664 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



singlet-triplet transitions in light atoms, only the n^/4 singlet states are of importance 

 for absorption. The longer the chain, the more numerous are the excited states. It 

 can be shown that the "center of gravity" of these states in the energy diagram remains 

 more or less unchanged, while the lowest excited states shift closer and closer to the 

 ground state as the chain grows longer. This is shown schematically in figure 21.38. 

 At the left we have the energy diagram of an A2-molecule, at the opposite end, that of an 

 infinite chain of A nuclei; the long- wave absorption limits are represented by the ar- 

 rows; they become shorter and shorter, i. e., the absorption shifts further and further 



to the red, with increasing chain length, until it 

 extends into the infrared. 



MuUiken has shown that, if the chain is a 

 straight as possible {i. e., if all carbon atoms 

 in the chain are in trann positions), the transi- 

 tion to the lowest excited state (arrow in fig. 

 21.38) is more piobable than all the other transi- 

 tions together. This means that the intensity 

 of the absorption band with the lowest fre- 

 quency must increase steadily with increasing 

 chain length. We have thus obtained theo- 

 retical interpretations, both of the gradual shift 

 of the absorption band to longer waves, and of 

 the increase in its intensity with the growing 

 length of the chain. 



Fig. 21.38. Shift of first absorp- 

 tion band with increasing length of 

 conjugated chain (thickness of ar- 

 row indicates intensity). 



The two or three separate maxima 

 observed in carotenoid spectra may 

 mean as many distinct electronic transi- 

 tions; but more probably they correspond to coexcitation of one or several 

 vibrational quanta. The distance between maxima (ca. 1500 cm.~0 is 

 of the order of magnitude of vibrational quanta in organic molecules. 



D. Absorption Spectra of the Phycobilins* 



The absorption spectra of the phj'cobilins have been observed in living 

 algae, in aqueous colloidal extracts of chromoproteids and in organic solu- 

 tions of chromophores. The results are somewhat confused because both 

 phycocyanin and phycoerythrin apparently occiu* in several modifications 

 of slightly different color. (These modifications might be due either to 

 minor variations in the structure of the chromophores, or to the association 

 of the same chromophore with different proteins.) 



The first extensive data on the absorption spectra of the phycochromo- 

 proteids were given by Schiitt (1888). Among the more recent papers on 

 this subject are those of Lemberg (1928, 1930), Svedberg and Lewis (1928), 

 Svedberg and Katsurai (1929), Dhere and Fontaine (1931), Svedberg and 

 Eriksson (1932), Roche (1933), Katz and Wassink (1939) and French and 

 co-workers (1948,1951). 



Bibliography, page 67 1. 



