732 LIGHT AND LIFE 



Much the same relationships involve the artificial isopigments, which 

 have 9-cis retinene as chromophore. On bleaching with light, these 

 too yield as product 2i\\-trans retinene, and again an isomerization 

 would be needed for their resynthesis. 



Mechanism of bleaching. 



The recent work of Hubbard and Kropf has made it clear that 

 these changes in the geometrical configuration of retinene represent 

 more than a curious complication in the reactions of visual excitation. 

 They are in fact the heart of this process. The only thing that light 

 does in any visual system we know is to isomerize retinene. Every- 

 thing else follows from this, by ordinary thermal ("dark") reactions 

 (21, 25, 26, 30) . 



Retinene contains as its only reactive site the terminal aldehyde 

 group ( — HC^O) . There is reasonably good evidence that in form- 

 ing rhodopsin this condenses with an amino group of opsin in a 

 Schiff base linkage: CjoHoyHC^O + HgN-opsin -^ CioHotHC^N— 

 opsin (1) . Such a bond anchors it to the protein, but leaves unex- 

 plained why the geometric configuration of the hydrocarbon portion 

 of the molecule is of such decisive importance. It is clear that what 

 this means is that this shape of retinene is effective because it fits 

 a corresponding portion of the surface of opsin. The point of fitting 

 is that it permits a sufficiently intimate approach of the side-chain 

 and ring of retinene to opsin to make possible strong interactions 

 between them: the operation of van der Waals' forces and some de- 

 gree of charge-displacement (21) . 



This interaction between 11-m retinene and opsin is illustrated dia- 

 grammatically in Fig. 4 (26) . At the left, we see rhodopsin, the alde- 

 hyde group of retinene having been joined to an amino group of 

 opsin in a Schiff base linkage, the hydrocarbon portion of the retinene 

 fitting closely the structure of opsin. Actually a Schiff base linkage, 

 in itself, rather than producing the enormous shift in spectrum be- 

 tween retinene with its absorption maximum at about 380 ni/i, and 

 rhodopsin with Amax 500 m^x,, would have resulted in a loss of color. 

 In order to account for this shift, it must be assumed that the Schiff 

 base linkage is protonated (i.e., has attached an H + ) , so introduc- 

 ing a charge into the conjugated system which results in a large en- 

 hancement of spectrum and shift toward the red (Ci9H^,7HCr:rNH+- 

 opsin) . Even this, however, is inadequate, for such protonated Schiff 

 bases of retinene ordinarily have Amax about 440 m/x. To bring the 

 spectrum still further toward the red, it is assumed that a negative 



