Amino acids self-assembling

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

Xiong, H., Bnckwalter, B., Shieh, H.-M., and Hecht, M. H., 1995. Periodicity of polar and nonpolar amino acids is the major determinant of secondary structure in self-assembling oligomeric peptides. Proceedings of the National Academy of Sciences 92 6349—6353. 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

To be successful in these applications, it is important that materials can self-assemble into precisely defined structures. Peptide-based polymers have many advantages over conventional synthetic polymers since they are able to hierarchically assemble into stable, ordered conformations [4]. Depending on the substituents of the amino acid side chain, polypeptides are able to adopt a multitude of... 

The characteristic coiled-coil motifs found in proteins share an (abcdefg) heptad repeat of polar and nonpolar amino acid residues (Fig. 1). In this motif, positions a, d, e, and g are responsible for directing the dimer interface, whereas positions b, c, and f are exposed on the surfaces of coiled-coil assemblies. Positions a and d are usually occupied by hydrophobic residues responsible for interhelical hydrophobic interactions. Tailoring positions a, d, e, and g facilitates responsiveness to environmental conditions. Two or more a-helix peptides can self-assemble with one another and exclude hydrophobic regions from the aqueous environment [74]. Seven-helix coiled-coil geometries have also been demonstrated [75]. 

The observation that a repeating, relatively short sequence of amino acids directs the spontaneous self-assembly of a large protein is shared by other structural proteins from mammalian systems [6] as well as of plant origin [16], Hence, such sequences may inspire the construction of nanostructures made of polypeptides and small proteins, as discussed later. 

Salt-dependent self-assembly of relatively short ionic polypeptides (8-32 amino acid sequences) into three-dimensional structures has recently been described [32],... 

Cyclic artificial polypeptides comprised of alternating d and l amino acids were recently synthesized and self-assembled into nanotubes exhibiting lengths of 1000-1500 A, formed at an air-water interface [34,35],... 

The general structural characteristics of ribosomes and their self-assembly process are discussed in Chapter 37. These particulate entities serve as the machinery on which the mRNA nucleotide sequence is translated into the sequence of amino acids of the specified protein. 

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