472 



S. S. Brody and M. Brody 



action between chlorophyll molecules, rather than between chlorophyll and 

 protein molecules, which gives rise to the spectral transformation observed 

 in vivo. While the heterogeneous interaction is possible, the fact that many 

 spectral properties found in vivo can be simulated by protein-less solutions 

 of chlorophyll lends support to our contentions. 



In the present analysis it was assumed that first long wavelength emission 

 band to appear (at 717 m|a,) arises from the smallest possible aggregate. 

 This assumption is supported by the following: if the distance between mole- 

 cular centers were the main parameter determining the position of the 

 emission maximum, then there would be a continuous shift in maximum 

 starting from the position of the monomer peak. However, a discontinuous 

 shift of 3Z m(j. is observed between the position of the monomer maximum 

 and the shortest wavelength at which emission from the aggregate occurs 

 On the other hand, the discontinuous shift (which actually obtains ) is to be 

 expected if a small aggregate is formed. As the concentration of larger 

 aggregates increases, a continuous shift in the emission band is to be anti- 

 cipated because the displacements between maxima decrease rapidly with 

 increasing size, e. g. , the displacement between dimer and trimer maxima 

 is only 9 my.. 



We are led to believe, therefore, that the spectral transformations obser- 

 ved in Euglena (and other organisms) represent changes in distribution of 

 monomer, dimer, trimer, etc., and that this distribution undergoes a con- 

 siderable modification between the time of initial formation of chlorophyll 

 and attainment of a "steady state". The steady state distribution seems to 

 depend upon the genus and species of the organism ( ). If these changes 



in distribution do occur, they would explain in large part the diversity of the 

 reported maxima for the "various forms" of chlorophyll in vivo (See Table I). 



By letting N go to infinity in Eq. 3, we calculate for v -p, °=, a limiting 

 value of 13, 338 cm"^ (^"f °= - ^50 m|jL). The corresponding absorption 

 maximum for an infinitely large aggregate, calculated f rom ~ p, o=, (see 

 Table II) , turns out to be 736 m(ji, a value which compares favorably with 

 the reported absorption maximum of large microcrystals of ethyl chloro- 

 phyllide a - 740 m|jL (34). This agreement lends support to our assumption 

 about R, a and the value taken for (l+cos^ o )/R^, and also indicates that the 

 internnolecular dimensions in the crystal and the aggregate are similar. 



It is of interest to note that the absorption maximum we calculated for 

 dimeric chlorophyll in vivo (705 m(x), corresponds to the wavelength report- 

 ed for the following kinds of studies: changes in absorption of irradiated 

 organisms, (18,36,60), differential absorption (1 5, Z8, Z9), and low temper- 

 ature absorption spectroscopy (16). 



The shift in emission nnaximum from 719 mfji to 73Z m|ji (measured at 77 K) 

 which occurs during the greening process in Euglena, can result from the 



