A viewpoint: Why chlorophyll a?

Abstract

Chlorophyll a (Chl a) serves a dual role in oxygenic photosynthesis: in light harvesting as well as in converting energy of absorbed photons to chemical energy. No other Chl is as omnipresent in oxygenic photosynthesis as is Chl a, and this is particularly true if we include Chl a(2), (=[8-vinyl]-Chl a), which occurs in Prochlorococcus, as a type of Chl a. One exception to this near universal pattern is Chl d, which is found in some cyanobacteria that live in filtered light that is enriched in wavelengths >700 nm. They trap the long wavelength electronic excitation, and convert it into chemical energy. In this Viewpoint, we have traced the possible reasons for the near ubiquity of Chl a for its use in the primary photochemistry of Photosystem II (PS II) that leads to water oxidation and of Photosystem I (PS I) that leads to ferredoxin reduction. Chl a appears to be unique and irreplaceable, particularly if global scale oxygenic photosynthesis is considered. Its uniqueness is determined by its physicochemical properties, but there is more. Other contributing factors include specially tailored protein environments, and functional compatibility with neighboring electron transporting cofactors. Thus, the same molecule, Chl a in vivo, is capable of generating a radical cation at +1 V or higher (in PS II), a radical anion at -1 V or lower (in PS I), or of being completely redox silent (in antenna holochromes).

Chlorophylls, Symmetry, Chirality, and Photosynthesis

“…Further critical differences in the biophysical dynamics of these two reaction centers are apparent. Firstly, that the primary donors are believed to be "accessory" chlorophylls, specifically ChlD1 in PS II [215] and either Chl AA or -AB in PS I [216], contrary to the situation in reaction centers, even so, the photo-generated cation radicals are localized to the both P680 and P700 after initial charge-separation [217]. Additionally, as hinted by these identifications, electron transfer is not as asymmetric in PS I when compared to the reactions centers from the purple bacteria, although this feature is retained by PS II [217].…”

Novel chlorophylls and new directions in photosynthesis research

“…In general, oxygenic photosynthesis driven by Chl a and Chl b (not red-shifted chlorophylls) has long wavelength absorbance spectra extending to limits of~700 nm due to the high energy requirements of splitting water and oxygen production (Björn et al 2009). Removal of electrons from water requires powerful oxidative potential and hence the presence of P680 (E m = 1.23 V) is vital for oxygenic photosynthesis (Dau and Haumann 2008), although the long wavelength light has an excited state redox potential that is sufficiently negative to power the reduction of the primary electron acceptor, such as anoxygenic (non-oxygen evolving) photosynthesis driven by bacteriochlorophylls (Blankenship and Prince 1985).…”

“…Chl a was thought to be the only chlorophyll that can generate enough energy to split water and evolve oxygen as a by-product of oxygenic photosynthesis (Björn et al 2009). It was thought Fig.…”

Govindjee at 80: more than 50 years of free energy for photosynthesis

“…Our joint publications deal with intriguing questions: Why chlorophyll a (Björn et al 2009a)? How did oxygenic photosynthesis evolve (Björn and Govindjee 2009)?…”