COMPLEXATION OF CHLOROPHYLL-A AND CYTOCHROME-C IN MONOLAYER AT AIR-WATER-INTERFACE

The chlorophyll (chi) alpha-cytochrome (cyt) c mixed system was studie d in Langmuir films at the air-water interface and Langmuir-Blodgett f ilms. Surface pressure-molecular area isotherms measured for pure and mixed components were discussed in terms of ideal mixing processes. Th e negative deviations from ideality observed at cyt c molar fractions lower than 0.1 can be interpreted as a result of pigment-protein compl exation. On the other hand, at protein molar fractions higher than 0.1 , progressive positive deviations were observed and could be due to cy t c aggregation in the mixed system. Furthermore, the spontaneous comp lexation of the chi a and cyt c (in solution) was observed in hydrated ethanol (water:ethanol 97:3). In this case, the protein molar fractio n was found to be 0.2. Langmuir-Blodgett films of the chlorophyll-prot ein mixtures were also analyzed using absorption, Fourier transform in frared, and fluorescent spectroscopic techniques. The infrared spectro scopy showed that at protein molar fractions higher than 0.1, cvt c mo lecules undergo drastic conformational changes from alpha-helix to bet a-sheet and turn structures. Such conformational changes are interpret ed in terms of protein aggregation and denaturation. Spectroscopic evi dence has also indicated the participation of the keto group in the ch lorophyll-protein interaction and the presence of chlorophyll molecule s in an aggregated form in the complexes. (C) 1995 Academic Press, Inc .