Peptides piperidine-2-carboxylic acid

Another competing cyclisation during peptide synthesis is the formation of aspartimides from aspartic acid residues [15]. This problem is common with the aspartic acid-glycine sequence in the peptide backbone and can take place under both acidic and basic conditions (Fig. 9). In the acid-catalysed aspartimide formation, subsequent hydrolysis of the imide-containing peptide leads to a mixture of the desired peptide and a (3-peptide. The side-chain carboxyl group of this (3-peptide will become a part of the new peptide backbone. In the base-catalysed aspartimide formation, the presence of piperidine used during Fmoc group deprotection results in the formation of peptide piperidines. 

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. 

Victor KG, Cafiso DS. Location and dynamics of basic peptides at the membrane interface electron paramagnetic resonance spectroscopy of tetramethyl-piperidine-N-oxyl-4-amino-4-carboxylic acid-labeled peptides. Biophys. J. 2001 81 2241-2250. 

Besides coupling of adjacent Aib residuest , the incorporation of other hindered amino acids into peptides such as 1-aminocyclohexane-l-carboxylic acid (Acscjb l and 4-amino-piperidine-4-carboxylic acid,b l as well as the coupling of 0,0-(dimethylphosphoro)tyrosine to these amino acids b was achieved by the use of the corresponding acid fluorides with solid-phase methods. 

The trityl linkers were introduced to permit anchoring of carboxylic acids and other nucleophiles to a solid support and to effect cleavage reactions under very mild acidic conditions [64-67]. Various trityl resins, such as Ib-le (Table 1), have been developed that differ in the substitution pattern of the aromatic ring substituents in order to modify the cleavage properties by their influence on the stability of the trityl cation. For carboxylic acids, amines, and phenols, the chlorotrityl resin Ic affords a more stable anchor [65-67] than does resin lb. Similarly, resin le, which contains both fluoro and carbonyl ring substituents, proved to be very stable toward nucleophiles and was fully compatible with piperidine / / -Fmoc (9-fluorenylmethoxycar-bonyl) deprotections used in a model peptide synthesis. Cleavage of acids from le could be effected using dilute TFA in dichloromethane [68]. 

The 9-fluorenylmethyl group, in the guise of the 9-fluorenylmethoxycarbonyl or Fmoc group, is the keystone in modern solid-phase peptide synthesis and we will examine its enormous influence in section 8.3.5. The great virtues of the 9-fluorenylmethyl group have also been adapted to the protection of carboxylic acids as 9-fluorenylmethyl esters (see section 6.5.3). Bodanszky and Bednarek realised the opportunity for extending the principle of 9-fluorenylmethyl activation, the aromaticity of the fluorenyl anion, to the protection of cysteine in peptide synthesis. 9-Fluorenylmethyl thioethers are stable towards iodine and acidic conditions, including HF, but they easily eliminate on treatment with piperidine or DBU. 

An alternative strategy for the introduction of spin labels can be used in protein and peptide chemistry by spin label building blocks, e.g., 4-amino-l-oxyl-2,2,6,6-tetramethyl-piperidine-4-carboxylic acid (TOAC, Fig. 1, right) which are directly incorporated into the peptide during chemical synthesis [20-25]. 

N chemical shielding in peptides and proteins is known to be sensitive to secondary structure as well as noncovalent interactions. Cai et al have recently employed DFT calculations with a polarizable continuum solvent model and explicit water molecules in the first solvation shell for A-formyl-alanyl-X amides, where X is one of the 19 naturally occurring amino acids excluding proline. This recent work suggests that the explicit water molecules incorporated in the calculations affect the isotropic amide N chemical shift, but not its anisotropy. A solvation model likewise appears to improve the correlation between calculated and observed C chemical shifts in the complex formed by piperidine-4-carboxylic acid and chlor-oacetic acid. Ksiazek et have evaluated the performance of the... 

Finally, the addition of an analogue chemically related to an amino acid found in actinomycin may result in competition between the endogenously synthesized substance and the exogenous compound incorporation of such related substances into the antibiotic peptides will result in the formation of new actinomycins. Piperidine-2-carboxylic acid, azetidine-2-carboxylic acid and 4-methylpro-line appear to be incorporated directly into actinomycin molecules in this manner. 

The number of new compounds synthesized in any given situation will vary with the amino acid, the concentration used, the degree to which the exogenous compound is metabolized, the enzymes involved, etc. On theoretical grounds, one would expect several new compounds to be produced in each instance where incorporation takes place. For example, with piperidine-2-carboxylic acid it should be possible to find actinomycin peptides containing 1. piperidine-2-carb-oxylic acid/piperidine-2-carboxylic acid, 2. proline/piperidine-2-carboxylic acid, 3. hydroxyproline/piperidine-2-carboxylic acid 4. oxoproline/piperidine-2-carb-oxylic acid and 5. sarcosine/piperidine-2-carboxylic acid. For sarcosine, one should find sarcosine/sarcosine and sarcosine/proline peptides and, in addition, sarcosine/hydroxyproline and sarcosine/oxoproline peptides. 

Figure 2.2 Modern solid phase peptide synthesis. Process begins with a-N terminal Fmoc deprotection of resin bound C-terminal amino acid residue with piperidine (mechanism illustrated). Peptide link formation follows (typical solvent Al-methylpyrrolidone [NMP]) by carboxyl group activation with dicyclohexylcarbodiimide (DCC) (mechanism illustrated) in presence of hydroxybenzotriazole (HOBt). HOBt probably replaces DCC as an activated leaving group helping to reduce a-racemization during peptide link formation. Other effective coupling agents used in place of DCC/HOBt are HBTU 2-(lH-benzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate Py-BOP benzotriazole-l-yl-oxy-tns-pyrrolidino-phosphonium hexafluorophosphate. The Process of a-N deprotection, and peptide link formation, continues for as many times as required (n-times), prior to global deprotection and resin removal.

---