Substituted residues

Fig. 2.38 sheet forming y-peptides. (A) Crystal structure of the two stranded antiparallel sheet formed by a,j -unsaturated y-dipeptide 152 with a-methyl substituted residues [208], Both intermolecular H-bonds are characterized by a N---0 distance of 2.84 A and an angle (N-H- -O) ofl54.2°. (B) Crystal structure of the infinite parallel sheet arrangement formed by vinylogous dipeptide 153 [208], Intermolecular H-bonds are characterized by a N -O distance of 2.88 A and 3.24 A and an... 

FIGURE 5.19 Approaches for synthesis of protected peptides using acid-sensitive protectors. For (A) the last residue is incorporated as the Cbz-derivative. (B) The chain is assembled on a substituted oxime resin,44 starting at the penultimate residue. The chain is then detached from the resin by displacement by the carboxy-terminal residue of the target peptide, which may be a free (terf-butylammonium salt) or carboxy-substituted residue. (C) An alternative for producing acids by use of the oxime resin is transesterfication of the peptide with A-hydroxypiperidine, followed by reduction to remove the piperidino substituent. 

Our original approach to polysaccharide C-13 n.m.r. spectral analysis consisted of making a minimum number of hypotheses about expected structure-to-spectra relationships (8). By then comparing spectra to known structure for a series of D-glucans, we attempted to establish the validity of these hypotheses and to establish how diverse a structural difference could be accommodated The hypotheses were as follows. Firstly, that each polymer could be considered as an assembly of independent saccharide monomers. Secondly, that these hypothetical saccharide monomers would be 0 alkylated (0 -methylated) in the same positions as the actual saccharide linked residues (it had previously been established that 0-methylation of any a-D-glucopyranosyl carbon atom position resulted in a down-field displacement of vlO p.p.m. for the associated resonance). Thirdly, that each differently substituted residue would have a completely different set of chemical shift values for each carbon atom position (different from the unsubstituted saccharide) but that only the carbon atom positions involved in inter-saccharide linkages would have A6 greater that 1 p.p.m. And, fourthly, that the hypothetical 0-alkylated residues would contribute resonances to the total spectrum proportional to their mole ratio in the polymers. 

A second example [IJ) is that of the anomeric spectral region of dextran B-742 fraction S, a polysaccharide for which per-methylation data indicate Structure 2, when n=0. This is an unusual polymer, as every backbone residue is 3-0-substituted. It is fortunate that this polymer exists, as the dextrans branching through 3,6-di-O-substituted residues present a problem in the anomeric spectral region, displaying only a single branching anomeric resonance in addition to the linear dextran resonance. 

The pentapeptide sequences were assembled on a Fmoc-Val-Pepsyn-KA-resin (0.08 mequiv-g 1) using a Biolynx synthesizer. Fmoc(Fmoc-Hmb)N-Gly-OH was incorporated using its pentafluorophenyl ester with HOBt catalysis for 45 min. Fmoc-Ala-OPfp was double coupled to the terminal (Hmb)tripeptide-resin for 2 h. All couplings not involving an Hmb-substituted residue were performed using standard 0.5 mmol scale coupling protocols. 

Fig. 17. Structure of a macromolecular polyornithine Gd-D03A complex, note that the 84 unsubstituted and the 30 substituted residues are randomly distributed on the polypeptide chain...

Fig. 6. Multiple alignment in region of known echinocandin mutations. Wild-type deviations from the consensus sequence are shown in bold type. The sequences of three known echinocandin-resistant mutations are listed at the bottom with the substituted residues underlined. Reproduced with permission from Thompson et al. (1999).

Figure 25. Crystal structure of the /3-tripeptide t Boc/ 3-HVal-/33 IIAla fp IILeuOMe (3) reported by Seebach223 showing an intrinsic preference for a turned conformation. Two of the /3-peptide residues are rendered as thick cylinders. The Newman protections about the Ca—Cp bonds for these residues are shown, indicating the apparent preference of the /33 substituted residue bearing the bulky substituents to adopt a sc conformation while the residue with the smaller methyl substituents takes on an ap conformation.

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