624 



ABSORPTION SPECTRA OF PIGMENTS 7.V VITRO 



CHAP. 21 



Subsequently, Calvin, Ball and Aronoff (1943) found indications that 

 the two "isomers" whose spectra are shown in figure 21.13 actually l)elong 

 to two different reckiction levels of the porphin system. Thus, in this case 

 at least, the dominant red band was confirmed as an indicator of partial 

 hydrogenation. 



3.0 X 10 



2.5 



2.0 



« 1.5 



H 



50Q 



650 



550 600 



WAVE LENGTH, m^ 



Fig. 21.13. Molar absorption spectra of two tetraphenylporphin "iso- 

 mers" (after Aronoff and Calvin 1943). 



No such explanation can as yet be given to another observation of 

 Aronoff and Calvin — that addition of hydrochloric acid to tetraphenylpor- 

 phin solutions with spectra of the etio type causes a reversible transition 

 to chlorin type (fig. 21.14). Aionoff and Calvin attributed this to salt 

 formation: porphin + 2HC1-^ (porphin H2)++(C1~)2. The addition of 2 

 hydrogen ions thus appears to have the same effect on the porphin spectrum 

 as does the addition of two hydrogen atoms. If this is true, the problem of 

 the spectroscopic difference between prophins and chlorins becomes cognate 



