New proposal for structure of special-pair chlorophyll

Abstract

A new model is proposed for the structure of the special pair of chlorophyll a molecules believed to correspond to the P700 species in green plants and algae. The proposed model, although admittedly speculative, is based upon exciton-theoretical considerations and on in vitro infrared and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, two chlorophyll a molecules are held together by (a) (Fig. 1 of ref. 13), the two Chl a molecules are held together by two water molecules, each of which coordinates to the Mg of one Chl a molecule and simultaneously hydrogen bonds to the carbomethoxy C=GO of the other Chl a molecule. The entire structure has C2 symmetry. We estimate from molecular models that the separation between the macrocycle planes in the Fong structure is approximately 5.6 A; thus, the 7r-systems of the two macrocycles in this model are also not in contact. In the Fong model the ring V keto C=O is not bound or used in special pair formation in any way.In the present paper we propose a new model for the structure of Chls based upon exciton-theoretical considerations and on the in vitro visible absorption and infrared spectra of a 700 nm absorbing Chl a-ethanol adduct. 700 nm absorbing chlorophyll a-ethanol adduct Brody and Brody (14) observed the formation of about 700 nm absorbing Chl a species in ethanol that fluoresces at about 720 nm. We have taken a new look at this species because of the close similarity between the position of its major visible absorption peak and the position of the major absorption peak of P700 in vivo. The visible absorption spectrum of a 0.094 M solution of Chi a in dry toluene with a 1.5 molar ratio of ethanol (about 0.14 M ethanol) at 298 K, 273 K, 247 K, 224 K, and 175 K is shown in Fig. 1. As the temperature is lowered there is a shift from a predominance of a species that absorbs near 670 nm to a species that absorbs near 700 nm. The C=O stretch region of the infrared spectrum of a 0.064 M Chl a solution in dry toluene with a 1.5 molar ratio of ethanol (about 0.096 M ethanol) at 298 K (spectrum A) and 183 K (spectrum B) is shown in Fig. 2. At the higher temperature, the 670 nm species predominates, and at the lower temperature, the 700 nm species predominates. As the temperature is lowered, there is very little change in the area of the ester C=O absorption, and the ester peak shifts only slightly from 1737 cm-1 to 1733 cm-1. On the other hand, the transformation of the 670 nm 700 nm species

Protein/lipid interactions in phospholipid monolayers containing the bacterial antenna protein B800–850

“…As the transition is electronically allowed, its transition dipole moment is not sensitively dependent on the environment of the fluorophore group [19]. This also holds for the dimer if the transition originates from the lower of the two energy levels that result from the electronic interaction of the monomer excited states [20,21]. As this argument is valid in the present situation the transition dipole moment quite probably remains unaffected by a pressure change.…”

Structural studies of the primary donor cation radical P 870 +· in reaction centers of Rhodospirillum rubrum by electron-nuclear double resonance in solution

“…5), which is similar to models proposed earlier (46,47,49), is characterized as follows. (i) The equivalence of the two halves of the dimer is assured by adopting a model with C2 symmetry.…”

Linear dichroism of light‐induced absorbance changes of reaction centers of Rhodospirillum rubrum

“…aEnvironmental-shift [22] of 480 cm-' bEnvironmental-shift of 620 cm-' 26 debye' and 12 debye' for the transitions at 870 nm, 800 nm, 760 nm and 810 nm, respectively, in reaction centers. The energy of the exciton splitting in the BChl-800 dimer is about t77 cm-' and the ratio of esOO/esle z 5 corresponds to an angle of 45" between the dipole-vectors of monomers.…”

“…. .H-O---Mg interactions [22] . In this structure if the distance between the molecule centers is about 5.34 A and the separation between the macrocycle planes is about 3.4 A, the exciton splitting energy is -780 cm-'.…”