Abstract
A MARKED, reversible effect of water on the absorption spectrum of crystalline chlorophyll a in ‘Nujol’ mull has recently been observed1. Thin films of such mulls, dried by prolonged evacuation or standing over phosphorus pentoxide, absorb at 672 nm, but this absorption shifts to about 743 nm when the film is exposed to water, or if the chlorophyll contains water of crystallization. The infrared spectra of the wet and dry films are correspondingly different1. The 743 form shows only a single carbonyl absorption at 1,647 cm−1, similar to the band assigned to an “associated” carbonyl at the C(9) position in dissolved chlorophyll2, while the 672 species exhibits an additional infra-red peak at 1,698 cm−1, which has been assigned to a “free” carbonyl. The intensity of this band is about two-thirds that of the 1,647 cm−1 peak. This suggests2 that the 672 form contains dimers or other stoichiometrically associated forms. The differences in spectrum (both red and Soret bands) between the 672 species and the chlorophyll a dimer definitely identified in solution3 may be attributable to effects of the medium or to exciton interactions in the solid phase. It is remarkable, however, that dimers in non-polar solvents are disaggregated by addition of small concentrations of Lewis bases (ethanol, pyridine, water)2,4, whereas in these chlorophyll crystal mulls, moisture increases the extent of dye interaction, as indicated by the bathochromic shift of the optical spectrum and, presumably, by the disappearance of the “free” carbonyl band. The water effect in solid chlorophyll has therefore been studied further, particularly to determine whether the 672–743 transition is sharp or whether intermediate forms can be obtained. Such intermediate forms are reported here.
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SHERMAN, G., LINSCHITZ, H. Chlorophyll – Water Interactions in the Solid State. Nature 215, 511 (1967). https://doi.org/10.1038/215511a0
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DOI: https://doi.org/10.1038/215511a0
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