6 - Visible Absorption and Fluorescence of Chlorophyll and Its Aggregates in Solution

Abstract

This chapter reviews visible absorption and fluorescence of chlorophyll, and its aggregates in solution. The high concentration of chlorophyll in the photosynthetic apparatus, and the difference in shape between the absorption spectra of chlorophylls in dilute solution and in vivo, have motivated investigations on effect of high pigment concentration on spectral properties. If a solution of Bchl or chlorophyll in methanol or acetone is diluted with water, an opalescent, nonfluorescent colloid forms. The absorption spectrum differs from the spectrum in solution, especially with Bchl. With chlorophyll a, no formation of a second long wavelength band is usually found, but the absorption spectrum of a colloidal chlorophyll a suspension may vary to a certain extent with conditions of preparation, purity, and time. The mechanism resulting in the spectral shifts related to the water and dioxane colloids might be a dispersion of unordered particles into more regular aggregates. The decrease in scattering and apparent increase in absorption coefficient may be caused by a decrease of the sieve effect, because of splitting up of large particles. Dioxane probably effects polymerization via the central Mg or Zn atom.