Amides infrared spectra

Dong AC, Huang P, Caughey WS. Protein secondary structure in water from second-derivative amide infrared spectra. Biochem 1990 29 3303-6. 

Quinoxalin-2-ones are in tautomeric equilibrium with 2-hydroxy-quinoxalines, but physical measurements indicate that both in solution and in the solid state they exist as cyclic amides rather than as hydroxy compounds. Thus quinoxalin-2-one and its A -methyl derivative show practically identical ultraviolet absorption and are bases of similar strength. In contrast, the ultraviolet spectra of quinoxalin-2-one and its 0-methyl derivative (2-methoxyquinoxaIine) are dissimilar. The methoxy compound is also a significantly stronger base (Table II). Similar relationships also exist between the ultraviolet absorption and ionization properties of 3-methylquinoxalin-2-one and its N- and 0-methyl derivatives. The infrared spectrum of 3- (p-methoxy-benzyl)quinoxalin-2-one (77) in methylene chloride shows bands at 3375 and 1565 cm" which are absent in the spectrum of the deuterated... 

A criterion for the position of the extent of the mesomerism of type 9 is given by the bond order of the CO bond, a first approximation to W hich can be obtained from the infrared spectrum (v C=0). Unfortunately, relatively little is known of the infrared spectra of amide anions. How-ever, it can be assumed that the mesomeric relationships in the anions 9 can also be deduced from the infrared spectra of the free amides (4), although, of course, the absolute participation of the canonical forms a and b in structures 4 and 9 is different. If Table I is considered from this point of view, the intimate relationship betw-een the position of the amide band 1 (v C=0) and the orientation (0 or N) of methylation of lactams by diazomethane is unmistakeable. Thus the behavior of a lactam tow ard diazomethane can be deduced from the acidity (velocity of reaction) and the C=0 stretching frequency (orientation of methylation). Three major regions can be differentiated (1) 1620-1680 cm h 0-methylation (2) 1680-1720 cm i, O- and A -methylation, w ith kinetic dependence and (3) 1730-1800 em , A -methylation, The factual material in Table I is... 

Figure 5.12 Dependence of peak frequencies in infrared spectrum of glucosamide bolaam-phiphiles NC( )GN-GLC (14) on methylene spacer length n. (a) The CH2 antisymmetric vas and symmetric vs stretching vibrations reveal gauche-included conformation for short chains and an all-trans conformation for longer chains, (b) Amide I and II frequencies show an even-odd effect for n > 10. Reprinted from Ref. 53 with permission of Wiley-VCH.

The infrared spectrum of 4-quinolizone shows an amide carbonyl band, which serves to emphasize the aromatic character in the system, and its position (6.06 p.) emphasizes the greater single bond contribution of the amide carbonyl due to resonance. The introduction of... 

The infrared spectrum of rabbit hair was similar to that of the wool fibers with readily apparent amide bands in the regions of 1650 cm-1 and 1530 cm-1 (Figure 6). 

With these compounds the presence of the halogen will have been detected in the tests for elements. Most acid halides undergo ready hydrolysis with water to give an acidic solution and the halide ion produced may be detected and confirmed with silver nitrate solution. The characteristic carbonyl adsorption at about 1800 cm -1 in the infrared spectrum will be apparent. Acid chlorides may be converted into esters as a confirmatory test to 1 ml of absolute ethanol in a dry test tube add 1 ml of the acid chloride dropwise (use a dropper pipette keep the mixture cool and note whether any hydrogen chloride gas is evolved). Pour into 2 ml of saturated salt solution and observe the formation of an upper layer of ester note the odour of the ester. Acid chlorides are normally characterised by direct conversion into carboxylic acid derivatives (e.g. substituted amides) or into the carboxylic acid if the latter is a solid (see Section 9.6.16, p. 1265). 

The mineral/matrix ratio is a measure of the mineral content and is calculated from the infrared spectrum as the ratio of the integrated phosphate Vi,V3 envelope (ca. 900-1200cm-1) to the collagen amide I envelope... 

The refined structure had residuals R=0.244, R"=0.274. The amide groups are approximately perpendicular to the chain axis and form hydrogen bonds N-H...0=C, as indicated by the polarized infrared spectrum. The 06-H group is close to the gt conformation but is not hydrogen bonded and has a short 06 05 contact. Elimination of the short contact with a non-bonded constraint increases the residuals to R=0.250, R"=0.288, an insignificant change. However the 06-H groups remain unbonded, contrary to infrared indications. 

There is the 3 methods for preparing of 8-azaspiro(4.5)decane-7,9-dione, 8-(4-(4-(2-pyrimidinyl)-l-piperazinyl)butyl) monohydrochloride (U.S. Patent 3,717,634). One of them is follows a mixture of 0.1 mole of the substituted glutaric anhydride, 0.1 mole of l-(4-aminobutyl)-4-(2-pyrimidinyl)piperazine (U.S. Pat. 3,398151), and 300 ml of pyridine was refluxed until imide formation was completed. The degree of reaction was readily followed by taking an aliquot portion of the reaction mixture, removing the solvent, and obtaining the infrared absorption spectrum of the residue. When reaction is complete, the spectrum exhibited typical infrared imide bands at 1701 and 1710 cm-1 whereas if incomplete, the infrared spectrum contains amide and carboxyl absorption bands at 1680, 1760 and 3300 cm 1. 

For verifying how the temperature affects the structure of the adsorbed molecules so that the direction of their dipole moment flips, we measured the infrared spectrum of an LC film at two temperatures as shown in Fig. 7. The peaks in Fig. 7a are of the amide-I (1,668 cm ) and amide-II (1,544 cm ) bands and those in Fig. 7b are of the amide-III (3,200-3,500 cm ) band (N-H stretching). Clearly a dramatic change occurs in the vibrations of the polypeptide upon cooling of the sample. The amide-I vibration is split and shifted to lower wavenumber. The amide-III band also splits but in addition it is shifted to higher wavenumbers. 

A difference infrared spectrum of a control sample of caeruloplasmin after the subtraction of 2H20 is shown in Fig. 7.2, where the amide I maximum is centred at 1641 cm-1 (Haris et al., 1989). This band frequency is consistent with the presence of predominantly / -sheet structure in this protein with a proportion of a-helix. 

The infrared spectrum of polyamide (nylon 6), shown in Fig. 4.1-2B, additionally exhibits the typical bands of a secondary amide an N-H stretching vibration at 3300 cm ... 

The infrared spectrum of a protein is dominated by its peptide backbone amide I (C=0) and amide II (C-N. NH) vibrations. Fig. 6.6-1 shows a typical IR absorption spectrum of a hydrated protein film, in this case bacteriorhodopsin. In addition to the strong amide I (1658 cm ) and amide II (1546 cm ) bands water also contributes largely to the absorption (3379 cm , 1650 cm ). 

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