Amides infrared absorption frequencies

Aldehydes Amides Carboxylic acid anhydrides Carboxylic acids Esters Ketones and functional groups, 56 infrared absorption frequencies, 519,... 

The UV-spectra of azolides have already been discussed in the context of hydrolysis kinetics in Chapter 1. Specific infrared absorptions of azolides were mentioned there as well increased reactivity of azolides in nucleophilic reactions involving the carbonyl group is paralleled by a marked shift in the infrared absorption of the corresponding carbonyl bond toward shorter wavelength. For example, for the highly reactive N-acetyl-tetrazole this absorption is found in a frequency range (1780 cm-1) that is very unusual for amides obviously the effect is due to electron attraction by the heterocyclic sys-tem.[40] As mentioned previously in the context of hydrolysis kinetics of both imidazo-... 

The wavelengths of IR absorption bands are characteristic of specific types of chemical bonds. In the past infrared had little application in protein analysis due to instrumentation and interpretation limitations. The development of Fourier transform infrared spectroscopy (FUR) makes it possible to characterize proteins using IR techniques (Surewicz et al. 1993). Several IR absorption regions are important for protein analysis. The amide I groups in proteins have a vibration absorption frequency of 1630-1670 cm. Secondary structures of proteins such as alpha(a)-helix and beta(P)-sheet have amide absorptions of 1645-1660 cm-1 and 1665-1680 cm, respectively. Random coil has absorptions in the range of 1660-1670 cm These characterization criteria come from studies of model polypeptides with known secondary structures. Thus, FTIR is useful in conformational analysis of peptides and proteins (Arrondo et al. 1993). 

One of tire distinguishing physical characteristics of the cephalosporins is the infrared stretching frequency of the /i-lactam carbonyl. This absorption occurs at higher frequencies (1770-1815 cm-1) than those of either normal secondary amides (1504-1695 cm-1) or ester carbonyl groups (1720-1780 cm-1). 

One of the first VCD studies we undertook in 1988 was work on the tripeptide L-Ala-L-Ala-L-Ala [23], This molecule, in neutral aqueous (D O) solution, exhibits a distinct, near-conservative VCD spectrum in the amide I region, shown in Figure 7. The infrared absorption shows two overlapping peaks in the amide I region, at 1650 and 1675 cm 1. Raman depolarization ratios indicated that the high frequency component is the symmetric stretching combination. 

Figure 9 shows a comparison of the infrared absorption and VCD spectra of (L-Ala), n = 3 - 5. The spectra are normalized for equal absorption intensity at 1595 cm 1, which is the frequency of the carboxylate anion antisymmetric stretching mode. The data show that the amide I intensity increases roughly linearly with the number of peptide linkages in the molecule, and that the VCD intensity increases similarly. However, the positions of the infrared absorption maxima are shifted from about 1654 cm 1 in the trimer to 1648 cm 1 in the pentamer. Similarly, the VCD zero crossing in the trimer occurs at 10 cm 1 higher frequency than in the tetramer and pentamer. We have interpreted these results [48] in terms of different solution conformations of the peptides the trimer seems to be stabilized by zwitterionic interactions, as discussed before, whereas formation of extended helices seems to occur at the level of the tetramer. 

The infrared absorption spectrum of sulfacetamide sodium has been determined in KBr disc (4). The principal peaks appear at 825, 1090,1145,1264,1552,1600 cm 1. The infrared stretching frequencies of the amino group have been used to calculate the force constant, the band angle and the "S" character of the nitrogen orbitals of the N-H band (23,24). Infrared measurements of sulfonamides have been performed to study the imide-amide tautomerism (25) and to see if there is any change in the electronegativity of the SO2 group (26,27). Sulfacetamide in eye-drops and ointments has been identified by attenuated total reflectance (ATR) infrared spectra (28). 

Infrared spectroscopy is also frequently used in the quantitative determination of the proportion of diads. As a rule, the assignment of the different diad signals is made possible from results with polymers or oligomers of known configuration. In certain cases, the calculation of the absorption frequency for the individual types has already been performed. The CH and CH2 deformational vibrations refer directly to various configurations, as is also often the case with the amide I bands of poly(a-amino acids). Since products of different stereoregularity crystallize to different extents, and since ir spectra are sensitive to crystallinity in the range 670-1000 cm then the diad content can also be determined by means of what are called crystallinity... 

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). 

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