Ind. Eng. Chem. Res. 2008, 47, 6391–6398
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Peptides As Functional Surfactants Annette F. Dexter and Anton P. J. Middelberg* Centre for Biomolecular Engineering, School of Engineering and The Australian Institute for Bioengineering and Nanotechnology, The UniVersity of Queensland, St Lucia QLD 4072 Australia
Peptides offer interesting alternatives to conventional surfactants in applications where renewability, biocompatibility, or added functionality may be desired. This review offers a brief overview of different classes of surface-active peptides and lipopeptides, covering molecules obtained from natural sources as well as those obtained by design. Bacterial lipopeptides are cyclic molecules containing a single fatty acyl moiety, which can exhibit ultralow interfacial tension as well as antimicrobial activities. Bacterial lipopeptides have been proposed for industrial applications such as bioremediation and oil recovery, but they suffer the dual disadvantages of being difficult to bioproduce at low cost and not being easily genetically engineered. A class of synthetic molecules related to bacterial lipopeptides are the peptide amphiphiles, in which a peptide headgroup is combined with a peptide or nonpeptide hydrophobic tail. Self-assembly of peptide amphiphiles has largely been studied in bulk solution rather than at interfaces, meaning that very little information is available on the interfacial properties of these designer molecules. A larger body of information is available for protein hydrolysates, products of the partial breakdown of low-cost proteins (usually food proteins) into a complex mixture of small peptides. Partial hydrolysis can improve the functional properties of many proteins, but the outcomes are difficult to predict or control, and useful functional properties may be associated with only a few minor components in the digest mix. Finally, designed peptide surfactants (Pepfactants), recently reported by the authors, are facially amphipathic molecules that self-assemble at fluid interfaces to give cohesive films stabilizing foams and emulsions. A change in the bulk solution conditions can switch off the interfacial film, leading to rapid foam or emulsion collapse. Pepfactants can be genetically engineered and bioproduced using standard methods, which represents an advantage over bacterial lipopeptides. While peptides have not been widely used in surfactant applications so far, recent developments may facilitate the incorporation of these interesting molecules into industrial and consumer products in the near future. Introduction Biosurfactants are bioderived or biomimetic surfactants that have potential as alternatives to synthetic surfactants obtained from petrochemical sources.1–4 Several classes of naturally occurring biosurfactants have been identified: glycolipids such as rhamnolipids, trehalose lipids, and sophorolipids; lipopeptides including surfactin and iturin; and a range of higher molecular weight polymers. Some of these natural surfactants display ultralow interfacial tension.1,3–5 Others possess useful biological activities and may find medicinal use as antivirals, antimicrobials, antifungals, antiadhesives, vaccine adjuvants, antiproliferative agents, or anticlotting agents, among other possibilities.1,2,6–8 In this paper, we review the class of biosurfactants comprising peptides and peptide-lipid conjugates. We will cover both naturally derived peptide surfactants and those obtained via de novo design, while restricting discussion to peptides of less than approximately 30 amino acids in length. The surfactant properties of larger polypeptides (i.e., proteins and lipoproteins) are beyond the scope of this paper, because of the complex and unpredictable changes in folded structure that occur in these molecules at interfaces. To understand the functional properties and design opportunities associated with surfactant peptides, it is important to understand their structural composition. Figure 1 illustrates structural features of peptide surfactants as compared to conventional surfactants. Conventional surfactants, as wellknown, contain one or more nonpolar tails linked to a polar headgroup that may be charged (cationic, anionic, or zwitteri* To whom correspondence should be addressed. Phone: +61-73346 4189. Fax: +61-7-3346 4197. E-mail:
[email protected].
Figure 1. Structural features of conventional surfactants as compared to peptide surfactants. (Top) Conventional surfactant with hydrophobic and hydrophilic moieties; (Bottom) peptide surfactant based on a polyamide backbone with hydrogen bonding capacity, chirally decorated with a defined sequence of hydrophobic and hydrophilic side-chains. The polyamide backbone may have a ring configuration, as occurs, for example, in surfactin.
onic) or uncharged (e.g., polyether or sugar). Inclusion of linker groups yields more complex surfactants, such as bolaamphiphiles9 or gemini surfactants.10 In contrast, peptides are best understood as short informational polymers. They comprise a polyamide backbone, stereospecifically decorated with sidechains of varying hydrophobicity or hydrophilicity, some of which may be weak acids or bases. In biological peptides and their synthetic equivalents, the backbone has a fixed length and
10.1021/ie800127f CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
6392 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008
Figure 2. Conformations of different classes of amphiphilic peptides. From top, surfactant-like peptide V6D2 in a hypothetical extended conformation;53 charge-complementary peptide EAK16-II in a β-sheet conformation;115 Pepfactant AM1 in an R-helical conformation;100 and cyclic lipopeptide surfactin.20,21
carries a fixed sequence of side-chains. Covalent modification of the peptide with fatty acyl chains gives a lipopeptide. Selfclosure of the backbone with an amide linkage, as occurs in some bacterial lipopeptides, yields a cyclic peptide, while closure with an ester linkage gives a cyclic depsipeptide. By appropriate design, peptide bolaamphiphiles and peptide gemini surfactants may also be obtained.11–14 Depending on the sequence and local environment, many peptides are capable of folding into ordered structures constrained by regular hydrogen bonding of the polyamide backbone. The two most common secondary structures of peptides are the R-helix, in which backbone amide groups hydrogen bond to each other in a coiled arrangement, and the β-sheet, an almost fully extended structure in which adjacent backbone strands hydrogen bond either intra- or intermolecularly. Examples of these conformations are given in Figure 2. Both R-helical and β-sheet secondary structures have been successfully incorporated into designed peptide surfactants. Lipopeptides Bacterial lipopeptides are naturally occurring cyclic peptides containing a single fatty acyl chain. They are secreted into growth media by a number of different microorganisms, including various species of Gram-positive Bacillus, Lactobacillus, and Streptomyces, as well as Gram-negative Pseudomonas and Serratia.2,15 In the natural context, lipopeptides are thought to play a role in bacterial swarming motility on semisolid surfaces, as well as on the formation of structured biofilms on solid surfaces.3,16–18 The first natural lipopeptide to be discovered was surfactin, reported in 1968 as a product of Bacillus subtilis and found to inhibit blood clotting.19 Surfactin has striking surface activity: at 0.005% (w/v), it lowers the air–water interfacial tension from 72.8 to 27.9 mN/m. Hydrocarbon–water interfacial tensions can be
