinuous with regard to water. A comprehensive discussion of the possibilities of dividing space into two interpenetrating continuous regions separated by 'periodic minimal surfaces' has been published by Schoen [9], and the existence of such surfaces separating water and the hydrocarbon moiety and forming 'equilibrium bicontinuous structures' in amphiphile-water systems was recently proposed by Scriven [10]. The cubic structures formed by such curved periodic minimal surfaces will have primitive cubic lattices, and one of the possibilities, Schwarz's periodic minimal surface (space group no. 221, Pm 3 m) is closely related to the cubic monoglyceride structure discussed above. If the hexagon-shaped discs are allowed to be deformed as shown in Fig. 3, the methyl-end group planes will form a Schwarz's primitive cubic minimal surface. If we instead of the plane consider a bilayer, and also take into account the symmetry of the lipid bilayer, the structural unit will be the corresponding monolayer unit and the lattice is no longer primitive but bodycentered (Ira 3 m). An attractive feature of this structure compared to the one with planar disc-like units (Fig. 2), where all the curvature occurs at the edges, is that the 'minimal surface' can separate two bulk phases which are in hydrostatic equilibrium. The membrane organization in the so-called prolamellar body occurring in plants might be related to this cubic structure. It is known that a three-dimensional ordered membrane system develops from etioplasts in darkness, and forms chloroplast lamellae when exposed to light [11 ]. The prolamellar body is considered to be a depot for formation of thylakoid bilayer membranes. It is for example known that ATP inhibits the dissociation between pigment and protein in the prolamellar body, and it also inhibits the rearrangement of the prolamellar body into thylakoids at exposure to light. The formation of prolamellar bodies in plastids is a normal process in the development of chloroplasts in the initial growing stage of, for example, cereal seedlings. The successive changes when a prolameUar body is deorganized and transformed into a chloroplast membrane have been described [11], and in that connection a cubic framework of tubular lamellae has been observed. The structure was derived at from the appearance of the electron micrographs [12,13]. The inner diameter of the tubes is about 100 ~, and the tubes are formed of membranes whose thickness is about 50 X. These tubes are connected to a structure resembling that of the cubic monoglyceride-water phase (Fig. 4). This tubular stlucture has two
K. Larsson et al., Structural phases in monoglyceride.water systems
325
Fig. 3. Comparison between two strucutre units of Schwar~'s periodic minimal surface, with primitive cubic symmetry, and the corresponding structure units of the cubic monoglyceride structure based upon hexagon-shaped bilayer discs. The figure shows two polyhedra along one water channel. Only the methyl end-group planes are shown (as in Fig. 2). If the surface formed by the methyl end-group planes is replaced by the bilayer, with its two-fold symmetry, the space group of Schwarz's surface (Pro 3 m) is changed to the body centered space group (Ira 3 m) suggested for the cubic monoglyceride-water phase [5].
Fig. 4. The cubic structure of the prolamellar body according to electron microscopy studied [12]. Membranes form tubular aggregates paraUel to the three principal axes and connected at the unit cell corners. If the curvature (which hardly can be determined from electronmicroscopy pictures) is slightly changed, the same structure as shown in Fig. 3 is obtained (i.e. a body-centered structure).
326
K. Larsson et al., Structural phases in monoglyceride-water systems
contInuous regions for the aqueous medium; inside and outside the membrane tube systems. The electron microscopical appearance is also in general agreement with the cubic structure formed by Schwarz's infinite periodic minimal surfaces (of. Figs. Figs. 3 and 4). The structure of the prolamellar body seems thus to be closely related to or identical with the structure of the cubic monoglyceride-water phase. The cubic monoglyceride-water phase is also of physiological interest. In vitro studies of triolein lipolysis, in an environment resembling the intestine, have shown that the cubic phase is an intermediate state between fat emulsion droplets and monoglyceride/fatty acid/bile acid mixed micelles [3]. The geometry of cubic to hexagonal transition will now be considered: The diffraction data of the thermal phase transition lamellar ~ cubic and cubic -~ hexagonal show approximately linear relations between the observed set of spacings. The agreement between the lamellar d(O01)-spacing and the cubic (222)-spacing was thus discussed above and Fig. 5 gives a DPT-diagram of the cubic ~ hexagonal transition. At 45°C the cubic phase exhibits the spacings 58.2, 47.4, 33.9 and 27.8/~. Above the transition cubic ~ hexagonal at 75°C the spacings are 47.9, 27.5 and 24.1 ,~ (ratio 1 : x/~ : Vc4). An indexing of the cubic reflections in compliance with earlier X-ray findings [5] gives (h 2 + k 2 + 12)-values of 12, 20, 42 and 58. The first spacing, cubic (222), appears to be connected with the hexagonal d(10)-spacing and the relation does not, within the experimental accuracy, vary with water composition. Lattice relationships between the lamellar, cubic and hexagonal phases
In order to discuss the lattice relations, let us for simplicity consider hypothetical lameUar, cubic and hexagonal phases, where the bilayers are so thin that we can °C
@< 0 Fig. 5. DPT-diagzam (diffraction pattern versus temperature) showing the transitions cubic to reversed hexagonal and to isotropic solution (L2-phase) in the system sunflower oil monoglycerides-water. Composition 60 : 40 (w/w).
K. Larsson et aL, Structural phases in monoglyceride-water systems
327
neglect their thickness. Such phases should show a linear relationship between the spacings of the lamellar and cubic as well as between the cubic and hexagonal phases. If we thus consider the bilayer areas within the cubic unit cell, the (222)planes correspond to an area of 2a2x/~, whereas all bilayer hexagons correspond to an area of 3/2a2vr3 (see 'The Cubic Phase' above). Similarly, the relations between such hypothetic lamellar and hexagonal phases are shown in Fig. 6 (a H = 2d(O01)). In fact, the experirnental data show a direct connection between the cubic (222)spacing (corresponding to the lamellar d(001)-spacing) and the first hexagonal spacing d(10) = a H X/3/2 (Fig. 5). According to this simplified model of bilayers with neglectable thickness there should be a reduction in molecular cross-section area in the bilayer in the series lameUar o cubic o hexagonal corresponding to 1 : 3/4 : 1/x/~ (1 : 0.75 : 0.58). It should in this connection be mentioned that inside the lamellar phases there is a marked decrease in spacing with increase in temperature which means an increase in the cross-section area per molecule. Near the transition temperature there is an increase in cross-section area per molecule of 0.5 -- 1 )k2 per °C. The cause for the phase transitions might be explained by a tendency with increasing temperature for chain divergence, or for an increasing amount of gauche-conformations, toward the methyl end groups. Within the lameUar phase this tendency must result in the observed strong increase in cross-section area per molecule, and (at a certain degree of 'loose' chain-packing) transition into the cubic phase. In the cubic phase it is possible to estimate the molecular divergence from the lattice parameters. Thus an estimation based on the data for 1-monoolein with 25% (w/w) of water [5] gives an increase of about 25% in the molecular cross-section area along the chain from the polar head to the methyl end group. In the hexagonal phase, the corresponding divergence can be estimated to amount to about 50% according to the dimensions shown in Fig. 5. The transition cubic to hexagonal might thus also be explained by an increasing tendency for chain divergence. Thus on a molecular level there seems in aqueous systems to be a close relation between the structures for the lamellar, cubic and hexagonal phases. The reason for this is requirements for a critical molecular packing at the interfaces between polar and nonpolar regions, evident among other things by the 'ideal' swelling of aH
d(O01) ~ Fig. 6. lUustraUon of geometric relations between a lamellar structure and a hexagonal one, which is formed when the lamellae axe cut up into long strips and joined into a structure with honey-comb cross-section. The lines represent the methyl end-group planes of the lipid bilayers. It is evident that the relation between the hexagonal axis a H and the lamellar period d (001) isa H = 2 d (001).
328
K. Larsson et al., Structural phases in monoglyceride-water systems
the lamellar phase with water. When the temperature of the lamellar phase is increased, the increased molecular mobility of the hydrocarbon chains results in a demand for an increased molecular cross-section area, and this puts a strain on the stability of the phase. This strain may be appeased by a transition to a cubic structure, so that a close molecular packing at the polar-non-polar interfaces can persist while the relative volume of the non-polar hydrocarbon region is increased. At still higher temperatures the same mechanism might result in the transition from cubic to a reversed hexagonal structure. Such an explanation is also in agreement with results of NMR-studies, showing that the surroundings of the polar groups in different phases of an aqueous amphiphile system are very similar [14,15]. Luzzati has discussed other cases of 'strained' structures in very much similar terms [16]. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
E.S. Lutton, J. Am. Oil Chem. Soc., 42 (1965) 1068. K. Larsson, Z. Phys. Chem. (Frankfurt-am-Main), 56 (1967) 173. J.S. Patton and M.C. Carey, Science, 204 (1979) 145. J.V. Boyd, N. Krog and P. Sherman, in A.L. Smith (Ed.), Theory and Practice of Emulsion Technology, Academic Press, 1976, p. 61. G. Lindblom, K. Larsson, L. Johansson, K. Fontell and S. Fors6n, J. Am. Chem. Soc., 101 (1979) 5465. E. Stenhagen, Acta Chem. Scand., 5 (1951) 805. V. Luzzati, H. Mustacchi, A. Skoulios and F. Husson, Acta Cryst., 13 (1960) 660. V. Luzzati and P.A. Spegt, Nature, 215 (1967) 701. A.H. Schoen, Nasa Technical Notes (1970) NASA TN D-5541. L.E. Scriven, Nature, 263 (1976) 123. H.I. Virgin, A. Kahn and D. Wettstein, Photochem. Photobiol. 2 (1963) 83. B.E.S. Gunning, Protoplasma, 60 (1965) 111. P. Horton and R.M. Leech,.Plant Physiol., 55 (1975) 393. B. Lindman and H. Wennerstr6m, Top. Curl Chem., 87 (1980) 1. H. Wennerstr6m and B. Lindman, Physics Rept., 52 (1979) 1. V. Luzzati, in D. Chapman (Ed.), Biological Membranes, Academic Press, 1976, p. 71.
STRUCTURAL RELATIONSHIPS BETWEEN LAMELLAR, CUBIC AND HEXAGONAL PHASES IN MONOGLYCERIDE-WATER SYSTEMS. POSSIBILITY OF CUBIC STRUCTURES IN BIOLOGICAL SYSTEMS
K. LARSSON, K. FONTELL and N. KROG Department of Food Technology, University of Lurid, Box 740, S-220 07 Lund (Sweden) and A/S Grindstedvaerket, DK.8220, Brabrand (Denmark) Received March 24th, 1980
accepted June 26th, 1980
The transitions lamellar ~ cubic ~ hexagonal in the aqueous system of sunflower oll monoglycerides are analysed. X-Ray diffraction data show linear relationships between the lattices of the three phases, which are discussed on the basis of structures formed by lipid bilayer units. The cubic structure is related to 'Schwarz's primitive cubic minimal surface' and consists of a three-dimensional continuous bilayer system separating two separate water channel systems. It is also pointed out that the three-dimensional membrane system in plant plastids, the prolarnellar body, which is involved in the formation of thylakoid membranes of chloroplasts, has a structure which is closely related to or identical with that of the cubic phase of monoglyeeride-water systems. Introduction Three different liquid-crystalline phases have been observed in binary monoglyceride-water systems [1,2]. They have lamellar, cubic and reversed hexagonal structures and their X-ray diffraction data reveal geometric relationships between them. These relationships and their structural aspects will be considered in the present paper, which concerns the system sunflower oil monoglycerides-water. This aqueous system was chosen for two reasons: it exhibits all these phases and the fatty acid pattern is rather similar to that o f chloroplast membrane. The cubic phase is o f interest with regard to biomembranes. Arguments will be given below that the ordered membrane system involved in the formation o f chloroplast lameUae in plastids o f plants, the so-called prolamellar body, has the same type o f structure as the cubic phase occurring in monoglyceride-water systems. Furthermore recent findings suggest that this cubic phase is an intermediate physical state in fat digestion [3].
Experimental The sunflower oil monoglyceride was prepared by molecular distillation and
321
322
K. Larsson et al., Structural phases in monoglyceride.watersystems
consists of about 90% of the 1-monoglyceride and about 10% of the 2-isomer. Its fatty acid pattern is: C 12:0 0.1%, C 14:0 0.2%, C 16:0 7.2%, C 17:0 0.1%, C 18:0 3.9% C 20:0 0.1%, C 22:0 0.5%, C 18:l 26.4%, C 20:1 0.1%, C 22:1 0.1%, C 18:2 60.8%, C 18:3 0.4% [4]. The sample preparation and X-ray technique were the same as in previous work [5]. The X-ray diffractograms were recorded by a DPT-camera [6] and by a Guinier-camera according to Luzzati [7].
Results and discussion The phase diagram
The formation of other hquid-crystalline phases than the lamellar one in monoglyceride-water systems is favoured when the monoglyceride has a long chain and also by cis-double bonds in the chain. A characteristics phase diagram for such systems is that of the sunflower oil monoglyceride-water system (Fig. 1). With increasing temperature the lamellar phase is transformed into a cubic phase, which in turn is transformed into a hexagonal phase. The hexagonal phases of the monoglyceride-water systems have been shown to be of the reversed type (a structure with water channels in a hydrocarbon chain continuous matrix) [2]. The cubic phase
Arguments have recently been presented for a structure of the cubic monoglyceride-water phase consisting of lamellar bilayer units [5]. In this structure hexagon-shaped bilayer discs are connected into a three-dimensional network. Relations between the cubic and lamellar phases, shown by X-ray diffraction data and by values of the translational diffusion coefficients of the lipid, were taken as evidence for the proposed structure.
80 o
t
c 60
40
20
40
60
80
-' % (w/w) H20 Fig. 1. Phase.diagram for the binary system sunfloweroil monoglyceridesand water (cf. Ref. 4).
K. Larsson et aL, Structural phases in monoglyceride-water systems
323
The body-centered cubic structure of the monoglyceride system has some features which merit further consideration: (1) The structure is in fact bicontinuous with regard to water. There are two separate three-dimensional water channel systems and the two sets of polyhedra which form the channels in the three axial directions correspond to the positions (0,0,0) and (1/2, 1/2, 1/2)of the cubic cell. (2) There is a geometric relation between the lattices of the lamdlar and the cubic phases as demonstrated in the previous paper [5], and we have also found a similar relation between the cubic and hexagonal lattices. The geometry of the lamellar to cubic transition will first be considered: The DPT-diagrams reported in ref. [5 ] showed that the lamellar d (00 l)-spacing connects with the cubic (222)-spacing. In order to analyse this relation, let us for simplicity consider only the methyl-end group planes of the lipid bilayer hexagons which form the cubic phase (Fig. 2). The connection implies that the interplanar spacing of the methyl-end group planes in the lamellar phase is equal (or nearly equal) to the interplanar spacing of the (222)-planes of the cubic phase. The crosssection area of the (222)-planes within the cubic unit cell is 2a2V~. Compared with the total area of the hexagons forming the methyl-end group planes of the cubic structure, 3/2a:x/~ per unit cell, this implies a reduction by 25% in the molecular cross-section area at the methyl-end group plane. A further reduction in the molecular cross-section area will ultimately result in a hexagonal structure (see below).
Fig. 2. Schematic illustration of the body-centered cubic structure in monoglyceride-water systems. The cubic unit cell axes are dotted. The structure unit is a hexagon-shaped lipid monolayer, there are in a cell sixteen equivalents, which degenerate to eight equivalent methyl contact planes. The planes have the positions +_ [(~1 P), (I I I), (IF I), (t IT)].
324
K. Larsson et al., Structural phases in monoglyceride-water systems
In this connection it may be mentioned that there exists another group of cubic phases whose structures also are body-centered. These phases have the rod-structure proposed by Luzzati and coworkers; the structure is built up of short rod-like aggregates, which are joined three and three at each end and form two interwoven but otherwise independent three-dimensional networks [8]. There is a difference between these two structure proposals in that NMR diffusion studies both of lipid and water dearly makes a distinction between a structure based upon rodaggregates and upon lamellar disc-aggregates [5]. The now discussed cubic structure is as mentioned bicontinuous with regard to water. A comprehensive discussion of the possibilities of dividing space into two interpenetrating continuous regions separated by 'periodic minimal surfaces' has been published by Schoen [9], and the existence of such surfaces separating water and the hydrocarbon moiety and forming 'equilibrium bicontinuous structures' in amphiphile-water systems was recently proposed by Scriven [10]. The cubic structures formed by such curved periodic minimal surfaces will have primitive cubic lattices, and one of the possibilities, Schwarz's periodic minimal surface (space group no. 221, Pm 3 m) is closely related to the cubic monoglyceride structure discussed above. If the hexagon-shaped discs are allowed to be deformed as shown in Fig. 3, the methyl-end group planes will form a Schwarz's primitive cubic minimal surface. If we instead of the plane consider a bilayer, and also take into account the symmetry of the lipid bilayer, the structural unit will be the corresponding monolayer unit and the lattice is no longer primitive but bodycentered (Ira 3 m). An attractive feature of this structure compared to the one with planar disc-like units (Fig. 2), where all the curvature occurs at the edges, is that the 'minimal surface' can separate two bulk phases which are in hydrostatic equilibrium. The membrane organization in the so-called prolamellar body occurring in plants might be related to this cubic structure. It is known that a three-dimensional ordered membrane system develops from etioplasts in darkness, and forms chloroplast lamellae when exposed to light [11 ]. The prolamellar body is considered to be a depot for formation of thylakoid bilayer membranes. It is for example known that ATP inhibits the dissociation between pigment and protein in the prolamellar body, and it also inhibits the rearrangement of the prolamellar body into thylakoids at exposure to light. The formation of prolamellar bodies in plastids is a normal process in the development of chloroplasts in the initial growing stage of, for example, cereal seedlings. The successive changes when a prolameUar body is deorganized and transformed into a chloroplast membrane have been described [11], and in that connection a cubic framework of tubular lamellae has been observed. The structure was derived at from the appearance of the electron micrographs [12,13]. The inner diameter of the tubes is about 100 ~, and the tubes are formed of membranes whose thickness is about 50 X. These tubes are connected to a structure resembling that of the cubic monoglyceride-water phase (Fig. 4). This tubular stlucture has two
K. Larsson et al., Structural phases in monoglyceride.water systems
325
Fig. 3. Comparison between two strucutre units of Schwar~'s periodic minimal surface, with primitive cubic symmetry, and the corresponding structure units of the cubic monoglyceride structure based upon hexagon-shaped bilayer discs. The figure shows two polyhedra along one water channel. Only the methyl end-group planes are shown (as in Fig. 2). If the surface formed by the methyl end-group planes is replaced by the bilayer, with its two-fold symmetry, the space group of Schwarz's surface (Pro 3 m) is changed to the body centered space group (Ira 3 m) suggested for the cubic monoglyceride-water phase [5].
Fig. 4. The cubic structure of the prolamellar body according to electron microscopy studied [12]. Membranes form tubular aggregates paraUel to the three principal axes and connected at the unit cell corners. If the curvature (which hardly can be determined from electronmicroscopy pictures) is slightly changed, the same structure as shown in Fig. 3 is obtained (i.e. a body-centered structure).
326
K. Larsson et al., Structural phases in monoglyceride-water systems
contInuous regions for the aqueous medium; inside and outside the membrane tube systems. The electron microscopical appearance is also in general agreement with the cubic structure formed by Schwarz's infinite periodic minimal surfaces (of. Figs. Figs. 3 and 4). The structure of the prolamellar body seems thus to be closely related to or identical with the structure of the cubic monoglyceride-water phase. The cubic monoglyceride-water phase is also of physiological interest. In vitro studies of triolein lipolysis, in an environment resembling the intestine, have shown that the cubic phase is an intermediate state between fat emulsion droplets and monoglyceride/fatty acid/bile acid mixed micelles [3]. The geometry of cubic to hexagonal transition will now be considered: The diffraction data of the thermal phase transition lamellar ~ cubic and cubic -~ hexagonal show approximately linear relations between the observed set of spacings. The agreement between the lamellar d(O01)-spacing and the cubic (222)-spacing was thus discussed above and Fig. 5 gives a DPT-diagram of the cubic ~ hexagonal transition. At 45°C the cubic phase exhibits the spacings 58.2, 47.4, 33.9 and 27.8/~. Above the transition cubic ~ hexagonal at 75°C the spacings are 47.9, 27.5 and 24.1 ,~ (ratio 1 : x/~ : Vc4). An indexing of the cubic reflections in compliance with earlier X-ray findings [5] gives (h 2 + k 2 + 12)-values of 12, 20, 42 and 58. The first spacing, cubic (222), appears to be connected with the hexagonal d(10)-spacing and the relation does not, within the experimental accuracy, vary with water composition. Lattice relationships between the lamellar, cubic and hexagonal phases
In order to discuss the lattice relations, let us for simplicity consider hypothetical lameUar, cubic and hexagonal phases, where the bilayers are so thin that we can °C
@< 0 Fig. 5. DPT-diagzam (diffraction pattern versus temperature) showing the transitions cubic to reversed hexagonal and to isotropic solution (L2-phase) in the system sunflower oil monoglycerides-water. Composition 60 : 40 (w/w).
K. Larsson et aL, Structural phases in monoglyceride-water systems
327
neglect their thickness. Such phases should show a linear relationship between the spacings of the lamellar and cubic as well as between the cubic and hexagonal phases. If we thus consider the bilayer areas within the cubic unit cell, the (222)planes correspond to an area of 2a2x/~, whereas all bilayer hexagons correspond to an area of 3/2a2vr3 (see 'The Cubic Phase' above). Similarly, the relations between such hypothetic lamellar and hexagonal phases are shown in Fig. 6 (a H = 2d(O01)). In fact, the experirnental data show a direct connection between the cubic (222)spacing (corresponding to the lamellar d(001)-spacing) and the first hexagonal spacing d(10) = a H X/3/2 (Fig. 5). According to this simplified model of bilayers with neglectable thickness there should be a reduction in molecular cross-section area in the bilayer in the series lameUar o cubic o hexagonal corresponding to 1 : 3/4 : 1/x/~ (1 : 0.75 : 0.58). It should in this connection be mentioned that inside the lamellar phases there is a marked decrease in spacing with increase in temperature which means an increase in the cross-section area per molecule. Near the transition temperature there is an increase in cross-section area per molecule of 0.5 -- 1 )k2 per °C. The cause for the phase transitions might be explained by a tendency with increasing temperature for chain divergence, or for an increasing amount of gauche-conformations, toward the methyl end groups. Within the lameUar phase this tendency must result in the observed strong increase in cross-section area per molecule, and (at a certain degree of 'loose' chain-packing) transition into the cubic phase. In the cubic phase it is possible to estimate the molecular divergence from the lattice parameters. Thus an estimation based on the data for 1-monoolein with 25% (w/w) of water [5] gives an increase of about 25% in the molecular cross-section area along the chain from the polar head to the methyl end group. In the hexagonal phase, the corresponding divergence can be estimated to amount to about 50% according to the dimensions shown in Fig. 5. The transition cubic to hexagonal might thus also be explained by an increasing tendency for chain divergence. Thus on a molecular level there seems in aqueous systems to be a close relation between the structures for the lamellar, cubic and hexagonal phases. The reason for this is requirements for a critical molecular packing at the interfaces between polar and nonpolar regions, evident among other things by the 'ideal' swelling of aH
d(O01) ~ Fig. 6. lUustraUon of geometric relations between a lamellar structure and a hexagonal one, which is formed when the lamellae axe cut up into long strips and joined into a structure with honey-comb cross-section. The lines represent the methyl end-group planes of the lipid bilayers. It is evident that the relation between the hexagonal axis a H and the lamellar period d (001) isa H = 2 d (001).
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K. Larsson et al., Structural phases in monoglyceride-water systems
the lamellar phase with water. When the temperature of the lamellar phase is increased, the increased molecular mobility of the hydrocarbon chains results in a demand for an increased molecular cross-section area, and this puts a strain on the stability of the phase. This strain may be appeased by a transition to a cubic structure, so that a close molecular packing at the polar-non-polar interfaces can persist while the relative volume of the non-polar hydrocarbon region is increased. At still higher temperatures the same mechanism might result in the transition from cubic to a reversed hexagonal structure. Such an explanation is also in agreement with results of NMR-studies, showing that the surroundings of the polar groups in different phases of an aqueous amphiphile system are very similar [14,15]. Luzzati has discussed other cases of 'strained' structures in very much similar terms [16]. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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