PORPHINS, CHLORINS, PHORBINS, PHYTINS, PHYLLINS 



445 



as the hydrogenation product of chlorophyll a in the form 16. Ill B. 

 Bacteriochlorophyll can easily be oxidized, with the removal of the two 

 excess hydrogen atoms in positions 3 and 4, giving 2-acetyl-chloro- 

 phyll a. Fischer and Stern (1940) suggested that this compound, with 

 a structure intermediary between bacteriochlorophyll and ordinary 

 chlorophyll, may be identical with hacterioviridin, the natural pigment of 

 green sulfur bacteria. 



CH3 



CO 



CH, 



H,C— Ci 



/'\/ \ 



^5s 



HC 



\ 

 / 



\ 



K^g 



\ 



\ 



,C8 



4C 



H2C=CH 



H 

 .0^ S-. 



CH, 



I ' 

 C 



H3C — c. 



/VV\ 



C— C2H5 



..x^\ 



H CH, I 10 



/ HC- 



CH, 



/\y 



5C— CH, 



\ I I / 



HC Mg XH 



\ \ / 



.C"^N N C 



II ^ 



H,C — C. 



C — CH, 



0=C 



CH, HC- 



o=c 



O 



I 



O CH3 



Formula 16.IV. 

 Bacteriochlorophyll structure 

 according to Hans Fischer. The 

 routing of the all-round conjugated 

 system is fixed by the hydrogenation 

 of a second nucleus (nucleus II, 

 positions 3 and 4). 



CH, 

 I ^ 



o=c 



I 



o 



I 



x 



o=c 



CH, 



^20*^39 



Formula 16. V. 

 Protochlorophyll structure after 

 Hans Fischer. (One of six possible 

 structures distinguished by the routing 

 of the conjugated double bond system; 

 the structure represented here is the 

 one derived from structure B in for- 

 mula 16.III). 



Protochlorophyll, whose possible role in the synthesis (or decomposi- 

 tion) of chlorophyll in nature was discussed in chapter 15 (page 404) is, 

 according to Fischer (1940), an oxidation product of chlorophjdl a, with 

 the two hydrogen atoms in positions 7 and 8 removed, as shown by 

 formula 16.V. 



4. Porphins, Chlorins, Phorbins, Phytins, and Phyllins 



Protochlorophyll, chlorophyll, and bacteriochlorophyll belong to 

 three successive reduction levels of Kiister's system. It will be noted that 

 the eighteen-membered, conjugated ring system is maintained in all 

 three of them. The parent substances of these three groups are porphin, 

 dihydroporphin, and tetrahydroporphin. The first has been prepared, 

 synthetically by Fischer (in 1935) and by Rothemund (1936). 



