Peptide basicity

HochweUer K, Sweenie CH, Anderton SM Immunological tolerance using synthetic peptides - basic mechanisms and clinical application. Curr Mol Med 2006 6 631-643. 

Neutral peptides Basic peptides Acidic peptides Peptides with add and basic amino acids Peptides containing proline and hydroigrproUne ... 

Boman, H. G. (2003) Antibacterial peptides basic facts and emerging concepts. Journal of Internal Medicine, 254, 197-215. 

Fig. 9.6. Composite diagram showing the elution position of some peptides and amino acids. Chromatographic conditions column, Lichrosorb Si 60 7 fim (treated with 0.1 M copper(II) sulphate/1 M ammonia) mobile phase, (A) water/acetonitrile (10 90)-0.1 M ammonia, 1 ppm Cu " ", (B) water-acetonitrile (60 40)-0.95 M ammonia, 1 ppm Cu " ". Elution was achieved with a concave gradient of 0% B to 100% B over 70 min flow rate, 2 ml/min detection, UV at 254 nm. 1, Phe-Phe 2, Ala-Ala-Ala 3, mixture 4, Ala-Ser 5, Pro-Glu 6, Phe 7, Gly-Gly-Gly 8, Lys-Phe 9, Leu 10, Leu 11, Glu 12, Ala 13, Ser-Ser-Ser 14, Gly-His-Gly 15, Arg-Glu 16, Lys-Gly 17, Arg-Tyr 18, Pro-Gly-Lys-Ala-Arg,Lys-Lys-Gly-Glu A, hydrophobic, large peptides B, dipetides C, amino acids, hydrophilic peptides, basic peptides.

Very interesting work was also reported by Piwnica-Worms and coworkers. They prepared Tat-peptide derivatives like 90 dual-labeled with Tc and fluorescein for scintigraphy and fluorescence microscopy (Scheme 5.45) [218]. Tat-peptide basic regions have been intensely studied because of their ability to translocate across cell membranes, seemingly independent of receptor-mediated endocytosis. The peptides were prepared by SPPS with an N-terminal His ligand, to which Tc(CO)3 or Re(CO)3 is coordinated in solution after cleavage and purification. As an alternative to histidine, diethylenetriamine-pentaacetic acid (DTPA) was also used as an N-terminal metal chelator. The C-terminal cysteine was reacted with fluorescein-5-maleimide already on the resin, thus providing both labels in the same peptide, as shown for 90 in Scheme 5.45. Cellular localization of these... 

The biologiccJ function of a protein or peptide is often intimately dependent upon the conformation(s) that the molecule can adopt. In contrast to most synthetic polymers where the individual molecules can adopt very different conformations, a protein usually exists in a single native state. These native states are found rmder conditions typically found in Uving cells (aqueous solvents near neutred pH at 20-40°C). Proteins can be unfolded (or denatured) using high-temperature, acidic or basic pH or certain non-aqueous solvents. However, this unfolding is often reversible cind so proteins can be folded back to their native structure in the laboratory. 

Dowex 1-X2 0.6 0.65 Strongly basic anion exchanger with S-DVB matrix for separation of small peptides, nucleotides, and large metal complexes. Molecular weight exclusion is <2700. 

Fig. 1. Schematic drawing of precursors for selected brain oligopeptides. Shaded areas represent the location of sequences of active peptide products which are normally cleaved by trypsin-like enzymes acting on double-basic amino acid residues. Precursors are not necessarily drawn to scale, (a) CRF precursor (b) proopiomelanocortin (POMC) (c) P-protachykinin (d) proenkephalin A (e) CGRP precursor (f) preprodynorphin, ie, preproenkephalin B. Terms are...

Biosynthesis. Two closely related genes encode the three mammalian tachykinins. The preprotachykinin A gene encodes both substance P and substance K, while the preprotachykinin B gene encodes neuromedin K (45—47). The active sequences are flanked by the usual double-basic amino acid residues, and the carboxy-terrninal amino acid is a glycine residue which is decarboxylated to an amide. As with most neuropeptide precursors, intermediates in peptide processing can be detected, but their biological activities are not clear (ca 1994). 

Biosynthesis. Somatostatin exists in longer forms in several biological tissues (95,96). One of the longer forms, which has been isolated from porcine intestine, has been characterized as a 28-amino acid peptide (97). Somatostatin is derived from a precursor containing 116 amino acids (98,99). The precursor contains one copy of the somatostatin tetradecapeptide, which is contained within the sequence of the 28-amino acid peptide at the carboxy-terminal end of the precursor. The 28-amino acid somatostatin is preceded by a single Arg residue, while somatostatin 1-14 is preceded by a pair of basic residues. 

CGRP has a wide distribution in the nervous system (19) and was the first peptide to be localized to motoneurons (124). It is also found in primary sensory neurons where it is colocalized with substance P (125). CGRP is derived from a precursor stmcturaHy related to the calcitonin precursor. The latter precursor produces two products, calcitonin itself and katacalcin, while the CGRP precursor produces one copy of CGRP (123). Like other peptides, CGRP is cleaved from its precursor by tryptic breakdown between double basic amino acid residues. 

CCK has been detected in two principal forms, ie, the traditional 33-amino acid peptide, and an octapeptide CCK-8. The intestine produces mainly CCK-33 (133) and the brain produces mainly CCK-8 (132). The CCK precursor contains one copy of CCK-33 (133,134) this peptide is flanked on both ends with double basic residues, whereas CCK-8 is formed from CCK-33 by cleavage of a single basic residue. 

Group II consists of the enkephalins which come from the 267-aniino acid piecuisoi pro-enkephalin A [88402-54-4] (Fig. 2). This proteia contains four copies of Met-enkephalin, one copy of Leu-enkephalin, and the extended peptides Met-enkephalin-Arg -Phe (the last Met-enkephalin sequence ia Fig. 2) and Met-enkephalin-Arg -Gly -Leu (the fourth Met-enkephalin sequence ia Fig. 2) (25,26). AH of these products ate formed by trypsin-like cleavage between pairs of basic residues. The extended enkephalin peptides are further cleaved by carboxypeptidase E (27) to form authentic Met-enkephalin. 

The Group III peptides come from the 256-amino acid precursor, pro-dynorphin [88402-55-5] (pro-enkephalin B). This group contains dynorphin A [80448-90-4] and B [85006-82-2] as weU as a-neoendorphin [77739-20-9] (Fig. 2), all of which can be further cleaved to form biologically active iatermediates, eg, dynorphin A g and P-neoendorphin [77739-21-0] (a-neoendorphin ) (28). The longer of these peptides are relatively basic because of the number of Lys and Arg residues. 

Peptide Synthesis. The literature on the enzymatic synthesis of peptides is enormous (120—124). Here the basic principles that govern peptide synthesis are illustrated and recent trends in this area reviewed. 

There are two basic strategies for enzyme-catalyzed peptide synthesis equiUbrium- and kineticaHy controlled synthesis. The former is the direct reversal of proteolysis and involves the condensation of an amino component with unactivated carboxyl component. The latter proceeds by the aminolysis of an activated peptide ester. 

Under basic coupling conditions an aspartyl peptide that has a /3-phenacyl ester i converted to a succinimide. The use of PhSeH prevents the a,/3-rearrangement c the aspartyl residue during deprotection. 

Mel, CH3CN morpholine or diethylamine, methanol, 76-95% yield. These conditions also cleave tlie 4 -pyridyl derivative. The Pet ester is stable to the acidic conditions required to remove the BOC and r-butyl ester groups, to the basic conditions required to remove the Fmoc and Fm groups, and to hydrogenolysis. It is not recommended for use in peptides that contain methionine or histidine since these are susceptible to alkylation with methyl iodide. 

Cycloalkyl esters have.been used to protect the /3-CO2H group in aspartyl peptides to minimize aspartimide formation during acidic or basic reactions. Aspartimide foimation is limited to 2-3% in TFA (20 h, 25°), 5-7% with HF at 0°, and 1.5-4% TfOH (thioanisole in TFA). Cycloalkyl esters are also stable to Et3N, whereas use of the benzyl ester leads to 25 % aspartimide formation during Et3N treatment. Cycloalkyl esters are stable to CF3COOH, but are readily cleaved with HF or TfOH. - ... 

Caibamates ate formed from an amine with a wide variety of reagents, of which the chloroformate is the most common amides are formed from the acid chloride. -Alkyl caibamates are cleaved by acid-catalyzed hydrolysis A/-alkylamides are cleaved by acidic or basic hydrolysis at teflux. conditions that cleave peptide bonds. 

The BOC group is used extensively in peptide synthesis for amine protection. It is not hydrolyzed under basic conditions and is inert to many other nucleophilic reagents. 

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