Control of Molecular Arrangement in Langmuir−Blodgett Films of Chlorophyll a Prepared in Various Gas Phases Studied by Ultraviolet−Visible and Infrared Spectroscopies and Atomic Force Microscopy,

Abstract

Attenuated total reflection (ATR)/Fourier-transform infrared (FT-IR) spectra were measured for one-monolayer Langmuir−Blodgett (LB) films of chlorophyll a (Chl-a) fabricated in an argon (Ar) atmosphere. The frequencies of CO stretching bands and marker bands for the coordination number of the central Mg atom suggest that Chl-a takes a five-coordinated dimer in the films prepared in the Ar atmosphere. Ultraviolet−visible (UV−vis) as well as IR spectra were obtained for multilayer LB films of Chl-a prepared in air, Ar, nitrogen (N2), and oxygen (O2) atmospheres. In the cases of the multilayer LB films, spectral features in the CO stretching band region suggest that Chl-a exists as a monomer in the film prepared in air while it assumes a dimer in the films prepared in the Ar, N2, and O2 atmospheres. The marker bands for the coordination number of the Mg atom in the IR spectra indicate that Chl-a is in a five-coordinated state. These results imply that one can control the molecular arrangement in the LB films of Chl-a by changing the atmosphere in which they are prepared. Both the UV−vis and IR spectra of the LB films fabricated in the Ar, N2, and O2 atmospheres are almost identical to each other. This means that O2 does not affect the stability and structure of Chl-a in the monolayer on the aqueous subphase. On the basis of the postulate that CO2, which exists only in air, caused a change in pH of the aqueous subphase, we investigated pH-dependent IR spectral changes for the LB films. The results indicate that the dimer of Chl-a changes into pheophytine a (Phe-a) below pH 6.0 and that the monomer species do not exist through pH 6.0−4.0; therefore, it is unlikely that CO2 changes the pH. Probably CO2 coordinates to the central Mg atom as a fifth ligand in the LB films prepared in air, preventing the keto carbonyl group of another Chl-a molecule from coordinating to it.