Aromatic summation bands

The out-of-plane aromatic C—H bending vibrations in the 1000-700 cm region give rise to relatively prominent summation bands in the infrared 2000-1650 cm" region, seen in Fig. 8.8. Since all the CH out-of-plane... 

Figure 3-14 shows the spectra of N-acetyl ethyl esters of all three of the aromatic amino acids and of cystine. To a first approximation, the absorption spectra of proteins can be regarded as a summation of the spectra of the component amino acids. However, the absorption bands of some residues, particularly of tyrosine and tryptophan, are shifted to longer wavelengths than those of the reference compounds in water. This is presumably a result of being located within nonpolar regions of the protein. Notice that the spectra for tyrosine, phenylalanine, and cystine in Fig. 

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