20 PROBLEMS IN PHOTOSYNTHESIS 



free electrons (excess electrons) are liberated and migrate through the 

 crystal. Conductivity is then produced by these negative carriers of current. 

 In thep type semiconductor there is an electron deficit. The deficit electrons 

 can be considered to be positively charged "holes" which act as current 

 carriers. In the ?i type semiconductor a current of electrons proceeds from 

 a negative to a positive pole, whereas in the p type semiconductor a current 

 of "holes" proceeds from a positive to a negative pole. Figure 6 shows the 

 principle of a type of transistor. Three zones A, B, and C can be distinguished. 

 A and C are n-conductive and B is/?-conductive. The conducting electrons 

 produced in B migrate through A and the resulting "holes" in B are occupied 

 by electrons coming from C. 



Fig. 6. The principle of a transistor. 



Szent-Gyorgyi (50, 51) pointed out the possibility that proteins may have 

 an electronic structure analogous to that of semiconductors. The theoretical 

 considerations of Evans and Gergely (19) supported this hypothesis. Eley 

 et al. (18) found semiconductivity in plasma albumin, fibrinogen and edestin. 

 In their experiments, however, the proteins used underwent such drastic 

 treatment that the data obtained cannot be applied to native proteins. 

 According to Vladimirov and Konev (57), there is insufficient evidence of 

 semiconductivity in proteins. 



The statement that energy can migrate through a protein molecule is not 

 new. It is well known that light energy can be absorbed in one part of a 

 molecule and provoke chemical change in another part of the same molecule. 

 This has been proved by Warburg and Negelein (59, 61 ) for the CO-compound 

 of the iron oxygenase (cytochrome oxidase). With the quantum require- 

 ment equal to 1, CO is photochemically dissociated from the iron of the heme 

 part, though the light energy is absorbed by tryptophan and tyrosine in 

 the distant protein part of the enzyme. Biicher and Kaspers (10) found a 

 similar behavior in the photodissociation of CO-myoglobin (see also § 25). 



Besides the semiconductor hypothesis, energy transfer by resonance seems 

 very probable. According to this hypothesis, the energy of an excited oscil- 

 lator is electrodynamically transferred to neighboring oscillators. Such 

 oscillators showing resonance coupling can be atom groups in the same 

 protein molecule or two molecules of different substances oscillating at the 

 same frequency. The energy donors must be fluorescent at frequencies which 

 the energy acceptor is able to absorb, or in other words, the fluorescence 

 spectrum of the energy donor must overlap the absorption spectrum of the 

 energy acceptor. Furthermore, donor and acceptor should be sufficiently 



