TAIL AND HEAD GROUP-INTERACTIONS IN PHOSPHOLIPID MONOLAYERS

The order/disorder transition of the phospholipid 1,2-dihexadecyl-sn-g lycero-3-phosphocholine and its derivatives was studied by X-ray refle ction and thermodynamic measurements. We find that increasing the leng th of an ethylene oxide spacer (zero to three EO groups) between the p hosphate group and the hydrophobic chains (i) induces a slight shift o f the main transition, (ii) reduces the influence of the head groups o n the chain lattice, and (iii) increases the head group length and pro motes its hydration. For DH(EO)(3)PC in the ordered phase we find that the small phosphate groups (which are the main source of contrast in X-ray reflectivity) are homogeneously distributed within the head grou p region. In the fluid phase the phosphate groups are preferentially o riented toward the air/water interface. (C) 1996 Academic Press, Inc.