Glu, side chain

M-L Valero, E.Giralt, D Andreu. Optimized Asp/Glu side chain anchoring in synthesis of head-to-tail cyclic peptides by Boc/OFm/benzyl chemistry on solid phase, in R Ramage, R Epton, eds. Peptides 1996. Proceedings of the 24th European Peptide Symposium, Mayflower, Kingswinford, 1998, pp 857-858. 

The observed ellipticities below 250 nm are characteristic of an a-helix and different from a 310-helix, according to the distinction described by Toniolo et alJ131 The CD spectrum indicates that two turns of a helix can be readily detected by this measure, in contrast to previous expectations.[132,133] The molar ellipticity shows no temperature dependence. It is, however, affected by pH as protonation of the template carboxylate and the Glu side chain leads to loss of the favorable dipole interactions and salt bridges, respectively. 

On-Resin Synthesis of a Cyclic Peptide Library Using Lys and Glu Side-Chains... 

Easy access to peptide methyl esters is offered by the transesterification of peptides linked to benzyl alcohol type resins such as the HMBA resin.t l Tertiary amines generally serve as catalysts,although even better results are obtained with potassium cyanideor lithium bromide/DBUt as base catalysts. Note that Asp or Glu side-chain esters are likewise transformed under these conditions. 

In the crystal structure of the c-Fos-c-Jun heterodimeric LZ, the conserved Asn at a position a on the c-Jun LZ forms H-bonds with the Glu side-chain at the flanking position g on the c-Fos LZ (5). Interestingly, a similar interaction could be predicted to occur in the c-Myc-Max heterodimeric LZ between Max Asn5a and c-Myc Glu 4g (Fig. IB). On the other hand, according to the data presented here, it appears that the side-chain of Max Asn5a points in the direction of c-Myc Glu 5a rafter than that of c-Myc Glu 4g. 

Figure 5. Variation of the protein MEP along the active sites of some enzymes. It a-chymotiypsin, 2t p-tiypsin, 3 porcine pancreatic elastase, 4 Streptomyces Griseus hydrolase, Si a-lytic protease, 6t subtilisin NOVO, 7i acetylcholinesterase, 8> lipase A, 9 lysozyme, lOi D-xyloie isomerase. Point A is at OG of die active serine in 1-8, at the bisector ofODl and OD2 of Asp-52 in 9, at Ol of the cyclic xylose m 10. Point B is atNE2 of the catalytic histidine in 1-8, in the first trisector of points A and Din 9, atNEl ofHis-S4in 10. Point C is at ND1 of the catalytic histidine in 1-8 and 10, at the second trisector of points A and D in 9. Point D is at the bisector of the carboxylate oxygens of the catalytic Asp or Glu side chains.

Diagnostic losses of certain neutral species are commonly observed from b- and j-ions. For example, Ser, Thr, Asp, and Glu side chains exhibit prominent loss of water (—18 Da), and the Asn-, Gin-, Lys-, and Arg-containing ions similarly show abundant loss of ammonia (—17 Da). The loss of 48 Da (HSCH3) is observed from Met-containing sequence ions, but if Met is in oxidized form, the mass of the expelled neutral ion becomes 64 Da. The Cys-containing ions show a loss of 34 Da (H2S), which shifts to 92 Da (HSCH2COOH) if the precursor protein is alkylated with iodoacetic acid prior to digestion. The peptides that contain a basic residue at the C-terminus are likely to expel the C-terminal residue to produce the (h , + OH) ion. 

Asp and Glu side chains have only two oxygen atoms available for receiving hydrogen bonds and can only be monoanionic, unlike the typical dianionic form of phosphate. Third, the pKa values of Asp and Glu are considerably higher than that of the phosphate monoanion - indeed Asp and Glu carboxylates can sometimes be found in the neutral form. Thus, interpreting results with Asp and Glu substitutions can be difficult. For phosphotyrosine, there are no really suitable replacements among the 20 natural amino acids. 

A similar concept for site-selective thiolysis of peptide bonds via backbone peptide bond activation can be found in Jensen s work (Fig. 18) [115]. The key step involves activation of the carboxy group of a Glu side chain by PyBrOP (19), resulting in the on-resin formation of the pyroglutamyl imide moiety. The activation renders the imide C-N bond susceptible to thiolysis, after which protected peptide thioesters were released from the solid support. 

The following are some typical experimenteil amide proton titration shifts between pH 2 and 6. In the tetrapeptide H-Gly(l)-Gly(2)-Glu(3)--Ala(4)-0(H), downfield shifts of 0.062 ppm and 0.172 ppm were observed for the amide protons of the residues 2 euid 3, respectively. The corresponding titration shifts in H-Gly-Gly-Glu-Ala-OCH were 0.077 ppm cuid 0.228 ppm, in H-Gly-Gly-Gln-Ala-OH 0.021 ppm cuid -0.006 ppm, in H-Gly-Gly-Met-Ala-OH 0.028 ppm and 0.020 ppm. From comparison of the pK values obtained from the amide proton titrations with those for the carboxylic acid groups measured independently by carbon-13 NMR, the large downfield shifts in the Glu peptides were shown to arise from hydrogen bonding with the Glu side chain carboxylate, while the titration shifts in the other peptides could be related to through space interactions with the C-terminus (Bundi and Wuthrich, 1978). 

Pyrodlutamate formation. May occur during AFFmoc removal with peptides containing N-terminal Glu, side-chain protected with benzyl-based groups... 

---