Amide A band

Absorption in the near infrared region is dominated by changes in vibrational energy levels. A typical wave number is that of the "amide A" band at 3300 cm 1 (3.0 pm wavelength), approximately 1014 s. ... 

While collapsed films of this polymer can be lifted off the surface on electron microscope grids, viewed under the light microscope they are seen to break under the action of surface forces within a few minutes. Electron diffraction observations are evidently not feasible, but good polarized IR spectra are obtainable (Figure 2). The parallel dichroism of the Amide A band (3300 cm ) and the Amide I band (1660 cm ), and the perpendicular dichroism of the Amide II band (1555 cm ) is strong evidence that the collapsed monolayer is in the a-helical conformation with the molecules aligned on the water surface more or less parallel to the barrier. There is not sufficient dichroism in the bands associated with the n-decyl side chain for it to be orientated predominantly either parallel or perpendicular to the backbone. Since the side chains are very flexible it is probable that during collapse of the monolayer the side chains fold to form a more compact non-dichroic structure. 

IR spectroscopy in solid stale (KBr pellets) has provided valuable information about the structural identity of Fc-modified peptides. The salient IR peaks of interest for compounds 17-25 are Usled in Table 5.3. The position of the Amide A band between 3,400 and 3,300cm is quite informative. Amide NH stretches below 3,400cm are diagnostic of the H-bonded amide NH [29,30]. The peak positions of all compounds except that for 83 are indicative of H-bonded structures. 

Protein Structure. In the IR spectrum, the absorption bands due to the proteiu backbone, consisting of amide groups R—CON—R and side-chain modes, are distinguishable. The amide A band at 3300 cm is due to N—H stretching. The absorption bands in the 1600-1700, 1510-1580-, and 1200-1350-cm regions are labeled amide I, II, and III (marked with a prime in the case of deuteration), respectively. Normal coordinate analysis [801, 802] has revealed that amide I is primarily (76%) the v C=0 mode with some contribution from CN (14%) and CCN (10%) deformation. In contrast, amide 11 and amide 111 are heavily mixed modes. The amide II is an out-of-phase combination of 5 pnH (43%) and v CN (29%) with minor contributions from 5 PC=0, v,yC—C, and v,fN—C. The amide III is an in-phase combination of 5 pN-H (55%) with some contributions from v C—C (19%), v C—N (15%), and 5 pC=0. The amino acid side-chain absorption (due to groups such as C—H, COOH, C=0, —NH2, and —NH3+) overlaps in many cases with amide I and II absorption. 

The infrared spectra of proteins and polypeptides comprise essentially four strong vibrational modes associated with the peptide link. These are the Amide A band near 3300 cm (vn-h)> the Amide I band near 1650 cm (vc=o), the Amide II band near 1550 cm " and the Amide III band near 1250 cm The latter two modes are both associated with combined stretch-... 

A strong band at about 3300 cm and a somewhat less strong one at about 3100 cm are referred to as the amide A and amide B bands, respectively [18, 33, 34]. The amide A band is attributed to the nNH vibration, whereas the amide B band is assigned to the first overtone of the amide II vibration, which is intensified by Fermi resonance with... 

The next step in assigning group frequencies is to establish the constancy of position of the group frequency in a series of related molecules. It has been reported [ ] that for amides a band near 1408 cm is indicative of the N—CH3 bend. Thus, it appears that for amides a band near 1410 cm is a group frequency of the N—CH3 group. 

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