Peptides unprotected

The intermediate (a) can also be arrived at directly from the unprotected linear peptide by applying the mixed anhydride method. After addition of one equivalent of acid, e.g. trifluoroacetic acid, the amino group will be protonated and the (still deprotonated) carboxyl anion will react with alkyl chloroformate to form (a), X = CO—O—Aik. Very conveniently, carbodiimides can be reacted with linear peptides unprotected at both ends to form cyclic peptides in satisfactory yields [26]. Since the carbodiimide method, particularly in the presence of V-hydroxybenzotriazole, is causing little racemization (see p. 93) this system is preferred in most laboratories. 

Amino Acids. Chloroformates play a most important role for the protection of the amino group of amino acids (qv) during peptide synthesis (32). The protective carbamate formed by the reaction of benzyl chloroformate and amino acid (33) can be cleaved by hydrogenolysis to free the amine after the carboxyl group has reacted further. The selectivity of the amino groups toward chloroformates results in amino-protected amino acids with the other reactive groups unprotected (34,35). Methods for the preparation of protected amino acids on an industrial scale have been developed (36,37). A wide variety of chloroformates have been used that give various carbamates that are stable or cleaved under different conditions. 

Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

A similar sequence of 12- and 10-membered turns is present in the structure of Boc-protected /S //S -peptides 96 and 97, the C=0 of the Boc group being engaged in the first 12-membered ring with NH of residue 3. The pattern of 10- and 12-membered turns is reversed for the fully protected / // -peptide 94 as well as the unprotected /S //S -dodecapeptide 98 which thus folds into a 10/12-helix, with the NH of residues 1 and 2, respectively being involved in the formation of an N-ter-minal 10-membered turn. 

Native chemical ligation has been used successfully to couple two unprotected peptides together during solid phase synthesis, wherein one of the peptides is attached to the resin using a thioester linkage and the other peptide is introduced containing a cysteine at its N-terminal... 

Camarero, J.A., Cotton, G.J., Adeva, A., and Muir, T.W. (1998) Chemical ligation of unprotected peptides directly from a solid support./. Pept. Res. 51(4), 303-316. 

Muir, T.W., Dawson, P.E., and Kent, S.B.H. (1997) Protein synthesis by chemical ligation of unprotected peptides in aqueous-solution. Meth. Enzymol. 289, 266-298. 

Tam, R, Lu, Y.-A., Liu, C.-F., and Shao, J. (1995) Peptide synthesis using unprotected peptides through orthogonal coupling methods. Proc. Natl. Acad. Sci. USA 92, 12485-12489. 

AMINOLYSIS OF SUCCINIMIDO ESTERS BY UNPROTECTED AMINO ACIDS OR PEPTIDES... 

M Schnolzer, SBH Kent. Constructing proteins by dovetailing unprotected synthetic peptides backbone-engineered HIV protease. Science 256, 221, 1992. 

A second approach that can be adopted to overcome the intrinsic requirement for cysteine at the N-terminus of C-terminal fragment utilizes the enzyme subtiligase, a double mutant of subtilisin, which is able to join two unprotected peptides. Thioester-modified proteins were shown to present good substrates of subtiligase [65]. However, although this approach could be potentially useful for general isotope labeling, the efficiency of this process remains to be proven. 

Amide and carbamate bonds. Unprotected peptides can be attached to the surface of liposomes by engaging, for example, their N-terminus into an amide... 

The acylated peptides (Myr)GCX-Bimane 31 a-e (X = G, L, R, T, V), which are found in certain nonreceptor tyrosine kinases and ct-subunits of several heterotrimeric G-proteins, were synthesized in solution using common solution-phase peptide synthesis with X-myristoylglycine as a building block. These model peptides were used for acylation studies with palmitoyl-CoA in phospholipid vesicles at physiological pH. For such uncatalyzed spontaneous reactions only a modest molar excess of acyl donor species (2.5 1) was necessary. Unprotected side chains of threonine or serine are not interfering with this S-acylation (Scheme 14). 

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