Long-wavelength fluorescence of tyrosine and tryptophan: a classic example of second order diffraction.

The recent report of long-wavelength fluorescence of tyrosine and tryptophan solutions [Macias et al. (1987) Biochem. Int. 15, 961-969] prompted an investigation to confirm its authenticity. Similar excitation and emission spectra were recorded in the absence of any filters, but completely disappeared when a 370 nm non-fluorescent cut-off filter was placed between the excitation monochromator and the sample. These results demonstrate that excitation spectra above 400 nm and emission spectra above 550 nm originate from second order diffraction effects.