Monomolecular films of bacteriochlorophyll and derivatives at an air-water interface: Surface and spectral properties

Monomolecular films of bacteriochlorophyll, bacteriopheophytin and 2-desvinyl-2-acetyl chlorophyll a were prepared and studied on aqueous subphases containing pH 7.8 buffer and 4·10⁻⁴ M ascorbate. These monolayers are mechanically stable in the dark and light at 15 °C. at surface pressures below about 18 dynes/cm the slope of the surface isotherm of bacteriochlorophyll is steeper than at pressures greater than 18 dynes/cm. The surface dipole moments of bacteriochlorophyll are less than half that reported for chlorophyll a. Compression of bacteriochlorophyll or bacteriopheophytin monolayers result in changes of their absorption spectra.

Compression of bacteriochlorophyll monolayers to 18 dynes/cm results in a shift of the pigment's red peak from 787 to 749 nm as well as the appearance of a new absorption maximum at 896 nm. Continued compression to 24 dynes/cm results in a slight decrease in peak height of the 794-nm maximum and further increase in the absorbance of the 896-nm maximum. With bacteriopheophytin the red maximum at 760 nm starts to shift when the film is compressed to a surface pressure of only 2 dynes/cm; further compression yields a new absorption maximum at 846 nm. Compression of a film of 2-desvinyl-2-acetyl chlorophyll a results in only a 10-nm shift of the absorption maximum at 690 nm.

An orientation of bacteriochlorophyll at an air-water interface is proposed that is different from that for chlorophyll a. Like chlorophyll a bacteriochlorophyll monolayers are closely packed, but different in that bacteriochlorophyll allows greater interaction between pigment molecules. In compressed monolayers bacteriochlorophyll appears to aggregate differently than in other model systems.