Direct Determination of Hydration in the Interdigitated and Ripple Phases of Dihexadecylphosphatidylcholine: Hydration of a Hydrophobic Cavity at the Membrane/Water Interface

Hydrophobic cavities at the membrane/water interface are stably expressed in interdigitated membranes. The nonsolvent water associated with 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (Hxdc(2)GroPCho) in the interdigitated (L(beta)I) and ripple (P(beta')) states and with its ester analogue 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (Pam(2)PtdCho) in the gel (L(beta')) and P(beta') states are determined directly. In the L(beta)I state at lower temperatures (4-20 degrees C), 16-18 water molecules per phospholipid are bound, consistent with water-filled cavities and hydrated headgroups. At 28 degrees C, the nonsolvent water decreases to 12, consistent with a reduction of the cavity depth by 0.34 nm due to increased chain interpenetration. This geometric lability may be a common feature of hydrophobic cavities. Only 5.4 waters are bound in the noninterdigitated P(beta') (40 degrees C), whereas the ester bound 8.1 waters in its P(beta') (37 degrees C), a difference of about one water per ester carbonyl. The relative dehydration of the ether linkage is consistent with it promoting more densely packed structures, which in turn, accounts for its ability to interdigitate.