Crystals liposomes

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

S-layer protein was crystallized on lipid monolayers lipid bilayer membranes and liposome. 

In order to enhance the stability of hposomes and to provide a biocompatible outermost surface shucture for controlled immobihzation (see Section IV), isolated monomeric and oligomeric S-layer protein from B. coagulans E38/vl [118,123,143], B. sphaericus CCM 2177, and the SbsB from B. stearothermophilus PV72/p2 [119] have been crystallized into the respective lattice type on positively charged liposomes composed of DPPC, HD A, and cholesterol. Such S-layer-coated hposomes are spherical biomimetic structures (Fig. 18) that resemble archaeal ceUs (Fig. 14) or virus envelopes. The crystallization of S-... 

Phospholipids or similar water-insoluble amphiphilic natural substances aggregate in water to form bilayer liquid crystals which rearrange when exposed to ultrasonic waves to give spherical vesicles. Natural product vesicles are also called liposomes. Liposomes, as well as synthetic bilayer vesicles, can entrap substances in the inner aqueous phase, retain them for extended periods, and release them by physical process. 

Cholesterol s presence in liposome membranes has the effect of decreasing or even abolishing (at high cholesterol concentrations) the phase transition from the gel state to the fluid or liquid crystal state that occurs with increasing temperature. It also can modulate the permeability and fluidity of the associated membrane—increasing both parameters at temperatures below the phase transition point and decreasing both above the phase transition temperature. Most liposomal recipes include cholesterol as an integral component in membrane construction. 

L. Alfonta, I. Willner, D.J. Throckmorton, and A.K. Singh, Electrochemical and quartz crystal microbalance detection of die cholera toxin employing horseradish peroxidase and GM1-functionalized liposomes. Anal. Chem. 73, 5287—5295 (2001). 

Talsma et al. [1.34] described the freezing behavior of certain liposomes by DSC measurements. Besides the expected influences of freezing and rewarming speeds, and of the CPAs (mannitol and mannitol in Tris-buffer solutions) it was shown, that the heterogeneous and homogeneous crystallization in mannitol solutions exists and the nucleation of ice depends also on the liposome size In small liposomes (e. g. 0.14 pm) mannitol suppressed the heterogeneous crystallization more effectively than in large (0.87 pm) liposomes. If in certain substances no crystallization or eutectic mixtures can be found by DSC (cephalosporin, Williams [1.35]) with the used experimental conditions, one has to seek different conditions [1.32]. 

Start of homogeneous ice crystallization indifferent CPAs (lipoid as in Fig. 3.19) at a concentration of 30-50 pmol/ml, lipoid size 0.3 pm 10 mM Tris buffer, pH 7.4 CPA and Tris buffer within or outside the liposomes [1.34, page 68]... 

Talsma shows that peak 2 changes only from -39.8 °C to —40.4 °C (liposomes, liposomes concentration, buffer and cooling speed as in (I), but no mannitol) if the liposomes size is decreased from 0.87 pm to 0.14 pm. With small liposomes, the start of the homogeneous crystallization is delayed. This can also be deduced from the weakly performed crystallization (Fig. 3.19 (d), peak 3), if mannitol is only within the liposome. 

The role of the ammonium salt anion is not the loading of the amphipathic weak base per se, but rather to control the stability of loading and the profile and rate of release of the amphipathic weak base from the liposome to the external aqueous phase. Two major factors that differentiate the different anions are, firstly, their ability to induce precipitation/crystallization/ gelation in the intraliposome aqueous phase (1,12), and secondly, their effect on the membrane/buffer and octanol/buffer partition coefficient of the amphipathic weak base (1). Regarding the precipitation, the higher the amount of precipitated amphipathic weak base, the more stable is the loading and the slower is its release rate (10-12,18,33,35) and (Wasserman et al.). There are also some risks involved in the precipitation which in some cases reduce the mechanical stability of the liposomes and change liposome shape (36). 

Another concern with freeze-drying LEH is the instability of liposome structure upon lyophilization. Vesicle formation occurs in the presence of bulk water and when water is removed, loss of structural integrity is inevitable. Fusion, crystal formation, and phase transition are observed, resulting... 

Similar to Voltaren" Emulgel, oily droplets of an eutectic mixture of lidocaine and prilocaine are dispersed in a hydrogel to provide local anesthesia to the skin for injections and siugical treatment (Emla cream). A further possibility is the dermal administration of a liposome dispersion as a spray (Heparin PUR ratiopharm Spriih-gel "). After administration, water and isopropylic alcohol evaporate partially resulting in an increase of concentration and in a transition from the initial liposome dispersion into a lamellar liquid crystal [32]. The therapeutic effect appears to be influenced favorably by the presence of lecithins rather than by the degree of liposome dispersion. 

Tiddy, G.J.T., Surfactant-water liquid crystal phases, Phys. Rep., 57 1-46 (1980). Gregoriadis, G., ed.. Liposome Technology, Vols. I-III, CRC Press Inc., Boca Raton, FL, 1993. 

Unilamellar liposomes are nanoparticles made of a bilayer, most often phospholipidic, entrapping an internal aqueous core. Formed spontaneously in the presence of an excess of water and above the gel-to-liquid crystal phase tran-... 

The appearance of tubular myelin-like structures in swollen lecithin was observed by light microscopy well before the systematic investigation of liposomes [351-352]. Similarly, it was also demonstrated some time ago that the addition of calcium ions converted phospholipid liposomes to cochleate cylinders [353]. Subsequent studies have, however, revealed that the system is extremely complex. For example, examination of the phase-transition behavior of synthetic sodium di-n-dodecyl phosphate [(C12H2sO)2PO2Na+ or NaDDP] and calcium di-n-dodecyl phosphate [Ca(DDP)2] showed the presence of many diverse structures [354]. In particular, hydrated NaDDP crystals were shown to form lyotropic liquid-crystalline phases which transformed, upon heating to 50 °C, to myelin-like tubes. Structures of the tubes formed were found... 

At the most fundamental level, monolayers of surfactants at an air-liquid interface serve as model systems to examine condensed matter phenomena. As we see briefly in Section 7.4, a rich variety of phases and structures occurs in such films, and phenomena such as nucleation, dendritic growth, and crystallization can be studied by a number of methods. Moreover, monolayers and bilayers of lipids can be used to model biological membranes and to produce vesicles and liposomes for potential applications in artificial blood substitutes and drug delivery systems (see, for example, Vignette 1.3 on liposomes in Chapter 1). 

O-antigen of 180 structures of 430 Lipoprotein(s) 58 bacterial 428 Liposomes 392, 392-394 NMR spectra 396 Liquid crystals 392-394 Liquid crystalline phases 392 Lithium salts, in treatment of manic-depressive illness 564 Lithostatine 443 Liverworts 29... 

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