Peptide bond magnitude

The classical methods for detection and quantitation of racemization require analysis of the chiral purity of the product of a peptide-bond-forming reaction. For example, the Anderson test is used to explore a variety of reaction conditions for the coupling of Z-Gly-Phe-OH to H-Gly-OEt (Scheme 6). 9 The two possible enantiomeric tripeptides are separable by fractional crystallization, so that gravimetric analysis furnishes the racemization data. This procedure has a detection limit of 1-2% of the epimerized tripeptide. A modification by Kemp,1"1 utilizing 14C-labeled carboxy components, extends the detection limit by two to three orders of magnitude by an isotopic dilution procedure. The Young test 11 addresses the coupling of Bz-Leu-OH to H-Gly-OEt, and the extent of epimerization is determined by measurement of the specific rotation of the dipeptide product. 

When AP-A was treated with trypsin only the Argl4 to Glyl5 peptide bond was cleaved [56]. The resulting derivative lacked cardiotonic activity but its binding affinity for the rat brain sodium channel was reduced by less than an order of magnitude (Llewellyn LE et al., unpublished results). Its overall structure, as... 

In the absence of tissue factor, factor Vila cleaves the Arg peptide bond in factor X at a rate so low that it is not readily detectable in vitro. In the presence of tissue factor and membrane surface phospholipids, factor Vila rapidly activates factor X. The estimates for the magnitude of the increase in activation rates in the presence of tissue factor... 

Interaction between even more remote spins is possible in conjugated systems such as alkynes and allenes. is also commonly observed in homoallylic fragments (H-C-C=C-C-H). Large couphngs of this type are observed in in l,4-cyclohexadienes. Small homoallylic coupling is also observed across amides. Its magnitude is a function of conformations across peptide bonds (Figure 12.81)." ... 

The secondary structure covers the spatially arranged conformations produced by hydrogen bonding between peptide bonds such as helix sequences and pleated sheet structures. Here, the tendency toward helix formation for the same amino acid residues in poly(a-amino acids) and proteins is mostly, but not always, of the same magnitude (Table 30-1). The peptide chains in the pleated sheet structure are mainly arranged antiparallel. Segments in the coil conformation are generally not included in the secondary structure. 

Each peptide bond in a peptide tends to be planar, which is a consequence of the planar nature of the amide unit. The R groups in 75 have a great influence on the magnitude of angles / and (p, and these angles of rotation define the conformation for that portion of the peptide. Structure 76B shows that the amide unit of one amino acid residue is anti to the amide unit of the adjacent amino acid residue. Peptide 76B also shows the carbonyl of the valine residue is anti to the carbonyl of the valine residue, which is anti to the carbonyl of the serine residue. The consequences of this observation are that a peptide chain assumes this alternating or anti pattern. 

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