Peptides at aqueous interfaces

From a theoretical point of view, studies on interactions of peptides with membranes have an additional, attractive feature. While most peptides are disordered in water, some of them can adopt an ordered structure at the water- 

Most of the difficulties outlined above can be avoided by considering a simplified system in which the membrane is replaced by a lamella of an alkane, e.g., hexane or octane, of the same width as the bilayer [80-84]. These membrane-mimetic systems capture the most important characteristic of the water-membrane system — the coexistence of a polar, aqueous phase and a nonpolar medium in close association. The utility of membrane-mimetics is underscored by experimental studies, which have shown that peptides built of L-leucine and L-lysine fold into the same secondary structures at a water-membrane and as at a water-hydrocarbon interface [85-87]. However, such model systems also have important limitations, chief among which are the absence of specific, electrostatic interactions between the protein and the lipid head groups and the effects of membrane ordering on protein behavior. 

The conformational equilibria of three terminally-blocked amino acids the N-acetyl-N -methyl-L-alanylamide (NANMA) and its L-leucyl (NANML) and L-glutamyl (NANMQ) analogs have been studied at a water-hexane interface [80], and NANMA was also investigated at the water-membrane interface [88]. For comparison, similar calculations were carried out in water and in the gas phase. The free energy profiles along z, obtained for all three peptides from equation (4), exhibit a minimum at the interface, indicating that all three peptides are interfacially active (Le tend to accumulate at the interface). A similar property has been noted for a wide range of small, polar solutes (see Section 3.1). The two-dimensional probability distribution functions of the peptide backbone tor- 

The unique role played by the segregation of polar and nonpolar side chains into different environments at the interface was further investigated in studies of an undecamer built from L-leucine and L-glutamine, LQQLLQQLLQL [90]. This peptide is sufficiently long to adopt an ordered structure at the water-hexane interface. It becomes amphipathic as an a-helix but not as a /J-strand. Thus, once placed in an interfacial region in the o-helical conformation, the peptide 

The results of the simulations of poly-L-leucine suggest a mechanism for the insertion of peptides into a membrane. Initially parallel to the interface, the pep- 

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Chipot C. and Pohoriiie A. Structure and dynamics of small peptides at aqueous interfaces - a multi-nanosecond molecular dynamics study. Theochem.-J. Mol. Str. 

This is another example of the hydrophobic effect which is manifested for peptides at aqueous interfaces as a tendency to segregate polar and nonpolar side chains into the aqueous and nonpolar phases, respectively. 

Other IRRAS applications to peptides and proteins. In addition to the pulmonary surfactant system, a variety of other applications employing IRRAS to study peptide and protein conformation and orientation have appeared. The secondary structure conversion of the amyloid (prion)-protein in the normal form into the abnormal form is the main cause of several human and animal diseases, such as Alzheimer s disease [68]. The secondary structure of the first 40 residues of the amyloid protein was detected by circular dichroism (CD) in aqueous solution and with IRRAS at the interface. A stable /1-sheet-enriched state of the amyloid is formed at the air-water interface, in contrast to the initial bulk solution containing high a-helix/random coil and low /l-sheet parts. The change in the pH going from bulk (alkaline pH) to the interface (neutral or slightly acidic pH) can have effects on the conformation at the interface. Another alternative might be the intrinsic hydrophobicity of the air-water interface, which is a hydrophobic-hydrophilic system with air as the hydrophobic part. 

Liposomes are microstructures composed of one or more concentric spheres of (phospho)lipid bilayer, separated by water or aqueous buffer compartments. Those particles can encapsulate and dehver both hydrophilic and lipophilic substances. Water soluble substances can be entrapped in the central aqueous core, lipid soluble substances in the membrane and peptide and small proteins at the hquid aqueous interface. The size of such a particle can differ from 20 nm to 10 pm. Liposomes are in general made synthetically e.g. by the lipid hydration method. Liposomal medicines are on the market for the treatment of systemic fungal infections, tumours and for vaccination. 

Desfosses et al. [328] measured binding of oxyphenyl betazone to the N-ter-minal peptic fragment of human serum albumin (HSA) in aqueous buffer and AOT/isooctane-RMs. The peptide affinity for the drug did not decrease in RMs. The interactions of HSA at membrane mimetic interface and its subsequent unfolding was suggested to constitute a drug release facilitating mechanism. 

Specific formulation strategies need to be employed for macromolecule compounds. An excellent review of protein stability in aqueous solutions has been published by Chi et al. (92). In addition to solution stability of proteins and peptides, aerosolization may result in significant surface interfacial destabilization of these compounds if no additional stabilization excipients are added. This is due to the fact that protein molecules are also surface active and adsorb at interfaces. The surface tension forces at interfaces perturb protein structure and often result in aggregation (92). Surfactants inhibit interface-induced aggregation by limiting the extent of protein adsorption (92). 

The high molar mass species reside mostly in the aqueous phase with a number of peptide groups residing in the oil/water interface [293]. Although these latter surfactants are less effective at reducing interfacial tension, they can form a viscoelastic membrane-like film around oil droplets or air bubbles. These tend to be used in the preparation of, for example, O/W emulsions. These trends are by no means exclusive, mixtures are the norm and competitive adsorption is prevalent. Caseinate, one of the most commonly used surfactants in the food industry, is itself a mixture of interacting proteins of varying surface activity [814],... 

Tonegawa et al. (2004) created a cationic polylysine with a tetrapeptide end sequence (glycine-tyrosine-glycine-lysine), which is a motif common to the consensus sequences of mussel adhesive proteins. They then cross-linked this with the anionic polysaccharide, gellan, enzymatically. The polyionic complexation between the cationic peptide and the anionic polysaccharide formed a hybrid fiber at the aqueous solution interface that, when cross-linked, mimicked the byssus gel that marine mussels use to adhere to surfaces, despite the presence of water and salt. 

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