Peptide bond resonance

Peptide bond resonance has several important consequences. First, it restricts free rotation around the peptide bond and leaves the peptide backbone with only two degrees of freedom per amino acid group rotation around... 

This is a variation of the proton-detected shift-correlation experiment via long-range couplings proposed by Bax and Summers (Bax and Summers, 1986), with the difference that the first C pulse is substituted by a frequency selective pulse (Fig. 7.14) (Bermel et al., 1989 Kessler et al., 1989b,1990). This significantly increases resolution in the F dimension. For example, this can be used to remove the overlap between the cross-peaks of the carbonyl resonances of peptide bonds in proteins that all occur within a... 

The peptide bond is characterized by a fixed planar structure, as was discovered by X-ray crystallography of peptides more than 60 years ago. The arrangement of the atoms in the peptide bond is due to resonance stabilisation the lowest-energy state of the system is that in which the four atoms forming the peptide linkage lie in a plane, while the C-N bond has partial double bond character. 

The H- and C-NMR spectroscopic data support the proposed primary structure of poly(Lys-25). The amide carbonyl resonances are particularly informative as these signals are well resolved in the C-NMR spectrum of poly(Lys-25) (Figure 4). An amide carbonyl resonance is observed at 174.9 ppm for poly(Lys-25) that does not appear in the spectrum of poly(Val-Pro-Gly-Val-Gly) [13]. The position and relative intensity of this resonance are consistent with a lysine amide carbonyl group within a peptide bond [14]. Moreover, the resonances of the amide carbonyl groups for other residues in the pentapeptide repeat are split due to the substitution of a lysine residue at position 4 in every fifth pentapeptide in Lys-25. In addition, the absence of splitting in amide carbonyl group of valine in position 4 (174.5 ppm) supports this assignment, as this residue is replaced by lysine in the fifth pentapeptide of the Lys-25 repeat. The presence of other resonances attributable to the lysine residue can be detected in the H- and C-NMR spectra of the Lys-25 polymer at levels commensurate with its... 

An important factor in the structure of protein-DNA complexes can be the peptide backbone. The amide bond can fimction as an H-bond acceptor as well an H-bond donor. Due to the reduced flexibility of the backbone vs. side chain (resonance stabilization of the peptide bond), H-bonds to the peptide backbone lead to a rigid and tight arrangement in the complex and contribute extensively to the exact fit between protein and nucleic acid. 

FIGURE 4-2 The planar peptide group, (a) Each peptide bond has some double-bond character due to resonance and cannot rotate. 

Extensive studies of enzyme-substrate complexes by resonance Raman spectroscopy (RR) have prompted the synthesis of new peptide bond modifications such as thionoesters and dithioesters (Scheme l7)t82-83l within simple model substrates. The resulting acyl-enzyme complexes are especially amenable to RR analysis with cysteine proteases such as papain due to formation of the transient dithioester intermediates. 

As expected, the l3C CP/MAS NMR spectrum of the l3C-enriched Gly-Tyr crystals displays the characteristic doublet (Fig. 91 A). However, the l3C CP/MAS difference spectrum between the CPA (which is also rich in C—N bonds, but which is not13C-enriched) and the CPA/Gly-Tyr complex (Fig. 91B) displays a single resonance. This signifies that, under the experimental conditions, the peptide bond of the Gly-Tyr has been cleaved. To confirm this result, the authors took the 1SN CP/MAS NMR spectrum of the... 

Amino acids can link together by a covalent peptide bond between the a-carboxyl end of one amino acid and the a-amino end of another. Formally, this bond is formed by the loss of a water molecule, as shown in figure 3.9. The peptide bond has partial double-bond character owing to resonance effects as a result, the C—N peptide linkage and all of the atoms directly connected to C and N lie in a planar configuration called the amide plane. In the following chap-... 

The peptide bond between the carbon and nitrogen exhibits partial doublebond character due to the closeness of the carbonyl carbon-oxygen double-bond allowing the resonance structures in Fig. lb to exist. Because of this, the C-N... 

Fig. 1. (a) Formation of a peptide bond, (b) resonance structures of the peptide bond, (c) peptide units within a... 

The physical and chemical properties of peptides are similar to those of amino acids except that the peptide bonds add another dimension to these compounds. Amino and carboxyl groups involved in the peptide linkage can no longer accept or donate protons. The peptide bond itself carries a partially planar (double-bonded) character because of the resonance effects that are possible ... 

Resonance stabilization of an amide accounts for its enhanced stability, the weak basicity of the nitrogen atom, and the restricted rotation of the C—N bond. In a peptide, the amide bond is called a peptide bond. It holds six atoms in a plane the C and O of the carbonyl, the N and its H, and the two associated a carbon atoms. 

The shortening of the C-N bond implies that hydrogen bonding confers more rigidity into polypeptide chains, not only by reason of the hydrogen bonds, per se, but also because of the additional Tt-bond character in the peptide bond. In terms of valence-bond resonance, it increases the importance of the representation shown below ... 

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