Fluorescence Anisotropy of Riboflavin vis-à-vis Protochlorophyllide in Lemon Juice

A violet laser of wavelength 405 nm is used to excite steady-state fluorescence in the juice of a lemon. Prominent wavelength peaks appear at 520 and 670 nm due to emissions from riboflavin (vitamin B₂) and protochlorophyllide (pchlide), the precursor of chlorophyll [Formula: see text], respectively. Using a polariser for excitation with vertically polarised light, and another one in parallel as well as in perpendicular positions to this one in the emission-path, counts at the two peaks in the high-resolution spectrometer are noted. Fluorescence anisotropies calculated from the obtained intensities at the two positions of the analyser reveal that the value at 520 nm peak is approximately twice that of the 670 nm one. This indicates that rotational mobility of riboflavin is much less than protochlorophyllide, even though the latter is much larger and more complex, in the low pH (high acidity) medium.