Amide plane

S—Cg is perpendicular to the amide plane of the / -lactam and therefore weakened. The S—bond, on the other hand, is not affected by electronic interactions with the benzamide plane. It was now thought, that a bridging of the thiazolidine moiety would bring the —S bond into a more orthogonal position with respect to the amide plane of the new lactam and make this bond more fragile. The tricyclic thiazolidine was synthesized as described above and fulfilled the predictions (J.E. Baldwin, 1978). 

Fig. 2.27 The two types of extended /1-peptide strands with conformation requirements around the C(a)-C(/1) bonds. (A) Parallel and antiparallel polar sheets with antiperiplanar conformations around the C(a)-C fl) bond are promoted by unlike-fi -ami-no acids with alkyl side-chains. Antiperiplanar side-chains at C(a) and C(/3) occupy positions approximately perpendicular to the amide planes. (B) Extended strands formed by alternating +)-sc and (-)-sc conformations...

Fig. 2.28 X-ray crystal structures of parallel sheet-forming and all-un//fce-/F -peptides 116 and 117 [10, 191]. Views along the parallel amide planes and crystal packing diagram show the parallel pleated sheet arrangement (view perpendicular to the amide planes).

Na+ CH30COCH3 214) RoNa+ = 3.00 60.0 minimum deviates from the ester resp. amide plane). 

In the solid state of cis-l,4-dibenzoyl-2,5-dimethylpiperazine, the Z,Z-form is present, with the phenyl groups twisted (77AX(B)3568) with respect to the amide plane. In solution (77TL2895), the cis-isomer of the 2,5-dimethyl and 2,5-diethyl derivatives shows the presence of only one conformer (Z,Z, with the piperazine ring in the twist-boat conformation), while the tra s-form consists of an equilibrium mixture of nearly equal amounts of the four axial alkyl rotamers. 

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-... 

Formation of a dipepetide from two amino acids, (a) Two amino acids, (b) A peptide bond (CO—NH) links amino acids by joining the a-carboxyl group of one with the a-amino group of another. A water molecule is lost in the reaction. It is conventional to draw dipeptides and polypeptides so that their free amino terminus is to the left and their free carboxyl terminus is to the right. The amide plane refers to six atoms that lie in the same plane. (Illustration copyright by Irving Geis. Reprinted by permission.)... 

Baldwin and Christie (18) have proposed that the origin of this difference almost certainly derives from a stereoelectronic factor. In 64 and 65, the Cg - S bond is more nearly orthogonal to the B-lactam amide plane than the C — S bons is, with respect to the thiazolidine amide plane (cf. 66). The Cg - S bond is therefore weaker than the CA - S bond and is preferentially cleaved. With tricyclic a-lactam 61, due to the five-membered ring, the C - S bond becomes more nearly orthogonal to the thiazolidine amide plane (cf. 67). As a consequence, the ordering of bond lability is reversed and the C — S bond is cleaved more readily. The stereoelectronically controlled step 61 63 has led to a stereospecific synthesis of a penicillin derivative from a peptide precursor. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

Figure 4.5 Assignment of atom types for the N and O atoms in Gin sidechains can be difficult and often relies on interpreting the H-bond partners in the local environment. For the two elastase structures lela (green carbon atoms) and lelc (yellow carbon atoms), a donor and an acceptor group in the ligand form close contacts with the Gin amide group, respectively. This allows unambiguous assignment of the O and N atom types. The position and orientation of the amide plane are almost identical in the two structures only the O and N positions are switched. The figure was prepared using RasMol [147].

Changes in the interaction type of Gin side chains can also occur without major spatial reorientations, as shown by a comparison of the blood coagulation factor Xa structures lfax and lxkb. In lfax, the Ne2 atom of Glnl92 forms an H-bond with the carboxylate group of the ligand. Conversely, in lxkb the face of the amide plane forms a Jt-s lacking interaction with one aromatic ring of the biphenyl moiety. 

You may have noticed that there is a lot more to Figure 9-4. In addition to labeling the alpha carbon atom and pendant R gronp, you can see that there is something called the amide plane and we have used Greek letters, 0 and 0 (pronounced fie and sie to rhyme with pie), to indicate that there are bond rotations around the C°-N and C -C bonds. What about the amide or peptide bond CO-NH To understand what s going on, we need to talk about the nature of die amide bond in a little more depth. 

H. C. Le, T. Hintennaim, T. Wcsscls, Z. Gan, D. Seebach, R R. Ernst, Determination of the Amide Plane Orientations in a Cyclo-P-Peptide by Magic-Angle-Spinning Deuterium Correlation Spectroscopy, and Comparison with the Powder X-Ray Structure , Helv. Chim. Acta 2001, 84,187 - 207. 

Apolar groups (i.e., alkyl, aryl) outside the amide plane cause a disturbance of this preferred arrangement in proportion to their steric bulk in a direction perpendicular to the amide plane. Such groups are classified as large and small by indices L and S, respectively. 

That member of a diastereomeric pair in which both faces of the amide plane are more shielded than the least shielded face in the other member is eluted first. 

A solid-state NMR study of [a- C, 0]benzamide has been presented. The orientations of the e.f.g. and chemical shift tensors have been determined from the analysis of MAS and static NMR spectra. It has been shown that the principal component of the chemical shift tensor with the least shielding is approximately 18° off the C=0 bond and that the component with the most shielding is perpendicular to the amide plane. 

The pseudo-double bond character of amides is much more pronounced than for esters due to the conjugation of the H-N-C=0 moiety and is correlated to the ability of distorted amides to be hydrolyzed to bases [19]. For this reason, the barrier to interconversion is significantly higher that for the ester series, with AGl typically ranging from 16 to 22 kcal mol-1 [17]. However, the rotational barrier is not solely due to conjugation and also partly arises from the orientation of the nitrogen lone pair which is perpendicular to the amide plane [20]. Therefore, the rates of isomerization are considerably slower than for esters. This means that both isomers can be observed by simple techniques, for example at room temperature by H and 13C NMR spectrometry and UV spectrophotometry [21]. 

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