Fluorescence spectra of lysozyme excited at 305 NM in presence of urea.

Tryptophan 108 of hen egg white lysozyme was selectively excited at 305 nm and fluorescence spectra were recorded as a function of pH (2-9) and concentration of urea (0-8 M). Urea at low concentrations (1-4 M) quenches markedly the Trp 108 fluorescence around pH 7; the gamma max, however, remained unaltered. The fluorescence quenching by urea is most likely due to local conformational changes around Trp 108 in active site region of the enzyme. Substantial unfolding of the enzyme, however, was brought about by 4 M urea below pH 3, and by 7 M urea at pH 10.3, as indicated by a marked red shift in the gamma max of the fluorescence emission.