Raman spectroscopy amide vibrations

Vibrational spectroscopy has been used in the past as an indicator of protein structural motifs. Most of the work utilized IR spectroscopy (see, for example, Refs. 118-128), but Raman spectroscopy has also been demonstrated to be extremely useful (129,130). Amide modes are vibrational eigenmodes localized on the peptide backbone, whose frequencies and intensities are related to the structure of the protein. The protein secondary structures must be the main factors determining the force fields and hence the spectra of the amide bands. In particular the amide I band (1600-1700 cm-1), which mainly involves the C=0-stretching motion of the peptide backbone, is ideal for infrared spectroscopy since it has an large transition dipole moment and is spectrally isolated... 

The new NIR FT Raman spectroscopy now allows the investigation of food (Keller et al., 1993). Fig. 4.1.20 shows typical Raman spectra of food components carbohydrates, proteins, and lipids. Fig. 4.1-20C shows the Raman spectrum of a banana with the out-of-phase and in-phase vibrations of the C-O-C groups of the carbohydrates at about 1100 and 850 cm. Fig. 4.1-20B, a Raman spectrum of turkey breast shows the amide I and III bands and some bands which can be directly assigned to amino acids. The Raman spectra of lipids allow the determination of the amount of cis and trans disubstituted C=C bonds Fig. 4.1-20A, butter (Keller et al., 1993). NIR Raman spectroscopy has good chances as tool for the investigation of living tissues, especially in medical diagnostics (Keller et al., 1994 Schrader et al., 1995). 

UV resonance Raman spectroscopy (UVRR), Sec. 6.1, has been used to determine the secondary structure of proteins. The strong conformational frequency and cross section dependence of the amide bands indicate that they are sensitive monitors of protein secondary structure. Excitation of the amide bands below 210 nm makes it possible to selectively study the secondary structure, while excitation between 210 and 240 nm selectively enhances aromatic amino acid bands (investigation of tyrosine and tryptophan environments) (Song and Asher, 1989 Wang et al., 1989, Su et al., 1991). Quantitative analysis of the UVRR spectra of a range of proteins showed a linear relation between the non-helical content and a newly characterized amide vibration referred to as amide S, which is found at 1385 cm (Wang et al., 1991). 

Previous infrared studies indicate that proteins with a large amount of 3-sheet structure absorb near 1240 cm and those with a-helix stucture absorb near 1280 cm l (22,32). Absorbances for denatured albumin, reportedly containing random and 3-sheet conformations, are found at 1240 and 1260 cvT (22). These assignments correlate with the more studied Raman spectroscopy of the amide III region which has vibrations at 1230-1250 cm l for 3-sheet structure, at 1260-1290 cm for a-helix structure, and at 1240-1265 cm l for unstructured polypeptide (34,35). 

FT-IR spectroscopy is complementary to Raman spectroscopy and the use of the Raman technique together with the infrared should always be considered. The IR spectra of DNA have been recorded and analysed (6). FT-IR spectra of DNA and the effect of a heavy metal, cis-Pt(NH3)2Cl2 on specific vibrations is shown in Figure 10. Important changes occur in the regions and 1700-1500 and 1200-900 cm in the shape of the carbonyl and amide band contour, as well as, in the phosphate or ribose-phosphate bonds and in particular the intensity of the vibration at 1054 cm (7) (Table 3, Fig. 9) diminishes drastically even with. 03 Pt/nucleotide. 

Vibrational (IR and Raman), UV-visible, photoelectron, NMR, and Mossbauer spectroscopy have all been reported for bis(bistrimethylsilyl-methyl)tin(II) and analogous tin(II) amides. Since an unusual tin-tin double bond has been proposed for the solid state of [Sn[CH(SiMe3)2]2]2 the Raman spectrum of this compound was of interest. Unfortunately, the compound decomposed in the laser beam however, an intense band at 300 cm-1 has been assigned as the Ge—Ge stretching frequency for the analogous germanium compound (68). 

Very few of the infrared studies of proteins have been carried out on aqueous solutions of the proteins. Except for the work of Koenig and Tabb (1), the few aqueous IR studies have been on single proteins. Correspondingly, most of the assignments of the backbone vibrations (the so-called Amide I, II, III, etc. vibrations) have been based on either Raman spectra of aqueous solutions (2) or on infrared spectra of proteins in the solid state (3). Where infrared solution spectra have been obtained, it has mostly been on D2O solutions (4) - not H2O solutions. Since these Amide I, II, III, etc. vibrations involve motion of the protein backbone, they are sensitive to the secondary structure of the protein and thus valid assignments are necessary in order to use infrared spectroscopy for determining the conformations of proteins. 

Since the nanocrystalline powders are washed with buffer solution and dried in a vacuum in order to perform the IR measurements, the existence of coupled stabilizers without connection to at least one nanocrystaUine surface can be excluded. However, it is conceivable that some of these amide bridges fall off the nanocrystal on one side. As the S-H bond vibrations are detected in neither Raman nor IR spectroscopy, this hypothesis cannot yet be verified. ... 

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