SPECTRAL CHANGES IN CHLOROPHYLL SOLUTIONS 83 



position, the monochromator slits and amplifier gain were adjusted 

 to give a suitable deflection on the screen at the desired wavelength. 

 The tube was then flashed and a sweep record obtained in which the 

 vertical deflection represents the total of transmitted and scattered 

 light reaching the photocell. The shutter was then closed, and a second 

 flash and sweep recorded, giving just the scattered light. The vertical 

 distance between the two traces then measures the net transmitted 

 light at the selected wavelength and time. A timing sweep, blanked 

 at 10,000 cycles by a calibrated audio oscillator, was also taken for 

 each measurement. Oscillograph deflections were converted to optical 

 densities by calibration records, using either pure solvent or the 

 measured absorption spectra of the test solutions. Linearity of the 

 photometric system was checked occasionally with a calibrated 

 screen. 



RESULTS 



Typical results, showing bleaching at the red peak and the appear- 

 ance of new absorption bands in the far red and green, are presented 

 in Fig. 2. It is obvious that flash-illumination leads to drastic changes 

 in absorption spectrum, confirming at least qualitatively the spectro- 

 graphic observations of Livingston d al. Systematic measurements at 

 various monochromator settings enable one to obtain the spectrum 

 of the metastable species. Our data are in general agreement with 

 those of Livingston and co-workers in showing a decreased absorption 

 at the red and blue peaks of chlorophyll and enhanced absorption in 

 the green (around 525 ni/x) and violet. Of particular interest, however, 

 is the new band in the far red (^700 mix), demonstrated in Fig. 2B. It 

 is probable that this is the same band first seen in the low-temperature 

 steady-state bleaching experiments, in rigid solvents (5). However, 

 the lifetime measurement afforded by the flash technique permits 

 us now to assign the far red absorption unequivocally to a metastable 

 species. 



The e.xistence of a reasonably intense far red band in the metastable 

 state has been predicted by Franck on the basis of a postulated struc- 

 tural analogy between the enolate ion and triplet state of chlorophyll 

 (6,7). In his most recent picture of the mechanism of splitting 

 of water bj^ excited chlorophyll, discussed elsewhere in this volume. 

 Professor Franck has also suggested that the function of this band is 

 to enable chlorophyll, in its metastable form, to become still further 



