Red shift of absorption maxima in chlorobiineae through enzymic methylation of their antenna bacteriochlorophylls

The bacteriochlorophyll d producing photosynthetic green sulfur bacteria Chlorobium vibrioforme forma thiosulfatophilum strain NCIB 8327 and C. vibrioforme strain B1-20 respond to reduced light conditions in culture by performing methylations at the 4- and 5-substituents, for example, converting the 4-Et into 4-n-Pr, 4-i-Bu, and even 4-neoPn. During this process, the absorption maximum in living cells of C. vibrioforme strain B1-20 red shifts from 714 to about 728 nm. Eventually, the C. vibrioforme forma thiosulfatophilum strain NCIB 8327 culture carries out a delta-methylation to produce the bacteriochlorophylls c (lambda max ca. 750 nm); the new UC Davis bacteriochlorophyll c culture is named C. vibrioforme forma thiosulfatophilum strain D. It is possible that the homologation process increases hydrophobic interactions between individual BChl molecules, giving rise to larger aggregates in the antenna system. Alternatively, the additional methyl units attached to the 4-position shift the absolute configuration of the 2-(1-hydroxyethyl) group from pure R in the case of 4-Et to pure S in the case of 4-neoPn, which in turn might determine the size of the in vivo aggregates due to the intrinsic nature of the pigment protein system. It is suggested that the bacteriochlorophylls c from Chloroflexus aurantiacus strain J-10-fl and the bacteriochlorophylls e from Chlorobium phaeovibrioides might have undergone similar meso methylation as a response to external environmental pressure such as low light intensity.