Analysis of the red absorption band of chlorophyll ain vivo

A precise and sensitive integrating spectrophotometer has been constructed in the shape of a dodecahedron with one photoelectric cell on each side. With the help of this instrument and a computer, the red chlorophyll $\text{a}$ absorption band of algae and chloroplasts was resolved (after subtracting the chlorophyll $\text{b}$ band) into two Gaussian components, with peaks at 668 and 683 nm^★★. The half-width of the two-band envelope is 32 nm; the half-width of each component, about 18 nm. In the blue-green alga, Anacystis and the red alga, Porphyridium (both containing no chlorophyll $\text{b}$), the two-component bands seem to be in the same positions, but are considerably wider. (However, preliminary analysis suggests that the red band in Anacystis can be interpreted instead as the sum of three components—two belonging to chlorophyll $\text{a}$, and a third one probably due to allophycocyanin.) The relative heights of the two chlorophyll $\text{a}$ components vary, in all plants used, only between 0.7 and 0.9, the 668-nm band always being the weaker one.

Broadening of chlorophyll $\text{a}$ absorption curves by the so-called “sieve effect” may to some extent change the analysis presented here, by causing the component bands in vivo to deviate from the Gaussian shape; this effect calls for further investigation but is unlikely to affect the qualitative conclusions.

A comparison of the absorption spectrum so analyzed with that of the “Pigment systems I and II” (Duysens, Frenchet al.) suggests that in Chlorella, a large portion of chlorophyll $\text{a}$ 668 nm belongs, together with a large part of chlorophyll $\text{b}$, to System II, while a large part of chlorophyll $\text{a}$ 683 nm must be identified with System I, although some of it probably belongs to System II. The simple identification of chlorophyll $\text{a}$ 668 nm with System II, and chlorophyll $\text{a}$ 683 nm with System I, as previously suggested, appears to be untenable. In red and blue-green algae, larger parts of both chlorophyll $\text{a}$ 668 nm and chlorophyll $\text{a}$ 683 nm seem to belong to System I.