Disulfide group vibrations

W. Qian and S. Krimm,/. Comput. Chem., 32, 1025-1033 (1992). Vibrational Studies of the Disulfide Group in Proteins. VI. General Correlations of SS and CS Stretch Frequencies with Disulfide Bridge Geometry. 

There are four disulfide bonds in short-chain (Type I) neurotoxins. This means that there are eight half-cystines. However, all Hydrophiinae toxins have nine halfcystines with one cysteine residue. An extra cysteine residue can be readily detected from the Raman spectrum as the sulfhydryl group shows a distinct S-H stretching vibration at 2578 cm" Some Laticaudinae toxins do not have a free cysteine residue as in the cases of L. laticaudata and L. semifasciata toxins. In long toxins (Type II) there are five disulfide bonds (Table III). 

The S-H vibration in proteins has been studied by Yu et and Li et alP This vibration, which occurs in the range of 2500-2600 cm, is far from any other Raman bands including the librational bands of water that extend from less than 1 cm to about 1750 cm . This permits one to observe the S-H vibration in proteins even though the concentration of the -SH group is very low and there is no resonance enhancement. (Sometimes, however, fluorescence may be a problem.) Yu et have studied the disappearance of the S-H vibration in lens on aging that is caused by the oxidation of the -SH groups to disulfide (—S—S—) groups. Li et have studied the effect of the environment on the frequencies... 

Both cysteine (disulfide) and methionine groups in proteins give rise to C-S stretching vibrations in the 600-750 cm regions. Hence, correlations proposed between Vq and conformation within the -C-C-S- moiety in disulfide and methionine are easier to apply when only one group is present. The C-S stretching frequency has been suggested, on the basis of studies on dialkyldisulfides, to lie near 630-670 cm when the site (X), trans to the sulfur atom, in... 

Figure 41 shows characteristic group frequencies of sulfides and disulfides. The stretching vibration of C-S bonds gives rise to a weak infrared but a strong Raman signal at 730-570 cm . Similarly, the S-S stretching vibration at 500 cm is a very strong Raman line but very weak infrared band (Fig. 42), Both Raman signals are very diagnostic in the conformational analysis of disulfide bridges in proteins.

The keratins are a specialized group of structural fibrous proteins that are characterized by their high cystine content and can be classified according to the amount of sulfur present in the protein. Structures such as wool, hair, hooves, horns, claws, beaks and feathers are classified as hard keratins because the sulfur concentration in these proteins is greater than 3%. Keratin proteins containing less than 3% sulfur, such as the stratum corneum (the outermost layer of skin), are classified as soft keratins. Although these keratin proteins have a similar molecular composition major spectral differences have been observed in the intensities of the C-S and S-S stretching vibrations the conformation of the disulfide bond and the position, line shape and bandwidth of vibrations associated with the proteins secondary structure. 

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