Absorption spectra cystine

Side chains of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan absorb ultraviolet light in the 240- to 300-nm region, while histidine and cystine absorb to a lesser extent. Figure 3-13 shows the absorption spectrum of a "reference compound" for tyrosine. There are three major absorption bands, the first one at 275 nm being a contributor to the well-... 

The molecular weight of 320,000 obtained for the muscle enzyme from sedimentation-diffusion data at 2-6 mg/ml and v = 0.75 (132) is to be compared with 270,000 obtained by Wolfenden et al. from s20,w = 11.1 S and D2 ,w = 3.75 X 10 7 cm2 sec1, and v = 0.731 calculated from the amino acid content (92). The rabbit muscle enzyme has a normal amino acid content, that is, no unusually low or large amount of a particular amino acid was found. Of the 32 cysteine/half-cystine residues per mole based on a molecular weight of 270,000, 6.2 were rapidly titrated with p-mercuribenzoate (92). Typical protein absorption spectra were reported for elasmobranch fish (126), carp (125), rat (127), and rabbit muscle enzyme (68). An E m at 280 nm = 9.13 has been reported for the rabbit muscle enzyme (133). The atypical absorption spectrum with a maximum at 275-276 nm observed by Lee (132) is indicative of contaminating bound nucleotides. 

The absorption spectra of several cystine-containing peptides as obtained by Otey and Greenstein (1954) are shown in Fig. 3. Some nonadditivity of absorptivities is evident for example, the spectrum of l-cystinyl-L-cystine is obviously not simply the sum of 2 cystine spectra, even at wavelengths greater than 2500 A, where the disulfide group is the only significant chromophore. The question, What is a suitable model for cystine absorptivity in a protein is apparently farther from a satisfactory answer than is the parallel question for aromatic amino acid residues. 

The visible absorption spectrum of a solution containing a known concentration of nitrated protein is measured in a solution buffered at pH 9.0, and the absorbance at the maximum (near 428 nm) used to calculate the nitrotyrosine content ( 428nm for the nitrophenoxide ion is 4200). The tyrosine and nitrotyrosine content of the modified protein should also be determined by amino acid analysis. If the sum of these values does not add up to the tyrosine content of the unmodified protein, intra- or intermolecular cross-linking may have occurred. The amino acid analysis may also reveal whether other side-reactions have taken place. Particular attention should be paid to the half-cystine, cysteine, methionine, histidine and tryptophan contents of the modified proteins. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate offers a rapid and highly sensitive way of detecting products of intermolecular cross-linking. Such products are readily removed by gel filtration. 

The absorption spectrum of wood (Fig. 4.23) depends very much on its origin, i.e. the composition of different amino acids. The absorption at 250-300 nm is due essentially to the presence of the amino acids tyrosine (4.129) and tryptophan (4.130), with minor contributions from cystine (4.131) and phenylalanine (4.132) [1576, 1578]. 

In the case of peptides and proteins the spectro-scop) of the amide bonds, the side chains and any prosthetic groups (such as haems) determines the observed UV-visible absorption spectrum. However, as with DNA, intensities and wavelengths can be perturbed by the local environment of the groups. UV spectra of proteins are usually divided into the near and far UV regions. The near-UV in this context means 250-300 nm and is also described as the aromatic region, though transitions of disulfide bonds (cystines) also contribute to the total absorption intensity in this region. The far-UV (< 250 nm) is dominated by transitions of the peptide backbone of the protein, but transitions from some side chains also contribute to the spectrum below 250 nm. 

L-Cystine (Figure 12.9) contains the S—S group, which gives rise to a very intense band in the Raman spectrum near 500 cm This mode has essentially no intensity in the IR. Because this structure contains highly polar groups, there is a rich collection of intense absorption bands in this region of the IR spectrum while the Raman spectrum is dominated by the single intense band from the nonpolar sulfur section of the molecule. 

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