Block synthesis peptide assembly

Similarly, iterative NRPSs operate in a linear fashion but utilize at least one domain or module multiple times for the synthesis of a single NRP product. Thus, peptides assembled by iterative synthetases contain short, repeating units of peptide building blocks. In such systems, the terminal PCP-TE (or infrequendy PCP-C) didomain is responsible for both condensation of the repeating peptide units and chain release from the assembly line. NRPs biosynthesized in this manner include enniatin, enterobactin, bacillibactin, " gramicidin and the depsi-peptides valinomycin and cereulide. Of these examples, condensation of the precursor peptides for both enterobactin and gramicidin S has been extensively studied and will be discussed in detail. 

The assembly of the p- and y-amino-acid building blocks to peptidic chains was achieved by simply using the established methods of peptide synthesis - in solution [6], on solid phase [11], or in a synthesizer machine [39] also, the so-called native ligation can be applied with p-peptides [54]. Furthermore, the methods of analyzing and studying the structures of a-peptides and natural proteins can mostiy be applied to P-peptides as well (the same is true for y-peptides [51,55-60]). These methods are CD [35,37] and NMR [6, 49] spectroscopy, mass spectrometry [27,35], X-ray analysis [6,21,24,25,36], molecular dynamics (MD) calculations [9,13,18,31,38] and biological investigations [6, 15,20,26,30,41-43,45,46,48]. All of this sounds like routine, but the results are rather spectacular. 

Yusufoglu, Y. Hu, Y. Kanapathipillai, M. Kramer, M. Kalay, Y. E. Thiyagarajan, P. Akinc, M. Schmidt-Rohr, K. Mallapragada, S., Bioinspired synthesis of self-assembled calcium phosphate nanocomposites using block copolymer-peptide conjugates. Journal of Materials Research 2008, 23, (12), 3196-3212. 

Compared with O-glycopeptides, aecess to A-glycopeptides on the solid phase is more readily possible via two different pathways [42]—conventional sequential incorporation of preformed and suitably protected building blocks into the solid-phase synthesis or assembly of the peptide chain, selective removal of the aspartic acid side-chain protecting groups, and formation of the amide linkage between glycosyl amines and the peptide backbone [81] on the solid phase. 

The synthesis of aldehydes and ketoamides was performed on solid phase as well as in solution (Scheme 2.2). A semicarbazone linker (6) was employed for the assembly of the aldehydes on solid phase whereas the corresponding aminoalcohol was coupled in solution to the tripeptide and oxidized to the aldehyde, which produced epimeric mixtures [137]. For the synthesis of the ketoamides, hydroxyester THP resins were used as solid support ((7), Scheme 2.2) [138]. In solution the peptide bond was formed using an aminohydroxycarboxylic acid building block [138, 147]. Oxidation of the free hydroxyl group yielded the final inhibitors ((8), Scheme 2.2). 

A second strategy is to attach a linker (also referred to as a handle or anchor) to the resin followed by assembly of the molecule. A linker is bifunctional spacer that serves to link the initial synthetic unit to the support in two discrete steps (Fig. 3). To attach a linker to a chloromethyl-PS resin, a phenol functionality such as handle 4 is used to form an ether bond (Fig. 4). To attach the same handle to an amino-functionalized support, acetoxy function 5 or a longer methylene spacer of the corresponding phenol is applied to form an amide bond. Both of these resins perform similarly and only differ in their initial starting resin [4], An alternative approach is to prepare a preformed handle in which the first building block is prederivatized to the linker and this moiety is attached to the resin. For peptide synthesis, this practice is common for the preparation of C-terminal peptide acids in order to reduce the amount of racemization of the a-carbon at the anchoring position [5],... 

Building blocks are amphiphiles, which have a delicate balance between the hydrophilic and hydrophobic group crucial to facilitate self-assembly. The peptide component serves to precisely control this balance, and the enzymatic reaction serves to alter it in favour of self-assembly. As illustrated in Fig. 3, the molecular switch may involve (1) phosphatase-catalysed removal of a (phosphate) group from the precursor to control the electrostatic balance (reaction (i) in Fig. 3) (2) hydrolysis of alkyl esters by hydrolases to change the amphiphilic balance (reaction (ii) in Fig. 3) or (3) condensation between two non-self-assembling precursors via a condensation reaction, e.g. involving protease-catalysed amide synthesis to alter the hydrophilic/hydrophobic balance (reaction (iii) in Fig. 3). A number of examples of each type are summarised in Table 1. 

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