Calculations of Bacteriochlorophyll g primary donors in photosynthetic Heliobacteria. How to shift the energy of a phototrap by 2000 cm{sup -1}

Journal Article · Thu Apr 02 04:00:00 EST 1992 · Journal of Physical Chemistry

DOI:https://doi.org/10.1021/j100186a031· OSTI ID:263818

Thompson, M A; Fajer, J ^[1]

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1. Pacific Northwest Laboratory, Richland, WA (United States)

Heliobacteria are a recently uncovered class of photosynthetic bacteria comprised of a novel chromophore, bacteriochlorophyll (BChl) g. Only the substitution of a vinyl; group for an acetyl group on ring I distinguishes the macrocycle of BChl g from that of the more common BChl b. The different substituents impart small differences of 30 nm or {approx} 500 cm{sup -1} in the Qy transitions of the chromophores in vitro but result in 2000-cm{sup -1} differences in the energies of the primary donors in reaction centers containing BChls b (960 nm) and BChls g (800)nm. INDO/s calculations are presented that consider whether this large spectral shift reflects a different mode of aggregation or architecture for the primary donor in Heliobacteria or whether the observed difference can be explained in terms of the dimers or special pairs found in organisms comprised of BChls b or a. Calculations based on the crystallographic coordinates of the BChls b in Rhodopseudomonas viridis with the acetyl groups replaced by vinyls yield good agreement with the observed Qy energies of BChl g monomers in vitro. The calculations are also extended to predict the spectral properties of an as-yet undiscovered organism comprised of BChls {open_quotes}h{close_quotes}, hypothetical vinyl-substituted analogues of BChls a. The calculations may also offer some guidelines for the considerable effort now devoted to chlorin-based artificial photosynthetic models. Introduction of acetyl functions or other polar substients that can conjugate with the chlorin {pi} system clearly provide simple synthetic avenues to modulate the optical properties of porphinoid monomers; effects that are further enhanced by dimerization, as evidenced by the large differences in the BCgls g and b in vivo. 31 refs., 1 fig., 1 tab.

• Sponsoring Organization: USDOE
• OSTI ID: 263818
• Journal Information: Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 7 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
40 CHEMISTRY
CHEMICAL REACTION YIELD
MATHEMATICAL MODELS
PHOTOSYNTHESIS
PHOTOSYNTHETIC BACTERIA