N-terminal amino group

Fig. 11.—pH-Dependence of the Resonances for the Di[ C]methylated N-Terminal Amino Groups of Glyco-octapeptide A ( , 6), Asialoglyco-octapeptide A (O, 8), and glycopentapeptide 10. (Taken from Ref. 61.)... 

The results concerning the N-terminal structures of glycophorins A" and A were based on the labels that were placed on the crucial, N-terminal amino group, and they clearly showed that the amino acid residues at position S may play only a minor, if any, role in determining the structure of the MN blood-group determinants. The carbohydrate residues appear... 

As mentioned in the Introduction, a variety of unusual carbohydrate -protein linkages is found in Nature. These may be placed in several categories (i) carbohydrates O-glycosylated to jff-hydroxy amino acids (ii) carbohydrates S-glycosylated to cysteine (Hi) carbohydrates N-gly-cosylated to the N-terminal amino group of the protein or (iv) in some... 

A-Acetylimidazole was found to be a very efficient terminating (capping) agent in the solid-phase synthesis of peptides.t40],[41] A terminating agent is used to block any N-terminal amino groups that have not reacted in the coupling steps.[40]... 

The N-terminal amino group can be modified by different reagents, including succinic anhydride, propionic anhydride, and /V-acetoxysuccinimide. Different approaches allow the use of different mass shifts between light and heavy populations and usually differ in fragmentation patterns. 

Silk fibroin contains no cystine and the content of lysine and histidine is also low (about 1% in total), but it does contain tyrosine phenolic (13%) and serine alcoholic (16%) sidechains. Since glycine accounts for 44% of the total aminoacid content, an N-terminal glycine residue is reasonably representative of most of the primary amino dyeing sites in silk fibres. Amino acid analysis of hydrolysed reactive-dyed silk indicates that the reaction between fibroin and reactive dyes takes place mainly at the e-amino group of lysine, the imino group of histidine and the N-terminal amino group of the peptide chain. In an alkaline medium, the hydroxy groups of tyrosine and serine also react [114]. 

This problem has been confirmed recently in a study of the mechanism of covalent reaction between nylon 6.6 and the sulpha toe thylsulphone dye Cl Reactive Blue 19 (7.37). Acid hydrolysis of the dyed fibre and HPLC analysis of the hydrolysate yielded the 6-aminohexylaminoethylsulphonyl derivative of Blue 19. Even when the dyeing procedure was optimised to achieve maximal exhaustion and fixation to the fibre [128], only about 30% of the N-terminal amino groups in the nylon 6.6 were accessible because of mutual blocking effects between these bulky anionic dye molecules. 

More controlled and efficient fixation is possible when the reactant is applied as a pretreating agent [146]. If nylon given such a pretreatment is subsequently dyed with the conventional chlorotriazine dye Cl Reactive Red 3 (7.2), the substantivity and fixation of the latter are markedly lowered because the anionic XLC residues have reacted with N-terminal amino groups in the fibre. Treatment of the modified nylon with ammonia, however, restores some degree of dyeability. Opposite effects are observed if Cl Reactive Red 3 is reacted with ethylenediamine to form an aminoalkyl derivative (7.131). This nucleophilic dye exhibits a high degree of fixation only on the modified nylon that has been pretreated with XLC. 

Interest in Fixing Agent P (7.123) has been revived in the context of nucleophilic aminoalkyl dyes of the 7.129 and 7.131 types. Nylon pretreated with this symmetrical trifunctional reactant contains residual acryloyl sites that will undergo an addition reaction with nucleophilic dye molecules [145]. Although in theory each N-terminal amino group in the fibre can give rise to two acryloyl sites (Scheme 7.73), crosslinking between two N-... 

Aspartame is relatively unstable in solution, undergoing cyclisation by intramolecular self-aminolysis at pH values in excess of 2.0 [91]. This follows nucleophilic attack of the free base N-terminal amino group on the phenylalanine carboxyl group resulting in the formation of 3-methylenecarboxyl-6-benzyl-2, 5-diketopiperazine (DKP). The DKP further hydrolyses to L-aspartyl-L-phenyl-alanine and to L-phenylalanine-L-aspartate [92]. Grant and co-workers [93] have extensively investigated the solid-state stability of aspartame. At elevated temperatures, dehydration followed by loss of methanol and the resultant cyclisation to DKP were observed. The solid-state reaction mechanism was described as Prout-Tompkins kinetics (via nucleation control mechanism). 

Homodetic cyclic peptides are classified according to the location of the ring juncture as shown in Scheme 1. The most common mode of cyclization is head-to-tail where ring closure occurs by amide bond between the N-terminal amino group and the C-terminal carboxy group. Additional ring closures involve side-chain functionalities such as head-to-side-chain, side-chain-to-side-chain or side-chain-to-tail cyclizations. Moreover, in backbone cycliza-tions 29 (see Section 6.8.3.2.4) the backbone amides may be linked to a second backbone amide or to side chains as well as to the termini of the peptide. 

The amino acids used for the covalent coupling are types with functional side chains e.g. Cys, Lys, Tyr and Asp, comprising thiol, amino, phenohc and caiboxylic groups. The reactivity of these residues will be determined by the chosen coupling chemistry and its conditions, where Lys and the N-terminal amino group are the most frequently used amino acids for covalent attachment. 

Unactivated COOH groups form peptide bonds directly with N-terminal amino groups in the presence of... 

A catalytic mechanism (Figure 14) was proposed on the basis of kinetic, spectroscopic and crystallographic data . The substrate peptide binds to Zn2 with its N-terminal amino group and to Znj with the carbonyl oxygen of the scissile peptide bond. Additionally, the N-terminus interacts with Aspl79. Upon substrate binding, the bridging water... 

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