Preparation and Properties of Monomolecular Films of Chlorophyll a and Pheophytin a

Abstract

Monomolecular films of chlorophyll a and pheophytin a on aqueous subphases have been prepared and studied. The preparation and preservation of these pigments in a state of high purity has required procedural refinements which are described. Chemical degradation in solution and in the monolayer can be prevented by appropriate precautions. At surface pressures below 22 dyn/cm for chlorophyll and 10 dyn/cm for pheophytin, the films on pH 8.0 buffer are mechanically stable in the dark at 20°C. At these pressures, there is little difference in the force—area curves of chlorophyll and pheophytin, although the surface potentials and surfacedipole moments of the two pigments are markedly different. The visible absorptionspectrum of the pigment monolayers on the aqueous subphase is rapidly altered by illumination, although under optimum conditions it shows little change in the dark over periods of several hours. These experiments were performed with a monolayer spectrometer, which permits measurements of the absorption spectra of monolayersin situ. This instrument is briefly described. The unified set of quantitative measurements of this study, together with previous observations in the literature, provide evidence for a monolayer structure in which the pigment molecules, with a folded configuration (both porphyrin plane and phytol sidechain rising above the water surface), are closely packed but randomly oriented in the two‐dimensional array. There is no evidence for any marked rearrangement on compression under our conditions. Anomalies in the absorption spectra are observed when pheophytin monolayers are compressed beyond the stable limit.