Origin and properties of fluorescence emission from the extrinsic 33 kDa manganese stabilizing protein of higher plant water oxidizing complex

The fluorescence properties of the isolated extrinsic 33 kDa subunit acting as 'manganese stabilizing protein' (MSP) of the water oxidizing complex in photosynthesis was analyzed in buffer solution. Measurements of the emissi on spectra as a function of excitation wavelength, pH and temperature led t o the following results: (a) under all experimental conditions the spectra monitored were found to be the composite of two contributions referred to a s '306 nm band' and 'long-wavelength band', (b) the excitation spectra of t hese two bands closely resemble those of tyrosine and tryptophan in solutio n, respectively, (c) the spectral shape of the '306 nm band' is virtually i ndependent on pH but its amplitude drastically decreases in the alkaline wi th a pK of 11.7, (d) the amplitude of the 'long-wavelength' emission band a t alkaline pH slightly increases when the pH rises from 7.2 to about 11.3 f ollowed by a sharp decline at higher pH, and (e) the shape of the overall s pectrum at pH 7.2 is only slightly changed upon heating to 90 degreesC wher eas the amplitude significantly declines. Based an these findings the two d istinct fluorescence bands are ascribed to tyrosine(s) ('306 nm band') and the only tryptophan residue W241 of MSP from higher plants ('long-wavelengt h band') as emitters which are both embedded into a rather hydrophobic envi ronment. (C) 2001 Elsevier Science B.V, All rights reserved.