Absorbency Spectra

Aliphatic amino acids exhibit no absorption in the UV region above 220 nm, with the exception of cystine (240 nm). The aromatic amino acids, phenylalanine, tyrosine, and tryptophan, have a UV absorbency above 220 nm. The exact position of the maximum and the molar extinction coefficient e are affected by the pH of the solution.

On the other hand, for infrared spectra, amino acids have common absorbency and specific absorbency. The common absorbencies of infrared spectra for amino acids are in the vicinity of 3070 cm"1 for NH3+, 1620 cm"1 for COOH, and 1560 to 1600 cm"1 for COO". The common absorbency nearly always shows a similar value irrespective of the state of the amino acid—solid or solution.

Table 7. Dissociation Constant and Isoelectric Point of Amino Acids

Amino acid

pK,

pK2

pK3

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