trans-2-Aminocyclohexanol as a pH-sensitive conformational switch in lipid amphiphileswz

Conformationally controlled molecular switches are central devices of molecular machines and provide a new and promising approach to materials with controllable properties. However, the concept of conformational switches has not yet been applied to the design of triggerable lipid amphiphiles. The potential advantage here is that the effect of the conformational switch in one molecule of lipid amphiphile can be amplified in its colloidal assemblies, such as liposomes. Acid-sensitive lipid amphiphiles have drawn much interest as building blocks of colloidal drug and gene carriers that release therapeutic payloads specifically at low-pH target sites, such as solid tumors and inflamed tissues. Although the reported acid-sensitive lipid amphiphiles have versatile chemical structures, their design falls into only two categories: (1) change of the hydrodynamic radius of the lipid headgroups by protonation with concomitant phase changes of the lipid bilayer, and more recently, (2) hydrolysis of acid-labile lipids. Since the resistance of lipid bilayers against diffusion mostly relies on the stacking of the hydrophobic lipid tails, a change of conformation of the lipid tails could also be exploited to trigger liposomes. Such consideration turned our attention to trans-2-aminocyclohexanols (TACHs), the protonation of which results in a strong hydrogen bond between the amino and the hydroxyl groups, forcing both these groups to adopt an equatorial conformation. Further, this change can be mechanically transmitted by the cyclohexane ring to induce conformational changes of its remote substituents. Previously, TACH-based pH-sensitive conformational switches were used to control the affinity of a crown ether and a podand to potassium ion. Here we present TACH derivatives 1 and 2 (Scheme 1) as the first attempt to design conformationally switchable lipid amphiphiles and their pH-sensitive liposomes. Based on our earlier studies on TACHs, we anticipated that both trans-dodecyloxycarbonyl groups of 1 would adopt the equatorial conformation while the morpholine and the hydroxyl groups would be both in the axial conformation (1A in Scheme 1). Upon protonation, the axial-to-equatorial switch of the morpholine and the hydroxyl groups would force both ester groups into axial positions (1BH), thereby drastically increasing the separation of the lipid tails. In liposomes comprising 1, this would loosen the packing of the lipid tails in the bilayer, leading to leakage and even to the collapse of the liposomes. To estimate the effect of such conformational change, the stereoisomer 2 (Scheme 1) with two dodecyloxycarbonyl groups in the cisconfiguration was also studied for comparison. We expected this isomer to be always predominantly in a conformation with three substituents in the equatorial position (2B), and to show little or no conformational change upon protonation (2BH, Scheme 1). Lipids 1 and 2 were prepared (Schemes S1, S2z) using the approach developed previously. The conformer populations (nA, nB) in the fast equilibrium [1A] $ [1B + 1BH ] (Scheme 1) were estimated from an averaged signal width (W = SJHH) measured as the distance between terminal peaks of a multiplet in H NMR: Wobserved = WAnA + WBnB. 7 We used mainly the signal of H4, geminal to the hydroxyl group (Scheme 1), which was usually better resolved and located in a region apart from other signals. The parameter WB was assumed