Hydrogen‐deuterium exchange of the tryptophan residues in bovine α‐lactalbumin

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Abstract
Effects of deuteration on the Raman spectrum of a tryptophan residue have been examined. The 1386 cm−1 line of deuterated tryptophan residue has been found to be useful for tracing the hydrogen‐deuterium exchange reaction of this residue in a protein. An examination on bovine α‐lactalbumin at pH 6.4 and at 20°C indicates that two of the four tryptophan residues exchange with a rate constant much greater than 9 × 10−4 sec−1, while the other two exchange with a rate constant of 4 × 10−5 sec−1. The latter two have been assigned to Trp 28 and Trp 108 of this protein. The kinetics of hydrogen‐deuterium exchange reaction of completely “free” tryptophan residue have been examined by a proton magnetic resonance study on tryptophan itself. By taking the result of this examination into account, the chance of exposure to the solvent for Trp 28 or Trp 108 has been estimated to be 3 × 10−6 at pH 6.4 and at 20°C.