Cooperative polymerization of photosynthetic pigments in formamide-water solution

The aggregation of bacteriochlorophyll a and bacteriopheophytin a into large oligomers with maximum optical absorption at 860 nm was studied in a 3:1 (vol/vol) formamide/water solution, using optical absorption spectroscopy and electron microscopy. The aggregation is cooperative and proceeds according to two equilibrium constants. Initially, two pigment molecules form a “seed” that absorbs at ≈860 nm. The equilibrium constant, Ka, governing this reaction equals 1.3×103 M-1 in the case of bacteriochlorophyll a (due to experimental limitations, Ka for bacteriopheophytin a could not be determined). The addition of monomers to aggregates consisting of two or more units is governed by an equilibrium constant, Kb, equal to 2.2×106 M-1 for bacteriochlorophyll a and ≈ 109 M-1 for bacteriopheophytin a. The enthalpy and entropy changes that drive the bacteriochlorophyll oligomer formation are -9.25 and ≈0.0 kcal/mol, respectively. Above a threshold concentration, the amount of oligomers remains constant but their length continues to increase. Each oligomer appears to consist of dimers that are associated by hydrophobic interactions among their alcohol residues, forming long strands. Single strands presumably coil into helices that are seen as cylinders. The bacteriochlorophyll a oligomers form cylinders with a constant diameter of 150Å and an average length of 2,000Å (at 1.5×10-5 M bacteriochlorophyll a). These cylinders contain 200–250 bacteriochlorophyll a dimers. The bacteriopheophytin oligomers coil into wider cylinders (≈400Å in diameter) which contain ≈600–700 bacteriopheophytin a dimers. In both cases, the separation between the dimers is ≈20Å. At such distances, the dipolar interactions among adjacent dimers are negligible and do not affect the optical absorption of each individual pair. Therefore, the optical absorption of these pairs can be a tool for investigating the absorption pattern of photosynthetic pigments in vivo