Combination bands amides

There are a number of methods based on using the spectra of proteins with secondary structures known from X-ray data [707]. An example is the method of Williams [8,116] which analyzes the amide I band of the Raman spectrum for a protein of unknown structure in terms of linear combinations of amide I bands for proteins with known X-ray structure. Significant correlations were observed between the Raman and X-ray diffraction estimates of helix, P-strand, turn, and undefined. Correlations were also observed between a-helix and disordered helix, and between parallel and antiparallel p-sheets. 

Amide II, NH bend CN stretch combination band and NH3+ symmetric deformation... 

Amide HI, NH bend CN stretch in plane combination band... 

Overtone and combination bands are usually weak in comparison with the fundamentals. However, when the frequency of such a combination falls close to that of another fundamental of the same symmetry species, a Fermi resonance interaction occurs, which results in a sharing of intensity between the two modes as well as frequency shifts in both. This occurs, for example, in the interaction between the NH stretch mode, Pa, and overtones or combinations of amide II modes, (Miyazawa, 1960b). From measurements on the frequencies and intensities of the observed bands, pa and Pb. it is possible to obtain the frequencies of the unperturbed fundamental, p, and combination, p. The relation is given by (Miyazawa, 1960b)... 

Figure 9.32 shows the FT-IR spectra of the PAll film before and after deuteration for 8.2 h. The intensity of the v(NH) absorption band at 3308cm" decreased, and simultaneously two new absorption bands developed at 2469 and 2411cm" . This band doublet was ascribed to Fermi resonance of the evolving v(ND) band and the Amide II (1470cm ) -f Amide III (975cm ) combination band of the deuterated PAll [67]. Furthermore, a significant decrease in the Amide II band of the... 

Fig. 63 Changes of the peak areas as a function of the pulse number for the following bands at 1780 cm-1 the C=0 stretching vibration of the imide ring ( ) at 1375 cm-1 the C-N stretching vibration of the amide ( ) at 1500 cm-1 the combination band C-N stretching vibration and N-H deformation of the amide (A) and at 1260 cm-1 the C -0-Car stretching vibration (T). Positive values correspond to decreasing absorption, and vice versa. REPRINTED WITH PERMISSION OF [Ref. 135], COPYRIGHT (2000) American Chemical Society...

There is some correlation between molecular structure and band position for certain bands, but because these are often overtone and combination bands, their positions are not as structure-dependent as the fundamental bands in the mid-IR. For example, primary amines, both ahphatic and aromatic, have two absorption bands, one at about 1500 nm and the second at about 1990 nm. Secondary amines have only one band at about 1500 nm. As expected, a tertiary amine has no NH band. Amides with an — NH2 group can be distinguished from R— NH—R amides by the number and position of the N—H bands. The reference by Goddu has a detailed table of NIR structure-wavelength correlations. 

In the polymer industry, packing material, laminates including multilayer films, pellets or molded products can be analyzed by NIR. Even polymer latex particles with up to 99 % water content may be analyzed. NIR provides information about reaction mechanisms, polymerization, crystallinity, orientation, water content and hydrogen bonding, even during the process of polymer manufacture. For example the disappearance of the double bonds in polyethylene and polypropylene can be monitored. In the NIR spectrum C=C bonds lead to a combination band at about 4740 cm and a first overtone at about 6170 cm NIR spectroscopy is applied to characterize ester-, nitrile-, or amide-based acrylic and methacrylic polymers. Other examples are the identification of polyvinylchloride, polyvinyl alcohol and polyvinyl acetates or the analysis of polymerization in epoxy and phenolic resins. 

Sutherland, 1953). Secondary amides have a characteristic band near 3100cm". Miyazawa (1960) describes this band as the result of Fermi resonance of N—H stretching with the combination band of C=0 stretching and N—H in-plane bending in cis-amides, and the result of Fermi resonance of the N—H stretching with the overtone of the amide II band in tra/ts-amides. (When an overtone or combination band is located near a fundamental frequency, the band intensity of the former may... 

The amide I band has been examined by Elliott et al. (1950) in native and denatured insulin, by Elliott et al. (1957) in lysozyme, and by Ambrose et al. (1951) in water-soluble silk. The band at 3200 cm" has also been investigated. Beer et al. (1959) have given a comprehensive list of proteins studied up to 1959, along with characteristic absorption bands. Bamford et al. (1956) have reviewed work done up to 1956 in the region between 5000 and 4500 cm (combination band of the N—H stretching frequency and that of the amide I or amide II band). The infrared dichroic properties of crystals of hemoglobin and ribonuclease have been observed in this region (Elliott and Ambrose, 1951 Elliott, 1952). 

The red-shifting effect of ester oxygen atom is weak and strongly dependent on substitution, possibly because the competition between the aforanentioned stereoelectronic effects is closely balanced. The carbonyl stretch in amides is very sensitive to substitution and its analysis is further complicated by the presence of combination bands (only the blue-shifled of the two bands is shown in Figure 12.6). 

Primary amines show characteristic first overtone bands at 1450, 1550 and 1000 nm which represent the first and second overtones of N-H stretching bands. Tertiary amines produce no N-H overtones, but do produce C-H and 0-H stretching combination bands at 1260-1270 nm. The first overtones of the N-H stretching bands of aliphatic amines are shifted to lower wavelengths. Primary amides may be characterized by combination bands at 1930-2250 nm and an N-H stretching overtone band at 1450-1550 nm. Secondary amides produce overtone bands in the 1350-1550 nm region and combination bands in the 1990-2250 nm region. 

Because of the cyclic structure the NH and C=0 groups are forced into the cis configuration so that no band comparable to the amide 11 band in trans secondary amides occurs in lactams. A characteristically strong NH stretching absorption near 3200 cm occurs in the infrared, which is only weak to medium in the Raman. A weaker infrared band near 3100 cm" is due to a combination band of the C=0 stretching and NH bending vibrations. 

The N-H functional group found in amines and amides has not been studied as extensively as hydroxyls, but it is important because it appears in many natural products, pharmaceuticals, and polymers. N-H participates in hydrogen bonding and therefore behaves differently in various solvents and matrices. The near-infrared region offers a special advantage in the measurement of the primary amine group NH2 due to a unique combination band. 

In primary amides, a single combination band of symmetric and asymmetric NH stretching appears at 6805 cm i (1470 nm). Several bands near 5100 and 4925 cm" (1960 and 2030 nm) have been assigned to the combinations of NH stretch and amide II and III deformations (these mid-infrared bands are generally considered to be due to different types of coupling of the CNH deformation and... 

The main combination bands of secondary amides are summarized in Table 8.5. These bands are important in the analysis of proteins. As explained by Murray in his chapter on spectral comparisons,24 two of the combination bands of proteins lie on either side of the OH combination band of carbohydrates. Therefore, the shape of the spectral region near 4760 cm- (2100 nm) is... 

Baughman, E.H. and Mayes, D., NIR applications in process analysis. Am. Lafe, 54-58, October 1989. Krikorian, S.E. and Mahpour, M., The identification and origin of N-H overtone and combination bands in the near-infrared spectra of simple primary and secondary amides, Spectrochim. Acta, 29A, 1233-1246, 1973. 

N-H [vN-H and amide II deformation (N-H in-plane bending) combination] for secondary amides in proteins CONHj combination of amide A and amide II N-H stretching and C=0 stretching (amide I) combination N-H (vN-H and 5N-H combination) for gamma-valerolactam N-H combination band found in the spectrum of native RNase A (C=0 amide I band)... 

N-H stretching and C=0 stretching (amide I) combination band in the spectrum of native RNase A... 

N-H in-plane bend and C-N stretching and N-H in-plane bend combination N-H native RNase A combination band at 4867 cm shifting due to thermal unfolding (C=0 amide I band)... 

N-H combination band from primary amides N-H [vN-H asymmetric and Amide N-H combination bmd 1 3... 

N-H/C-N combination band from urea (NH2-C=0-NH2) N-H [vN-H asymmetric and amide Urea ... 

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