Self-assembled molecules amino acids

This chapter will introduce polymer systems containing either naturally occurring maaomolecules (polysaccharides, proteins, DNA) or their subunits (bioanalogous molecules, amino acids, short peptides and peptide derivatives, polypeptides, polynucleotides), respectively. The natural building blocks can be connected by covalent bonds or by self-assembly and either can be used alone (see, e.g.. Section 5.4) or in combination with synthetic polymer units (biohybrids). Alternatively the building block itself may be a hybrid of a natural and synthetic molecule (bioconjugate cf. Section 3.5), as, for instance, a PEG-peptide conjugate. 

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). 

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

More recent examples of biomimetic synthesis are the syntheses of thebaine [11] and usnic acid [12], as well as strychnine [13], morphine alkaloids [11] [14] and a great number of terpenic compounds [15]. On the other hand, hypothetic prebiotic considerations may also simplify tremendously the synthetic plans. Such is the case, for example, of the work of Eschenmoser on vitamin B12 who, after synthesising it in collaboration with Woodward by a linear sequence of almost fifty steps [16], investigated the prebiotic origen of this complex molecule. The experimental work undertaken in this direction demonstrates that the amount of "external instruction" required for "self-assembling" the different structural elements present in this molecule is surprisingly small. This fact could eventually lead to a very simple synthesis of vitamin Bj2 starting from a-amino nitriles which would involve only a few steps [17]. 

Self-assembly of chiral molecules may result in organized aggregates displaying a remarkable enhancement of ophcal achvity. The best known examples are amino-acid residues that assume a periodic conformation - an a-helix or a p-sheet chain. In this case, the enhancement of optical activity is due to the onset of a parhcular rigid conformation. 

Figure 11. (a) The chemical structure of a 24-membered macrocyclic molecule composed of alternating D- and L-amino acids, cyclo(Gln-(D-Leu-Trp)4-D-Leu 7. (b)A self-assembled tubular structure spanned across the bilayer lipid membrane. Flat ring-shaped units in the antiparallel configuration stack to form a tubular structure through extensive inter subunit hydrogen bonding and peptide side chain-lipid interactions. ... 

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