CONTROL OF MOLECULAR ARRANGEMENT IN LANGMUIR-BLODGETT-FILMS OF CHLOROPHYLL-ALPHA PREPARED IN VARIOUS GAS PHASES STUDIED BY ULTRAVIOLET-VISIBLE AND INFRARED SPECTROSCOPIES AND ATOMIC-FORCE MICROSCOPY

Attenuated total reflection (ATR)/Fourier-transform infrared (FT-IR) s pectra were measured for one-monolayer Langmuir-Blodgett (LB) films of chlorophyll a (Chi-a) fabricated in an argon (Ar) atmosphere. The fre quencies of C=O stretching bands and marker bands for the coordination number of the central Mg atom suggest that Chi-a takes a five-coordin ated dimer in the films prepared in the Ar atmosphere. Ultraviolet-vis ible (UV-vis) as well as IR spectra were obtained for multilayer LB fi lms of Chi-a prepared in air, Ar, nitrogen (N-2), and oxygen (O-2) atm ospheres. In the cases of the multilayer LB films, spectral features i n the C=O stretching band region suggest that Chi-a exists as a monome r in the film prepared in air while it assumes a dimer in the films pr epared in the Ar, N-2, and O-2 atmospheres. The marker bands for the c oordination number of the Mg atom in the IR spectra indicate that Chi- a is in a five-coordinated state. These results imply that one can con trol the molecular arrangement in the LB films of Chi-a by changing th e atmosphere in which they are prepared. Both the UV-vis and IR spectr a of the LB films fabricated in the Ar, N-2, and O-2 atmospheres are a lmost identical to each other. This means that O-2 does not affect the stability and structure of Chi-a in the monolayer on the aqueous subp hase. 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-de pendent IR spectral changes for the LB films. The results indicate tha t the dimer of Chi-a changes into pheophytine a (Phe-a) below pH 6.0 a nd 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 Chi-a molecule from coo rdinating to it.