Structuration of poly(ethyleneglycol) grafted lipids at the air water-interfase

EDUARDO M. CLOP; ANAHÍ DEL V. TURINA; MARÍA V. NOLAN; MARÍA A. PERILLO

Los Cocos, Córdoba, Argentina

Congreso; XXXVIII Reunión de la Sociedad Argentina de Biofísica; 2009

Sociedad Argentina de Biofísica

In aggregates of grafted lipids with poly(ethylene glycol) (PEG) the polymer moiety form a hydrophilic layer with large excluded volume and high degree of conformational entropy [1][2] and is expected to impose strong constraints to the orientation of the lipid part or the molecule thus affecting the type of self-assembling structures they may form. To further study this structural problem, in the present work Langmuir films composed of dipalmitoyl phosphatidylethanolamine grafted poly(ethyleneglycol) (PE-PEG) with PEG mean molecular weight (MMW) of 350, 1000 and 5000 Da were analysed through surface pressure (p)-area (A) compression isotherms, Brewster Angle Microscopy (BAM) and Polarization Modulation IR Reflection Absorption Spectroscopy (PM-IRRAS) spectroscopy. All PE-PEGs p-A isotherms were gaseous at low p,  with lift-off areas increasing with the MMW of the PEG moiety (660, 1000 and 6000 A2/molec, respectively). At 15 mN/m they all acquired more packed arrangements with molecular areas 140, 180 and 500 2/molec. and PE-PEG1000 and PE-PEG5000 completed a smooth phase transition. PE-PEG5000 collapsed at 42 mN/m while PE-PEG350 and PE-PEG1000, after a phase transition at 25 and  30 mN/m, collapsed at 60 and 55 mN/m respectively. The transition from the highly expanded to a more condensed phase with relative small minimal molecular areas (40, 50 and 140 A2/molec) suggested the transition from the mushroom to linear brush conformations already described in bilayers vesicles. BAM images and PM-IRRAS spectra, acquired at an angle on incidence of 80o, were done with the three PE-PEG monolayers packed at varyied p. The PM-IRRAS for the three PE-PEGs assayed, showed a spectrum compatible with carbohydrate chains increasingly disordered as the PEG´s MMW increased, with the main transitional vibrational mode corresponding to the asymmetrical stretching of unbounded oxygen from highly oriented phosphate groups (nas(PO-2)) . For PE-PEG1000, a negative to positive inversion of a nas(PO-2) peak in the PM-IRRAS spectrum was observed upon compression which correlated with the appearance of circular domains in the originally homogeneous PE-PEG monolayer as revealed by BAM. In the case of PE-PEG350 similar domains were present at all the studied p, and no inversion of nas(PO-2) peak was observed (always positive). PE-PEG5000 monolayers exhibited homogeneous BAM images within the whole p range analysed and a peak inversion (from positive to negative) occurred upon compression. The turning point of phosphate dipole transition moment from the parallel to perpendicular orientation with respect to the monolayer [3] precludes the mushroom-brush phase transition of the molecular polar head group evidenced by white dots in BAM images PEG350 and PEG1000 or a sol-gel transition of the hydrophilic layer in PE-PEG5000.