Spectral Properties of Chlorophyll a Monolayers in the Presence of an Exogenous Electron Donor and Acceptor

Abstract

Chlorophyll a monolayers are studied at a nitrogen-water interface in the presence of a reducing or oxidizing agent : sodium ascorbate and benzyl viologen, respectively. Absorption spectra of the films are measured directly on the aqueous surface. With the aid of a computer, fourth derivative and difference spectra are determined. In the presence of ascorbate, a bathochromic shift of the absorption maximum to 693 nm can be induced as compared to 683 nm for a chlorophyll monolayer without any additives. In the presence of ascorbate, chlorophyll species at 676, 712 and 750 nm (present in a pure chlorophyll monolayer) are decreased or diminished. Illumination causes no change in the position of these absorption maxima; however, there is an increase of the absorbance of the main red absorption band.In the presence of benzyl viologen there is a hypsochromic shift of the red absorption maximum to 679 nm. Chlorophyll species at 670, 694, 712 and 740 nm (present in pure chlorophyll monolayers) are decreased or diminished upon addition of benzyl viologen. Upon illumination, there is a decrease in absorbance at 686 nm. It appears that the redox reagents induce the formation of specific chlorophyll aggregates, in the interfacial system, which might be analogous to the various chlorophyll species observed in green plant photosynthesis.The chloroplast thylakoid has been described as containing a monomolecular array of chlorophyll interfaced with endogenous redox reagents [l]. Therefore, studies of chlorophyll monolayers at various interfaces serve as a useful model to investigate interactions between chlorophyll and redox reagents in vivo which are normally immiscible in the same solution.Absorption spectra of chlorophyll monolayers have been reported on aqueous and solid surfaces [2 -41. Spectral properties of chlorophyll and pheophytin monolayers have been described in an earlier work [5]. By varying the chlorophyll concentration (or surface pressure), it was shown by difference and fourth-derivative absorption spectra, that species are formed in the monolayer system with spectral properties similar to those detected in vivo [6].In this paper we examine the effects of benzyl viologen and sodium ascorbate on the spectral properties of the chlorophyll monolayer. Benzyl viologen and ascorbate serve as artificial electron acceptor and donor, respectively, for photosystem I.

A Quantitative Comparison of Chlorophyll Bilayers Formed With and Without Solvent

“…Various aggregates of chl have been reported in compressed monolayers (Hirsch and Brody, 1978). Perhaps similar aggregates occur in solvent-free bilayers.…”

A tribute to Seymour Steven Brody: in memoriam (November 29, 1927 to May 25, 2010)

“…We concluded that in the monolayer, compression gives rise to various chlorophyll aggregated species (Hirsch and Brody 1979). The amount and specific chlorophyll species could be further induced by compression in the presence of reducing or oxidizing agents, with implications of chlorophyll orientation and complexation (Hirsch and Brody 1978.…”