Phenylene bolaamphiphiles: Influence of the substitution pattern on the aggregation behavior and the miscibility with classical phospholipids

Abstract

The synthesis of two symmetric single‐chain phenylene‐modified bolaamphiphiles with meta and ortho phenyl substitution pattern is described. The aggregation behavior of both bolaamphiphiles in aqueous suspension was investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), Fourier‐transform IR (FTIR) spectroscopy, and small angle neutron scattering (SANS). We could show that a change in the substitution pattern from para to meta or ortho leads to a change in the aggregation behavior so that small micelles instead of nanofibers are formed. Furthermore, the mixing behavior of these bolaamphiphiles with classical bilayer forming phospholipids, such as 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC) and 1,2‐distearoyl‐sn‐glycero‐3‐phosphocholine (DSPC) was studied by means of DSC, TEM, cryo‐TEM, and small angle X‐ray scattering (SAXS). The aim was to stabilize bilayer membranes formed by classical phospholipids by incorporation of bolaamphiphiles to obtain liposomes with improved stability suitable for drug delivery purposes. We could show that the phenylene‐modified bolaamphiphiles are indeed miscible with DPPC and DSPC; however, closed vesicles as observed for pure DPPC and DSPC were not found. Instead, small disk‐like aggregates are formed. In the case of mixtures of phenylene‐modified bolaamphiphiles with DPPC, these bilayer disks have a higher DSC transition temperature compared to pure DPPC indicating an increased stability of the ordered gel phase inside the disks. Practical applications: Vesicles composed of classical phospholipids are widely used as drug delivery systems for pharmaceutical applications. The stability of these vesicles in, e.g., body fluids is an important prerequisite for long‐term applications and it can possibly be enhanced by the incorporation of bipolar amphiphiles (bolaamphiphiles) into phospholipid bilayers. With the use of novel single‐chain phenylene‐modified bolaamphiphiles, synthesized in this work, we are able to generate small lipid‐disks in mixtures with classical phospholipids that have an increased stability of the ordered gel phase inside these disks compared to pure phospholipids. These disks might be further used as nano‐dimensioned drug carrier systems. The incorporation of ortho‐substituted phenylene‐modified bolaamphiphiles (PC‐C17oPhC17‐PC) into DPPC bilayer leads to the formation of small disk‐like aggregates with an increased stability compared to pure DPPC vesicles.

Aggregation behaviour of a single-chain, phenylene-modified bolalipid and its miscibility with classical phospholipids

“…But, closed lipid vesicles (liposomes) and a pronounced stabilization of the lamellar structure – prerequisites for the use as drug delivery vehicle – could not be observed, and bilayer fragments as well as elongated micelles were formed instead [ 35 ]. The isomers PC-C17mPhC17-PC and PC-C17oPhC17-PC, bearing a meta or ortho substitution at the central phenyl ring, showed the formation of disk-like aggregates in mixtures with DPPC and DSPC, respectively, with partly increased thermal stability [ 34 ]. The mixing behaviour of PC-C18pPhC18-PC with phosphatidylcholines was now studied by means of DSC and TEM.…”

“…8 ). Scattering data do not show sharp diffraction maxima, which are observed for binary DPPC/water mixtures at similar temperature and concentration range [ 34 ]. The transformation from sharp maxima to broad ones points on structural changes of the lipid system from multilamellar objects for pure DPPC to separated lamellae for the PC-C18pPhC18-PC:DPPC mixtures.…”

“…In a similar manner to a procedure from [ 34 ], the appropriate amount of the bolalipid PC-C18pPhC18-PC was suspended in H 2 O (Milli-Q). Homogeneous suspensions were obtained by heating to 90 °C and vortexing.…”

“…To evade these packing problems, we expanded the cross-sectional area of the alkyl chain of the bolalipid in order to fill the void volume. Besides heteroatoms [ 29 – 30 ], acetylene [ 31 ] or diacetylene groups [ 32 ], or methyl branches [ 31 , 33 ], also phenyl- or biphenyl rings were inserted into the long alkyl chain [ 34 – 37 ]. The insertion of a phenylene group led to PC-C16pPhC16-PC [ 36 ], which self-assembles at room temperature into small ellipsoidal micelles, and to PC-C17pPhC17-PC [ 35 ] that forms nanofibres with a significantly reduced thermal stability compared to PC-C32-PC.…”

Bolalipid-Doped Liposomes: Can Bolalipids Increase the Integrity of Liposomes Exposed to Gastrointestinal Fluids?

“…One possibility to overcome this packing problem is the enlargement of the hydrophobic part of the bolalipid. Several attempts including, for example, the incorporation of a phenyl group into the alkyl chain of the bolalipid [47][48][49][50][51] have failed. Hence, we synthesized a set of single-chain bolalipids bearing a lateral alkyl chain of different length right next to the PC headgroups, the PC-C32(1,32Cn)-PC with n = 3, 6, 9 [52] (Figure 1).…”