| Abstract: | Monomolecular layers of chlorophyll a at the air-water interface were investigated by absorption spectroscopy and simultaneous thermodynamic measurements. Whereas at temperatures near 20°C at all pressures, only a liquid phase exists, at a temperature of 4°C, a liquid-crystalline phase transition is observed at a surface pressure of 5 dynes/cm. Pressure-induced changes in the chlorophyll arrangement become evident from a drastic change in the absorption spectra, accompanying the phase transition. The crystalline phase exhibits an extremely narrow absorption band (halfwidth below 9 nm) centered at 698 nm, indicative of a coplanar chlorophyll arrangement in a well-ordered environment. It is highly probable that in these model membranes a chlorophyll arrrangement could be established that is equivalent to the one proposed for the reaction center P700. |
