358 



Discussion 



DISCUSSION 



The Structure of Haem a and Haem a. 



The Structure of Porphyrin a 



By M. Morrison (Rochester) 



Morrison: Barrett {Nature, Lond. 183, 1185, 1959) and Clezy and Barrett (Biochim. 

 biophys. Acta 33, 584, 1959) recently reported that porphyrin a contains a hydroxy! 

 group that can be acetylated and that this group is present as — CHOH — group 

 adjoining the porphyrin ring. Haemin a also can be acetylated by the procedure of 

 Barrett. It is converted to two products, one with increased chromatographic mobility 

 and the other with decreased chromatographic mobility in non-polar solvents. This 

 acetylation causes a spectral shift of the a-band of the pyridine haemochrome to 

 shorter wavelengths (cf. Table 1). This indicates that an electron-withdrawing group 

 has been removed from resonance with the porphyrin nucleus. The position of the 

 a-peak of the pyridine haemochrome is an excellent index of the number and type of 

 electron-withdrawing groups in resonance with the porphyrin nucleus. Two of these 

 groups are known to be formyl and vinyl. A comparison of haemochrome a (a-peak 

 587 m/0 with Spirographis haemochrome (a-peak 583 m//) and of the haemochromes 

 of their respective oximes (571 m/f and 561 m//) also indicates the presence of a third 

 electron-withdrawing group in addition to formyl and vinyl. It should be noted that 

 the effects of the electron-withdrawing groups except vinyl are lost on reduction with 

 borohydride (cf. Table 1). 



Table 1 



Since the infra-red spectrum of haemin a suggested the presence of another 

 carbonyl group in addition to formyl, one can speculate that the third group is 

 — CH(OH) — CO — R. The effect of this group is abolished by borohydride reduction 

 and by acetylation. Since the formyl and vinyl groups must occupy opposite pyrroles, 

 this group must occupy position 2, and if the formyl group occupies position 8 

 (according to the evidence of Piattelli quoted by Lemberg, p. 357), the formula of 

 Lemberg et a/., p. 350, Fig. 3 (11), but replacing -CHOH— CHj-R, in position 2 by 

 — CHOH-CO — R, can be assumed for porphyrin a. From the molecular weight of 

 chlorohaemin a (880) it can be calculated that the R group has a molecular weight of 

 182 and might represent a 13 carbon saturated alkyl: — CH(OH) — CO — (CH2)i2CH3. 



Clezy: Morrison's formula shows an a ketol conjugated with the porphyrin ring system. 

 It is difficult to reconcile this structure with the dehydration experiments reported in 

 the paper by Lemberg, Clezy and Barrett (this volume, p. 349). Porphyrin a and its 

 derivatives can be dehydrated with />-toluene sulphonyl chloride to a compound 

 showing spectroscopic properties in accord with the introduction of a carbon-carbon 

 double bond conjugated with the porphyrin ring system. This reaction requires at 

 least one hydrogen atom on the carbon atom ^ to the porphyrin ring system. 



Lemberg : The conclusions of the Rochester school on the structure of porphyrin a as 

 reported by Morrison are now in close agreement with ours, with the exception of 



