Biological vesicles

SNARE proteins A family of enzymes that facilitate the fusion of biological vesicles with cell membranes. SNARE = Soluble N-ethylmaleimide-sensitive factor attachment protein receptor. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

There is quite a large body of literature on films of biological substances and related model compounds, much of it made possible by the sophisticated microscopic techniques discussed in Section IV-3E. There is considerable interest in biomembranes and how they can be modeled by lipid monolayers [35]. In this section we briefly discuss lipid monolayers, lipolytic enzyme reactions, and model systems for studies of biological recognition. The related subjects of membranes and vesicles are covered in the following section. 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Phospholipids are a major component of all biological membranes together with glycolipids and cholesterol. Due to their polar nature, i.e. hydrophilic head and hydrophobic tail, phospholipids form in water vesicles or liposomes. 

Recently, unique vesicle-forming (spherical bUayers that offer a hydrophilic reservoir, suitable for incorporation of water-soluble molecules, as well as hydrophobic wall that protects the loaded molecules from the external solution) setf-assembUng peptide-based amphiphilic block copolymers that mimic biological membranes have attracted great interest as polymersomes or functional polymersomes due to their new and promising applications in dmg delivery and artificial cells [ 122]. However, in all the cases the block copolymers formed are chemically dispersed and are often contaminated with homopolymer. 

Bioencapsulation is a technology aimed toward the immobilization and incorporation (entrapping) of a biologically active compound on or inside solid particles (microspheres) or liquid vesicles in order to stabilize, stracture, and protect the active compound and allow control of its release. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

As an example of suspected endocrine disruptors (EDs), studies of the estrogenic action of bisphenol A (BPA) have been in progress in medical, physiological, and biological fields. In this situation, physicochemical approaches are required to get the structural information of BPA trapped in biomembranes. Most recently, we have determined the site and the orientation of BPA trapped in phospholipid vesicles by NMR, using the HCS rule [47]. In particular, we have succeeded in monitoring the NMR spectral changes of phospholipid vesicles, which are induced by the BPA delivery. 

The fluidity is one of the most vital properties of biological membranes. It relates to many functions involved in biological system, and effective biomembrane mimetic chemistry depends on the combination of both stability and mobility of the model membranes. However, in the polymerized vesicles the polymer chain interferes with the motion of the side groups and usually causes a decrease or even the loss of the fluid phases inside the polymerized vesicle (72,13). 

The octanol-water partition model has several limitations notably, it is not very biological. The alternative use of liposomes (which are vesicles with walls made of a phospholipid bilayer) has become more widespread [149,162,275, 380—4441. Also, liposomes contain the main ingredients found in all biological membranes. 

The second model of a biological membrane is the liposome (lipid vesicle), formed by dispersing a lipid in an aqueous solution by sonication. In this way, small liposomes with a single BLM are formed (Fig. 6.11), with a diameter of about 50 nm. Electrochemical measurements cannot be carried out directly on liposomes because of their small dimensions. After addition of a lipid-soluble ion (such as the tetraphenylphosphonium ion) to the bathing solution, however, its distribution between this solution and the liposome is measured, yielding the membrane potential according to Eq. 

Probes based on macrocyclic amines include dansylamidoethylcyclen, which can detect zinc at sub-nanomolar concentrations and probes containing xanthene as the chromophore with an ideal wavelength range for intracellular studies.953,962 It is also important that the sensors can differentiate between the alkali metals, which are present at higher concentrations, and zinc. 2, 7-dichlorofluorescein-based sensors have been used in biological systems and reveal concentrations up to ca. 0.3 mM in hippocampal nerve synaptic vesicles.963... 

Waggoner AS (1985) Dye probes of cell, organelle, and vesicle membrane potentials. In Martonosi AN (ed) The enzymes of biological membranes, 2nd edn. Plenum, New York... 

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