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Kinetics of Racemization of Amino Acid Residues in Casein
We thank J.L. Bada, S. Steinberg and W.H. Ward for helpful comments. Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval of the product to the exclusion of others which may also be suitable.
ABSTRACT
Exposing casein to alkali (0.1N NaOH at 65°C) partly racemized the amino acid residues. A plot of D/L ratios for seven amino acids versus time of treatment shows fast initial racemization rates that decrease after about 1 hr. A linear free energy relationship exists between racemization rates and inductive effects of amino acid side chains or R-substituents (σ*). Kinetic studies over the temperature range 25–75°C yields activation energies (in Kcal/mole) for aspartic acid (20.8), phenylalanine (28.7), glutamic acid (32.5), and alanine (32.4). Racemization rates increase with hydroxide ion concentration but not with protein concentration. Acetylation of amino groups prevents lysinoalanine formation but not racemization, permitting studies which distinguish between effects of these two alkali-induced reactions. Understanding principles that govern racemization should help in designing food processes that minimize undesirable, deteriorative changes in food proteins.