Amides contributing structures

In a reaction which is mechanistically related to the stilbene-dihydrophenanthrene photocyclisation, the anilide derivative of (178), compound (179), photocyclises to give (180). As mentioned in the Introduction to this chapter, such enamide cyclisatlons have now been reviewed. Other new examples of this reaction are the formation of (181) from (182) and the formation of the regioisomers (183) and (184) from (185) by irradiation of the enamldes in the presence of sodium borohydride. These enamide photocyclisation reactions can be viewed as an electrocyclic closure of the amide contributing structure (186) to give (187) in the presence of borohydride the iminlum function in (187) is reduced to the observed products. 

On the other hand, there are data suggesting that amides have a rather high value for association as pure liquids. Trifan and Terenzi measured IR spectra of several polyamides and polyurethanes from 25°C to 300°G. They deduce an average —AH of 8.5 1.2 kcal/mole for the polymers. Until more data are available we must draw the provisional conclusion that —AH for amide N—bonds in crystals (and possibly in pure liquids) may be much larger than in solutions. Under any circumstances, however, we arc reminded that the data of Davies establish empirically the substantial contribution of the amide groups to the stability of the amide crystal structures. 

As this area was not covered in Volume 31, the present chapter provides an overview of the literature published over the two years between July 1998 and June 2000. The report is structured in terms of the principal classes of tervalent phosphorus acid derivatives, viz halogenophosphines, tervalent phosphorus esters, and amides. Attempts have been made to minimise the extent of overlap with other chapters, in particular those concerned with the synthesis of nucleic acids and nucleotides to which the chemistry of tervalent phosphorus esters and amides contributes significantly (see Chapter 4), the use of known halogenophosphines as reagents for the synthesis of phosphines (see Chapter 1), and the reactions of dialkyl- and diaryl-phosphite esters, in which the contribution of the phosphonate tautomer, (R0)2P(0)H, is the dominant aspect. 

Two fundamental contributing structures determine protein spectra. The amide (peptide) bonds that compose the linear head-to-tail arrangement of the polypeptide chain have strong absorbance in the deep UV with an absorbance... 

Draw three contributing structures of the following amide and state the hybridization of the highlighted 0, C, and N. In which orbitals do the three lone pairs drawn reside ... 

C. Which of the following are reasonable contributing structures for the amide bond of the molecule shown on the left in the example above ... 

Only two contributing structures can be drawn for cation B. Of these, B-2 requires creation and separation of unlike charges and places positive charges on adjacent atoms. It therefore makes little contribution to the hybrid. Thus, the positive charge in cation B is essentially localized on the amide nitrogen. 

The carbonyl stretching of amides occurs at 1630-1680 cm at a lower frequency than for other carbonyl compounds. This observation can be explained by a weakened carbonyl C=0 bond as described through the three most important resonance-contributing structures of amides, two of which indicate single bond... 

The fact that the six atoms of an amide bond are coplanar with bond angles of 120° means that the resonance structure on the right makes a significant contribution to the hybrid and that the hybrid looks very much like this third structure. Inclusion of the third contributing structure explains why the amide nitrogen is sp hybridized and therefore trigonal planar. Also, the presence of a... 

In amides, the N atom is sp hybridized, owing to a third significant resonance contributing structure that places a double bond between the acyl C and N atoms. 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

Fig. 9. Comparison of FTIR absorption spectra of four proteins in H20 (left, amide I + II) and D20 (right, amide F + IF). Comparison between protein spectra for dominant secondary structure contributions from a-helix (myoglobin, MYO, top), /Fsheet (immunoglobin, IMUN), from both helix and sheet (lactoferrin, LCF) and from no long-range order (o -casein, CAS, bottom). The comparisons emphasize the high similarity, differing mostly by small frequency shifts of the amide I with the changes in secondary structure.

Figures 9 and 10 represent a selected comparison of amide V and I+II FTIR and VCD for four proteins in D2O solution. Of these, myoglobin (MYO) has a very high fraction of a-helix, immunoglobulin (IMU) has substantial /1-sheet component, lactoferrin (LAF) has both a and j3 contributions, and a-casein (CAS) supposedly has no extended structure. The FTIR spectra of these proteins change little, the primary difference...

The RAHB effect may be illustrated by the ubiquitous C=0- -H—N hydrogen bond of protein chemistry. As shown in Section 5.2.2, the simplest non-RAHB prototype for such bonding, the formaldehyde-ammonia complex (5.31c), has only a feeble H-bond (1.41 kcalmol-1). However, when the carbonyl and amine moieties are combined in the resonating amide group of, e.g., formamide, with strong contributions of covalent (I) and ionic (II) resonance structures,... 

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