CONSPECTUS: Defining the two-dimensional structure of cell membranes represents one of the most daunting challenges currently facing chemists, biochemists, and biophysicists. In particular, the time-averaged lateral organization of the lipids and proteins that make up these natural enclosures has yet to be established. As the classic Singer-Nicolson model of cell membranes has evolved over the past 40 years, special attention has focused on the structural role played by cholesterol, a key component that represents ca. 30% of the total lipids that are present. Despite extensive studies with model membranes, two fundamental issues have remained a mystery: (i) the mechanism by which cholesterol condenses low-melting lipids by uncoiling their acyl chains and (ii) the thermodynamics of the interaction between cholesterol and high- and low-melting lipids. The latter bears directly on one of the most popular notions in modern cell biology, that is, the lipid raft hypothesis, whereby cholesterol is thought to combine with high-melting lipids to form "lipid rafts" that float in a "sea" of low-melting lipids. In this Account, we first describe a chemical approach that we have developed in our laboratories that has allowed us to quantify the interactions between exchangeable mimics of cholesterol and low- and high-melting lipids in model membranes. In essence, this "nearest-neighbor recognition" (NNR) method involves the synthesis of dimeric forms of these lipids that contain a disulfide moiety as a linker. By means of thiolate-disulfide interchange reactions, equilibrium mixtures of dimers are then formed. These exchange reactions are initiated either by adding dithiothreitol to a liposomal dispersion to generate a small amount of thiol monomer or by including a small amount of thiol monomer in the liposomes at pH 5.0 and then raising the pH to 7.4. We then show how such NNR measurements have allowed us to distinguish between two very different mechanisms that have been proposed for cholesterol's condensing effect: (i) an umbrella mechanism in which the acyl chains and cholesterol become more tightly packed as cholesterol content increases because they share limited space under phospholipid headgroups and (ii) a template mechanism whereby cholesterol functions as a planar hydrophobic template at the membrane surface, thereby maximizing hydrophobic interactions and the hydrophobic effect. Specifically, our NNR experiments rule out the umbrella mechanism and provide strong support for the template mechanism. Similar NNR measurements have also allowed us to address the question of whether the interactions between low-melting kinked phospholipids and cholesterol can play a significant role in the formation of lipid rafts. Specifically, these NNR measurements have led to our discovery of a new physical principle in the lipids and membranes area that must be operating in biological membranes, that is, a "push-pull" mechanism, whereby cholesterol is pushed away from low-melting phospholipids and pulled toward high-melting lipids....

Heteroditopic monomers containing an arylboronate ester and a dialkyl-4-aminopyridine group aggregate via dative boron-nitrogen bonds to give main chain supramolecular polymers. The degree of polymerization can be tuned by changing the electronic and steric properties of the boronate ester.

A quantitative assessment has been made of the interaction between exchangeable mimics of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol in the liquid-ordered (l0) and the liquid-disordered (ld) states using the nearest-neighbor recognition (NNR) method. This assessment has established that these lipids mix ideally in the l0 phase (i.e., they show no net attraction or repulsion toward each other) but exhibit repulsive interactions in the ld phase. The implications of these findings for the interactions between unsaturated phospholipids and cholesterol in eukaryotic cell membranes are briefly discussed.

Nearest-neighbor recognition measurements have been made using exchangeable mimics of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in the liquid-ordered (lo) and liquid-disordered (ld) states. In the ld phase, the net interaction between these two lipids is repulsive. In the lo phase, their interactions are neither attractive nor repulsive. These results, together with previous nearest-neighbor measurements, imply that the overall driving force for lipid domain formation in bilayers composed of high-melting lipids, low-melting lipids, and cholesterol, corresponds to a strong pull (attraction) between the high-melting lipids and cholesterol, a significant push (repulsion) between the low-melting and high-melting lipids, and a significant push between the low-melting lipids and cholesterol. In a broader context, these results provide strong support for the notion that repulsive forces play a major role in the formation of lipid rafts.

A C(3) symmetric cyclic pseudohexapeptide containing 2-aminopicoline-derived subunits and 1,5-disubstituted 1,2,3-triazole rings is introduced as a potent anion receptor. This macrocycle was designed to mimic both the conformation and the receptor properties of a previously described cyclic hexapeptide containing alternating L-proline and 6-aminopicolinic acid subunits. Conformational analyses demonstrate that the cyclic peptide and the cyclic pseudopeptide are structurally closely related. Most importantly, both exhibit a converging arrangement of the NH groups, hence a good preorganization for anion binding. As a consequence, the pseudopeptide also very efficiently interacts with halide and sulfate ions, and this is the case even in competitive aqueous solvent mixtures. However, there are clear differences in the structures of both compounds, which translate into characteristic differences in receptor properties. Specifically, (i) the pseudopeptide possesses an anion affinity intrinsically higher than that of the cyclopeptide, (ii) the pseudopeptide is well preorganized for anion binding in a wider range of solvents from aprotic to protic, (iii) anion affinity in aprotic solvents is very high and associated with complexation equilibria that are slow on the NMR time-scale, (iv) the propensity of the pseudopeptide to form sandwich-type 2:1 complexes with two receptor molecules surrounding one anion is significantly lower than that of the cyclopeptide. A solvent-dependent calorimetric characterization of the binding equilibria of both compounds provided clear evidence for the stabilizing effect of hydrophobic interactions between the receptor subunits in such 2:1 complexes. The pseudopeptide thus represents the first member of a new family of anion receptors whose properties may be fine-tuned by varying the side chains in the periphery of the cavity.