ENERGY EXCHANGE IN PHOTOREACTIONS 57 



acid residues, of which there are two and four, respectively, in one mole- 

 cule of myoglobin (Wyman, 1948). If these residues are randomly dis- 

 tributed in the protein, some mechanism must act to convey at least 

 15 per cent of the energy of each absorbed quantum to the heme group. 

 Franck and Livingston (1949) have suggested that this mechanism is a 

 direct transfer of electronic quantum through field coupling a la Forster 

 (Sect. 4-3). On the other hand, if the absorbing residues are localized in 

 the immediate neighborhood of the heme, energy transfer may be accom- 

 plished by the migration of vibrational quanta. Under the latter cir- 

 cumstances, Weiss's mechanism (Sect. 4-3) is also a possibility. Other 

 direct processes for the transfer of electronic quanta are probably dis- 

 allowed by the fact that the amino acid residues show the same spectra 

 in and out of the protein, thus indicating the absence of strong electronic 

 coupling among them. 



Szent-Gyorgyi (1947) has discussed a series of experiments carried out 

 by his coworkers in which dried gelatin suspensions of various dye mole- 

 cules, such as eosin W and rhodamine B, manifested phosphorescence 

 and a severalfold increase in electrical conductivity on illumination. 

 Phosphorescence was observed when gelatin was replaced by other pure 

 proteins. This behavior is usually associated with the existence of bands 

 of upper, unfilled electronic states characteristic only of tightly bound 

 crystals. Jordan (1938) and Moglich and Schon (1938) have proposed 

 that these bands occur in single molecules of viruses, genes, and proteins. 

 On the usual theory of photoconduction (Seitz, 1940), electronic energy 

 would be made available at most parts of the protein by electron migra- 

 tion following excitation to a higher-lying band. There does not appear 

 to be any reason to expect these bands in protein molecules, which depend 

 for much of their configuration on weak hydrogen bonds and van der 

 Waals' interactions. Similar considerations mediate against an expla- 

 nation based on exciton migration or any other of the special processes 

 we have discussed. 



The final example of possible anomalous behavior involves electron 

 migration and energy transfer among the various cytochromes. The 

 action of these pigment-protein substances as intermediate catalysts and 

 oxidizing-reducing agents in oxidative metabolism depends on their 

 mutual linkage in a well-ordered secjuence. Since the cytochromes are 

 frequently found rigidly suspended in particles of inactive material and 

 since the heme group of at least one member of the sequence, cytochrome 

 C, appears to be buried in protein, as determined by its lack of chemical 

 reactivity, the question has been raised as to the energy linkages that 

 make functioning possible (Evans and Gergely, 1949; Geissman, 1949). 

 If the need for a special mechanism continues to be demonstrated by 

 further experiments, quite possibly the following explanation suggested 

 by Geissman's work may apply: Suppose there exists a complete chain of 



