Peptides pipecolic acid

The chemistry of pipecolic acid (piperidine-2-carboxylic acid, homoproline, 6) and thus the protecting groups and coupling methods closely resemble those of proline, although steric hindrance due to the larger ring size makes its use in standard peptide chemistry more difficult. 

The Z-protected derivative, again prepared by standard methods using benzyl chloroformate,t208 may serve in the case of racemic pipecolic acid for resolution into the pure enantiomers by fractional crystallization with L-tyrosine hydrazide/208 Acylation with N-protected pipecolic acid or of pipecolyl peptides is performed by standard procedures via the active ester methods, e.g. A-hydroxysuccinimide ester/121 by the mixed anhydride method, e.g. with isobutyl chloro-formate 95-114 or pivalic acid chloride/121 as well as by DCC/HOBt/118 In the synthesis on solid support, longer coupling times are required when compared to N-protected proline.1[235 ... 

In addition to the 20 amino acids most frequently found in proteins a large group of amino acids occur in plants, bacteria, and animals that are not found in proteins. Some are found in peptide linkages in compounds that are important as cell wall or capsular structures in bacteria or as antibiotic substances produced by bacteria and fungi. Others are found as free amino acids in seeds and other plant structures. Some amino acids are never found in proteins. These nonprotein amino acids, numbering in the hundreds, include precursors of normal amino acids, such as homoserine and diaminopimelate intermediates in catabolic pathways, such as pipecolic acid d enantiomers of normal amino acids and amino acid analogs, such as azetidine-2-carboxylic acid and canavanine, that might be formed by unique pathways or by modification of normal amino acid biosynthetic pathways. 

Synthesis and Application of Proline and Pipecolic Acid Derivatives Tools for Stabilization of Peptide Secondary Structures... 

The preferred geometry of the N-terminal peptide bond in pipecolic acids can therefore be triggered by choice of substituents attached to the heterocycle and their relative configuration. 

Fmoc amino acid chlorides are used in the presence of DIPEA for the acylation of hindered secondary amines such as the tetrahydro-(3-carbolines,l l where other methods including PyBroP, PyBroP/HOAt, HATU, and amino acid fluorides only led to poor results. In the case of difficult acylations, e.g. of pipecolic acid and other secondary anoino acids under solid-phase conditions, Fmoc amino acid chlorides can be successfully applied, by shaking a solution of the acid chloride in dichloromethane with the peptide resin for 5 min, while adding a tertiary amine, e.g. DIPEA, and adjusting the supernatant to pH An alter-... 

An enzyme has been isolated from the FK520 producer which is believed to be the key one responsible for inserting pipecolic acid into the macrocycle [114]. It is reported to be dimeric and activates pipecohc acid and several structural analogues in an ATP-dependent reaction to give an enzyme-bound amino-acyl adenylate. There is evidence that this then reacts to form a thioester linkage to the enzyme. This mechanism of activation is the same as that found in the non-ribosomal biosynthesis of peptide natural products such as gramicidin [112]. 

As mentioned above, peptide 153 exists in two or three stable conformational states in solution. However, the proline analog 224 showed only one conformation in various solvents, and extensive NMR experiments revealed that this conformation is very similar to the major conformer of peptide 153. Since analog 224 showed potent cytotoxicity, it is concluded that the major conformation of peptide 153 is at least in part responsible for the activity. The pipecolic acid analog 225 showed similar conformational and biological tendencies. 

Neoefrapeptins, neoefrapeptin A Ac-Pip-Aib- Pip- Iva-Aib- Leu-/3 -Ala-Gly-Acc-Aib-Pip-Gly-Leu-Iva-aX, a group of peptides with insecticidal activity isolated from the fungus Geotrichum candidum. AU 12 neoefrapeptins (A-I, L-N) contain the very rare amino acid 1-amino-cyclopropane-carboxylic acid (Acc), and some of them (F, I, L, M) also contain (2S,3S)-3-methyl-proline instead of pipecolic acid (Pip) in position 11. Further unusual building blocks are isovaline (Iva) and the C-terminal amide moiety X = 2,3,4,6,7,8-hexahydro-l-pyrrolo [ 1,2-a]pyrimidine. Neoefrapeptins show a close sequence similarity to the efrapeptins [A. Fredenhagen et al., J. Antibiot. 2006, 59, 2006]. 

C5 H,9N0,3, Mr914.19, cryst., mp. 183-185°C, [aJu -58.2° (CH3OH), a 31-membered peptide lactone in which a long-chain carboxylic acid is cyclized with l- pipecolic acid as a bridge and produced by Strepto-myces hygroscopicus. R. has antifungal, antineoplastic, and immunosuppressive activities, it is structurally related to FK-506 and exhibits a similar mechanism of action in the development of the immune response. R. was first marketed in 1999 in combination with cyclosporin and tacrolimus for use in transplantation medicine. The first total synthesis of R. was realized in 1993. 

The ninhydrin reaction is not restricted to a-amino acids, however. It is well known that imino acids, notably proline, but also for example pipecolic acid, in solution produce a different color with an absorption maximum at 440 nm. Primary aliphatic amines react likewise with ninhydrin to give Ruhe-man s purple but the color yield is lower than for a-amino acids. Peptides, amino acids with secondary amino groups, mainly N-methylamino acids, and secondary amines also react with ninhydrin, although often more slowly and with smaller yield than the a-amino acids. Tertiary amines and all aromatic amines do not react with ninhydrin. Ammonia itself reacts to give a color with ninhydrin, although with a rather low yield. 

Certain changes in the proportion of different actinomycins synthesized by a given Streptomyces sp. may result from changes in the amino acid composition of the culture medium. Studies by Katz and Goss have indicated that addition to the medium of proline, hydrox5 roline, or sarcosine respectively may result in an increased synthesis of actinomycins with these particular residues at sites A and A, and that the addition of pipecolic acid or azetidine-2-carboxylic acid results in the production of new actinomycins in which residues of the added compounds occupy these sites. It seems that the peptide synthesizing mechanism does not distinguish perfectly, at sites A and A, between amino acids of similar structure. 

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