1804 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



13.0 



FRACTION*ls 

 FRACTION •3^ 



FRACTION *2 



380 



420 



460 



500 



540 



580 



620 



660 



700 



WAVELENGTH M;1 



Fig. 37C.9. Absorption spectra of three fractions of allomerized chlorophyll in ether 

 (after Holt and Jacobs 1954). Red peaks arbitrarily matched in height. 



Livingston, Pariser, Thompson and Weller (1953) measured the ab- 

 sorption spectrum of pheophytin a in methanol in the presence of acids 

 or bases. In acid solution, a reversible conversion of the "neutral" into 

 an "acidic" form was revealed by spectroscopy; in basic solutions, an 

 irreversible conversion into a "basic" form took place. In dilute solutions 

 of strong bases, or in solutions containing weak bases (e. g., aliphatic 

 amines), the conversion into the basic form was gradual. Neutralization 

 of the base changed the spectrum of the basic form, but did not convert 

 it back into that of the neutral (or acidic) form, 



Neuberger and Scott (1952) interpreted the spectral changes, obtained in several 

 porphyrins by pH variation, as evidence of transformation of the free base, P, first into 

 a monovalent cation, PH+ (often around pH 7) and then into a divalent cation, PH2 "*""*■ 

 (often at about pH 4). This would mean that porphjTins are stronger bases than pyri- 

 dine; the authors attributed this increased proton affinity to additional resonating struc- 

 tures that become possible when one or two H + ions are added to the pyrrole nitrogens. 

 Scott (1952) suggested that a similar addition of H+ ions to ring nitrogens accounts for 

 the effect of acidity on tetraphenylporphin spectrum (cf. fig. 21.14), with the peculiarity 

 that the addition of the second H ■•" ion to the ring causes, in this particular case, the ap- 

 pearance of a "chlorin type" spectrum. (The spectrum of the intermediate, low- 

 acidity form seems to be similar to that of the monocations of other porphyrins.) 



Holt and Jacobs (1953) also measured the absorption curve of allo- 

 merized chlorophyll a in ether, and found it very similar to that given by 

 Livingston for the same material in methanol (fig. 21.4A). The main 

 peaks are at 420 m^ (Soret band) and 650 m/i, with minor peaks at 610, 

 567 and 520 mju. The most notable feature of this spectrum is the absence 

 of the "satellite" band on the short-wave side of the Soret band. 



Closer investigation by Holt and Jacobs (1953) confirmed the findings 



