Spectroscopic properties of crystals and monolayers of chlorophyll and related compounds

Abstract

Microcrystals of ethyl chlorophyllides and pheophorbides a and b, and of methyl bacteriochlorophyllide, have been prepared and their absorption spectra measured. The main long-wave bands are shifted toward longer waves by 1–2 × 10 3 cm. −1 compared to their (extrapolated) position in free molecules ( i.e., by 40–80 mμ from their position in organic solvents). The shift is a function of the size of the microcrystals, reaching “saturation” in crystals about 0.5 μ in diameter. An only slightly smaller shift is observed in “crystalline” monolayers, indicating that the interactions responsible for the shift occur mainly in one crystallographic plane. In “liquid” monolayers, the shift is much smaller—similar to that in amorphous colloidal solutions. A theory of the band shift is given, based on electrostatic interaction in an isotropic array of (virtual) dipoles, created by light absorption (without consideration of the overlapping of eigenfunctions and consequent resonance effects). The absolute value of the maximum shift in “large” microcrystals, and, in particular, the shape of the curve showing the dependence of the shift on crystal size, support the assumption of a predominantly two-dimensional interaction. The significance of these results for the hypothetical chlorophyll monolayers in vitro and the possible migration of excitation energy in them is discussed. Absence of fluorescence suggests that migration must be very restricted—if at all existent—in crystals and crystalline monolayers; it could be more extensive in noncrystalline monolayers. The existence of monolayers of the latter type is compatible with the absorption spectrum of chlorophyll in vivo.