Galactosyl headgroup interactions control the molecular packing of wheat lipids in Langmuir films and in hydrated liquid-crystalline mesophases

Abstract

The behavior of the two major galactolipids of wheat endosperm, mono- (MGDG) and di-galactosyldiacylglycerol (DGDG) was studied in aqueous dispersion and at the air/liquid interface. The acyl chains of the pure galactolipids and their binary equimolar mixture are in the fluid or liquid expanded phase. SAXS measurements on liquid-crystalline mesophases associated with the electron density reconstructions show that the DGDG adopts a lamellar phase L(alpha) with parallel orientation of the headgroups with respect to the plane of the bilayer, whereas MGDG forms an inverse hexagonal phase H(II) with a specific organization of galactosyl headgroups. The equimolar mixture shows a different behavior from those previously described with formation of an Im3m cubic phase. In comparing monolayers composed of the pure galactolipids and their equimolar mixtures, PM-IRRAS spectra show significant differences in the optical properties and orientation of galactosyl groups with respect to the interface. Furthermore, Raman and FTIR spectroscopies show that the acyl chains of the galactolipid mixture are more ordered compared to those of the pure components. These results suggest strong interactions between MGDG and DGDG galactosyl headgroups and these specific physical properties of galactolipids are discussed in relation to their biological interest in wheat seed.

Biomimetic monolayer films of monogalactosyldiacylglycerol incorporating ubiquinone

“…Also, our results for the collapse pressure are in agreement with previous results for saturated MGDG [29] and for pure UQ [10]. On the contrary, reported results in literature for unsaturated MGDG [11,27,28] showed a continuous LE phase according with the double bonds present in its structure, and also showed a collapse pressure of ca. 44 mN·m -1 , which is lower than the values obtained in our results due to the unsaturations.…”

“…Bottier et al [27] have also observed similar structures for the MGDG and MGDG:DGDG mixtures at high surface pressures (35 mN·m -1 ). Table II where the monolayer coverage area for 20:1 and 10:1 are also close and 5:1 is far from these values, at low surface pressures.…”

“…Unsaturated galactolipids form monolayers on water surface exhibiting an homogeneous liquid-expanded (LE) phase in agreement with the degree of double bonds in the acyl chains [27,28]. Bottier et al [27] studied MGDG, DGDG and MGDG:DGDG mixture films, and also studied these films inserting proteins (puroindolines) [38].…”

“…3 Several studies have been performed on galactolipids due to their relevance in photosynthesis and other natural processes like cell growth modulator [20], anti-inflammatory [21,22] and anti-cancer agent [23][24][25][26]. One of the most relevant model membrane information is the galactolipid phase behaviour, especially its liquid-crystalline packing properties which has been widely investigated [27][28][29][30][31][32][33][34][35]. The most important difference between membrane galactolipids from different organisms is the saturation grade of these lipid acyl chains, being commonly polyunsaturated for higher plants and monounsaturated or fully saturated for microorganisms [29,36,37].…”

“…Unsaturated MGDG has been more widely used because needs no further treatment after isolation, but has the inconvenience of the different saturation grade depending on the natural source and a lower stability. The extent of double bonds in lipid monolayer formation is associated with preventing the formation of ordered phases [12,27].…”