A simple, rapid and sensitive method has been applied on three amino acids viz., methionine, cystine and cysteine, to estimate the ionization constant. Spectrophotometric measurements were carried out on the absorbance of each amino acid at different pH values. The results showed that the cystine and cysteine have one pKa, while the methionine gave two different pKa values by using this method.
Key words: amino acids, ionization constant, spectrophotometry, cystine, cysteine, methionine.
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