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Pseudoproline Dipeptides
Pseudoproline a powerful tool for improving the quality of synthetic peptides. Pseudoproline dipeptides have greatly increased the success rate for synthesizing both long and difficult peptides. Pseudoproline dipeptides can be introduced in the same manner as other amino acid derivatives. The routine use of pseudoproline (oxazolidine) dipeptides in the FMOC solid phase pepdide sysnthesis (SPPS) of serine- and threonine-containing peptides leads to remarkable improvements in quality and yield of crude products and helps avoid unnecessary repeat synthesis of failed sequences. Pseudoproline dipeptides have proven particularly effective in the synthesis of intractable peptides, long peptides/small proteins, and cyclic peptides, enabling in many cases the production of peptides that otherwise could not be made. These dipeptides are extremely easy to use: simply substitute a serine or threonine residue together with the preceding amino acid residue in the peptide sequence with the appropriate pseudoproline dipeptide. The native sequence is regenerated on cleavage and deprotection.
Peter White, John W. Keyte, Kevin Bailey, Graham Bloomberg. Expediting the Fmoc solid phase synthesis of long peptides through the application of dimethyloxazolidine dipeptides. Journal of Peptide Science Volume 10, Issue 1, pages 18–26, January 2004.
Click Chemistry for Peptide Synthesis
Click chemistry provides a number of avenues for peptide or protein modifications and could easily be combined with other techniques to make complex structures and multi-component functionalised systems. The chemistry could be performed in different ways. For example, peptides can be converted post-synthetically to an azido derivative, which can be clicked with an appropriate substrate containing a clickable alkynyl group or vice versa.
Peptides can also be made by inter- and intramolecular click reactions using azide- or alkyne-containing amino acids or building blocks during peptide synthesis. Building blocks containing clickable moieties will be instrumental in constructing side-chain modified peptides, interside-chain peptide chimera, peptide small molecule conjugates and cyclic peptides. Solid phase resins modified with clickable groups can also be used to make clickable or modified peptides. Click chemistry is compatible with various protected amino acid side chains used in peptide synthesis.
Many reagents and building blocks can be used for click chemistry. These include: azido -amino acids (Fmoc-protected, for example), non-fluorescent azides like dabcyl-azide, azido propylamine, DNP-azide; PEG and spacer azide and alkynes, such as maleimide-PEG3 azide, acetylene PEG maleimide and azido pentanoic acid; azides and alkynes of fluorescent dyes; quencher dyes; nucleosides and nucleotides; alkyne- and azide-containing chemical modification reagents, such as propargyl amine, pentynoic acid and 2-azido-3-methyl propanoic acid; diazo transfer reagents like imidazole-1-sulfonyl azide; and, propargyl amino acids.
The simplicity and reliability of CuAAC, as well as the bioorthogonality of starting reactants, has made the reaction an asset to a hugely varied range of scientific applications in peptide science. The most important applications of click chemistry in peptide science include chemical ligation, cyclisation and bio-conjugation. Other applications include imaging, the synthesis of peptidomimetics based on a triazole backbone and conformational and back bone modifications.
Native chemical ligation
Native chemical ligation is carried out in aqueous solution and frequently gives near-quantitative yields of the desired ligation product. The challenge is the preparation of the necessary unprotected peptide-thioester building block. Peptide-thioesters are usually prepared by Boc chemistry SPPS; a thioester-containing peptide cannot be synthesized using a nucleophilic base, thus disfavoring Fmoc chemistry. Fmoc chemistry solid phase peptide synthesis techniques for generating peptide-thioesters are known; they make use of modifications of the Kenner safety catch linker. In making peptide segments for use in native chemical ligation, protecting groups that release aldehydes or ketones should be avoided since these may cap the N-terminal cysteine. For the same reason, the use of acetone should be avoided, particularly prior to lyophilization and in washing glassware.
A feature of the native chemical ligation technique is that the product polypeptide chain contains cysteine at the site of ligation. For some proteins, homocysteine can be used and methylated after ligation to form methionine, although side reactions can occur in this alkylation step. The cysteine at the ligation site can also be desulfurized to alanine; more recently, other beta-thiol containing aminoacids have been used for native chemical ligation, followed by desulfurization. Alternatively, thiol-containing ligation auxiliaries can be used that mimic an N-terminal cysteine for the ligation reaction, but which can be removed after synthesis. The use of thiol-containing auxiliaries is not as effective as ligation at a Cys residue. Native chemical ligation can also be performed with an N-terminal selenocysteine residue.
The payoff in the native chemical ligation method is that coupling long peptides by this technique is in many cases nearly quantitative and provides synthetic access to large peptides and proteins otherwise impossible to make, due to length or decoration by post-translational modification. Native chemical ligation forms the basis of modern chemical protein synthesis, and has been used to prepare numerous proteins and enzymes by total chemical synthesis.
Polypeptide C-terminal thioesters produced by recombinant DNA techniques can be reacted with an N-terminal Cys containing polypeptide by the same native ligation chemistry to provide very large semi-synthetic proteins. Native chemical ligation of this kind using a recombinant polypeptide segment is known as Expressed Protein Ligation. Similarly, a recombinant protein containing an N-terminal Cys can be reacted with a synthetic polypeptide thioester. Thus, native chemical ligation can be used to introduce chemically synthesized segments into recombinant proteins, regardless of size.

Use of the Hmb backbone-protecting group in the synthesis of difficult sequences.
Simmonds RG. Int J Pept Protein Res. 1996 Jan-Feb;47(1-2):36-41.
Aggregation due to hydrogen-bonded interchain association is thought to be the cause of difficult sequences in solid-phase peptide synthesis. Hmb (2-hydroxy-4-methoxybenzyl) was introduced recently as a backbone-protecting group for Fmoc/tBu strategies which inhibits this association.
Synthesis of an O-acyl isopeptide by using native chemical ligation to efficiently construct a hydrophobic polypeptide
Youhei Sohma, Hitomi Kitamura, Hiroyuki Kawashima, Hironobu Hojo, Masayuki Yamashita, Kenichi Akaji, Yoshiaki Kiso. Tetrahedron Letters, Volume 52, Issue 52, 28 December 2011, Pages 7146-7148

4-Methoxybenzyloxymethyl group as an Nπ-protecting group for histidine to eliminate side-chain-induced racemization in the Fmoc strategy
Hajime Hibino, Yuji Nishiuchi. Tetrahedron Letters, Volume 52, Issue 38, 21 September 2011, Pages 4947-4949

Quantitative Assessment of Preloaded 4-Alkoxybenzyl Alcohol Resins for Solid-Phase Peptide Syntheses by 1D and 2D HR-MAS NMR.
Rentsch D, Stähelin C, Obkircher M, Hany R, Simeunovic M, Samson D, Loidl G, Dick F., ACS Comb Sci. 2012, 14, 613−620
Peptide and protein PEGylation a review of problems and solutions
Francesco M. Veronese, Biomaterials 22 (2001) 405-417


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