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476 Tawfik<br />

2. A molar excess of p-hydroxyphenylglyoxal in the range of 50–500 is usually sufficient for<br />

a first trial.<br />

3. The absorbance of p-hydroxyphenylglyoxal modified arginines changes with the pH.<br />

Maximal absorbance is observed at 340 nm at pH ≥ 9.0 (ε = 18,300 M –1 cm –1 ) (1).<br />

4. Prolonged dialysis in neutral or mildly alkaline buffers may cause a significant release of<br />

the modifying group. Purification of the protein to determine the extent of modification<br />

should therefore be performed using exclusion chromatography (1).<br />

References<br />

1. Yamasaki, R. B., Vega, A., and Feeney, R. E. (1980) Modification of available arginine<br />

residues in proteins by p-hydroxyphenylglyoxal. Analyt. Biochem. 109, 32–40.<br />

2. Rogers, T. B., Børresen, T., and Feeney, R. E. (1978) Chemical modification of the arginines<br />

in transferrins. Biochemistry 17, 1105–1109.<br />

3. Stoughton, D. M., Zapata, G., Picone, R., and Vann, W. F. (1999) Identification of Arg-12 in<br />

the active site of Escherichia coli K1CMP-sialic acid synthetase. Biochem. J. 343, 397–402.

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