Thermodynamics and structures of amide phospholipid monolayers

Monolayers of newly synthesized amide phospholipids, l-O-hexadecyl-2-N-palmitoyl-2-amino-2-deoxy-snglycero-3-phosphocholine (L-ether-amide-PC) and 1-palmitoyl-2-N-palmitoyl-2-amino-2-deoxy-sn-glycero3-phosphocholine (L-ester-amide-PC) were investigated at the air/subphase interface by Brewster angle microscopy combined with a conventional LB trough and Grazing incidence X-ray diffraction (GIXD). Both systems investigated exhibit two-dimensional domains with characteristic shape in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) phases. Especially L-ether-amide-PC shows highly chiral domains depending on temperature. The largest chiral domains (400 μm) growing in a clockwise direction are obtained at 25°C. Such large domains have rarely been observed for phospholipids before. Thermodynamic data derived from π-A isotherms provide information about critical temperature (T_c) and phase transition enthalpy ΔH) for both amide phospholipids. The amide group leads to pronounced differences in the thermodynamic parameters compared with DPPC. The two-dimensional structures of the monolayers were investigated by GIXD. The results show that both amide phospholipids exhibit chiral in-plane structures. In the case of L-ether-amide-PC, the tilt angle of the aliphatic chains decreases only slightly with increasing pressure. On the contrary, the tilt angle of the aliphatic chains of L-ester-amide-PC shows strong dependence on surface pressure similar to DPPC. Possible hydrogen bonds involving the amide group seem to enhance the chiral interaction and play therefore a key role in the formation of extremely large chiral domains. The affects of substitution in the sn-1 position on structure formation are discussed in detail.