Thermal Stabilization of Bicelles by a Bile-Salt-Derived Detergent: A Combined ³¹P and ²H Nuclear Magnetic Resonance Study

Abstract

The properties of bicelles composed of mixtures of long-chain lipids dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG), stabilized by zwitterionic bile salt analogue 3-[(3-cholamidopropyl)dimethyl-d6-ammonio]-2-hydroxy-1-propanesulfonate (CHAPSO-d6), deuterated at both amino methyls, were investigated by a combination of (31)P and (2)H NMR, focusing on the behavior of CHAPSO as a function of temperature. For compositions of molar ratio q = [DMPC + DMPG]/[CHAPSO] = 3, R = [DMPG]/[DMPC + DMPG] = 0, 0.01 and 0.10 and lipid concentration CL = 25 wt % lipid at temperatures of between 30 and 60 °C, magnetic alignment was readily achieved as assessed via both (31)P NMR of the phospholipids and (2)H NMR of CHAPSO-d6. Increasing temperature yielded higher values for the chemical shift anisotropy of the former and the quadrupole splitting of the latter, consistent with the progressive migration of CHAPSO from edge regions into planar regions of the bicellar assemblies. However, relative to dihexadecyl phosphatidylcholine (DHPC), CHAPSO exhibited lower miscibility with DMPC, although the presence of DMPG enhanced this miscibility. At 65 °C, thermal instability became evident in the appearance of a separate isotropic component in both (31)P and (2)H NMR spectra. This isotropic phase was CHAPSO-enriched but less so as a function of increasing DMPG. These findings indicate that the enhanced thermal stability of CHAPSO- versus DHPC-containing bicelles arises from a combination of the larger surface area that edge CHAPSO is able to mask, mole for mole, and its relative preference for edge regions, plus, possibly, specific interactions with DMPG.

Chemically Locked Bicelles with High Thermal and Kinetic Stability

“…According to the 31 P NMR spectra, the system was composed solely of aligned bilayers from 25 °C to over 90 °C (Figure 1 d (ii–vii)), where the signals of isotropic mixed micelles or vesicles were not detected at all (Figure 1 d (ii–vii)). Before the present work, there have been only two bicellar systems that show an aligned phase at temperatures higher than 65 °C, which were reported very recently 5f,g. In addition, even when the system was cooled to 20 °C, where DMPC is known to readily miscible with surfactants owing to the transition to a gel state, our bicellar mixture did not change to isotropic mixed micelles (Figure 1 d (i)).…”

Phase Transitions in Small Isotropic Bicelles

“…31 P NMR is actively used to study the state of lipids in membrane mimetics, however, mostly for the systems with large particles. One of the most common applications of 31 P is monitoring of the orientation of large bicelles in strong magnetic field. , This process is temperature-dependent, and, at some point, the isotropic chemical shift of phosphorus (close to 0 ppm) becomes anisotropic (shifted by 10–20 ppm), which indicates the orientation of particles. 31 P spectra were also utilized to measure the concentration of the monomeric detergent in IsoBs. , Thus, we expected to observe the changes in 31 P chemical shifts or line widths upon the lipid phase transition or as a manifestation of mixing between the lipid and detergents in either the rim or planar area of IsoBs.…”

“…One of the most common applications of 31 P is monitoring of the orientation of large bicelles in strong magnetic field. 31,32 This process is temperature-dependent, and, at some point, the isotropic chemical shift of phosphorus (close to 0 ppm) becomes anisotropic (shifted by 10−20 ppm), which indicates the orientation of particles. 31 P spectra were also utilized to measure the concentration of the monomeric detergent in IsoBs.…”

“…The proposed hypothesis may also explain several facts regarding the behavior of large anisotropic bicelles. First of all, the spontaneous orientation of bicelles is known to occur only at temperatures higher than phase transition temperature of bilayer lipid, and reproducible orientation is obtained only at relatively high concentrations of bicelles (25% w/w) . Apparently, below the critical temperatures and at low concentrations anisotropic bicelles are too small to orient spontaneously, while the fusion of bicelles at high temperatures and in concentrated samples results in the formation of very large particles that undergo the orientation at strong magnetic fields.…”

Steroidal Surfactants: Detection of Premicellar Aggregation, Secondary Aggregation Changes in Micelles, and Hosting of a Highly Charged Negative Substance

“…[6][7][8] Their singular properties as surfactants are a consequence of their particular structure. They are derived from bile salts and also contain a tail with a dipolar sulfobetaine head group with one positive and one negative charge.…”