Spectroscopic properties of bacteriochlorophyll c in Langmuir monolayers in the absence and presence of amphiphilic peptides

Abstract

The spectroscopic properties of bacteriochlorophyll (BChl) c alone, and together with synthesized, amphiphilic peptides, were measured in a Langmuir film at the air–water interface. Analysis was done using surface pressure–area isotherms, Brewster angle microscopy and absorption in situ. The BChl c alone, after spreading and compressing at low surface pressure, showed the typical absorption pattern for the monomeric form. The pigment aggregates that absorbed at 731 nm were formed on the water subphase when the isotherm of BChl c reached the kink point around 13 mN/m and the average area occupied by pigment molecule was smaller than the area of a porphyrin ring. The effect of the amphiphilic peptides on BChl c Langmuir film was also examined by injection of peptides beneath the pigment monolayer compressed at a certain pressure. The mixed aggregated pigment–peptide complex prepared at 20 mN/m, showed optical properties similar to those of chlorosomes with the Q y absorption band at 739 nm. A possible structure of this complex is discussed based on the above results. In this report, we show that the results obtained by applying the spectroscopic methods in situ provide a new approach to study the chlorosome-like structure from the in vitro two-dimensional model preparation.