NH-------O=C hydrogen bond

Figure 6. Schematic diagram of n - 1 intra- o molecular NH O—C hydrogen bonds. When n = 3, then C5 and C j are the same atom. ...

Proposed conformational change of [Z-His-NHC5H4]Fe(C5H4CO-Asp-NH-r-Bu)] by deprotonation. In the sheet structure, two inter-chain NH - 0=C hydrogen bonds exist (left). When the carboxyl group releases a proton, ImNH--O C is formed instead of terminal NH - O C hydrogen bond to form a turn structure (right). 

As illustrated in Scheme 2, aU NH and O-C sites of ima ligands form intermolecular NH O-C hydrogen bonds with adjacent units. These interactions yielded onedimensional hydrogen-bonded framework along the -axis. These chains are further associated with jt-jt interactions between phen ligands in the adjacent chains. Their chains... 

C —H — O=C hydrogen bonds, and led to the association of modified NH and CH force constants with such bonds (Dwivedi and Krimm, 1982c). In fact, the presence of two types of CH2 groups suggests the prevalence of the antiparallel-chain crystal structure (see above), and this was tested in detail by a calculation of the normal modes of both unit cells. The results are given in Table XVII, and compared with observed Raman and IR data on (Gly),II. 

Although NH s is a highly localized mode, and therefore not likely to be sensitive to chain conformation, its frequency depends strongly on the strength of the N—H O=C hydrogen bond, and it can be expected that this will be a sensitive reflection of structure and its variations. 

The conformational diange of [S] -> [S ] upon the addition of metal salts was accompanied by the concurrent rotations around Pro-C —C and Gly-N—C bonds by ca. 80°. Durii the internal rotations, the intramolecular 1 <- 3 hydrogen bond (7-tum) (16,26,141,142) is broken which supported the conformation [S], and the Gly-NH protons became hydrogen-bonded with solvent. In addition to the internal rotations, three Pro-C=O oxygens come together on one side of the cyclic peptide and other three Gly-C = O oxygens on the opposite side of the cyclic peptide. Thus the... 

Helices of smaller and larger diameter than that of the a helix are possible. The most important is the 3jq helix (or S.O o helix), which has exactly three residues per tum. T42-i44 Each NH forms a hydrogen bond to the third C=0 on down the chain thus, the 3jo helix is tighter than the a helix. 

For phenols of pXa ranging from 10.3 to 4.5, OH- - -0=C hydrogen bonds are formed with 3-methyl-4-pyrimidone (59). With picric acid (pXa = 0.4) protonation occurs at the Ni nitrogen atom. For phenols of intermediate pXa valnes, there is no preferred site of interaction, both ArOH- - -0=C and NH+- - -O Ar bonds being formed in solution. ... 

Typical polyurethanes use a diol extender such as 1,4 butane diol. The NH groups form hydrogen bonds with C=0 groups on neighbouring chains, inside very small, hard blocks. The soft blocks are crosslinked polyethers or polyesters. A typical polyether is poly(propylene oxide) —[—O—CH2— C(CH2)H—] pre-polymerised to a molecular mass Mn of about 6000, a degree of polymerisation n = 140. The PPO has a glass transition temperature of about —60 °C, so the crosslinked PPO is a rubber. This rubbery phase is connected, via the polyurethane molecules, to the crystalline phase. 

Intermolecular OH...3 and CHjOH hydrogen bonds Intermolecular C(2,)NH.O=C(73> hydrogen bonds Intermolecular C(6,)OH.HOC (62) hydrogen bonds N-H stretching C-H stretching C=0 stretching... 

Recently, the structure of the first N- and C-protected tryptophan derivative, i.e., N-acetyl-L-tryptophan methyl ester, was solved by Cotrait and Barrans (97). There is one hydrogen bond NH. .. O = C of 2.86 A between the NH indole group and the acetyl C = O of different molecules. Molecular packing in the crystal is established by van der Waals forces between the two hydrophobic methyl groups and the aromatic ring. The conformation of the tryptophanyl side chain is very close to that of the L form of DL-tryptophan formate. Fig. 5 shows bond lengths and angles of N-acetyl-L-tryptophan methyl ester. 

Ramachandran plot (see Figure 1.7a). The a helix has 3.6 residues per turn with hydrogen bonds between C =0 of residue n and NH of residue n + 4 (Figure 2.2). Thus all NH and C O groups are joined with hydrogen bonds except the first NH groups and the last C O groups at the ends of the a helix. As a consequence, the ends of a helices are polar and are almost always at the surface of protein molecules. 

Figure 2.5 Schematic illustrations of antiparallel (3 sheets. Beta sheets are the second major element of secondary structure in proteins. The (3 strands are either all antiparallel as in this figure or all parallel or mixed as illustrated in following figures, (a) The extended conformation of a (3 strand. Side chains are shown as purple circles. The orientation of the (3 strand is at right angles to those of (b) and (c). A p strand is schematically illustrated as an arrow, from N to C terminus, (bj Schematic illustration of the hydrogen bond pattern in an antiparallel p sheet. Main-chain NH and O atoms within a p sheet are hydrogen bonded to each other.

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