Evaluating the Nature of So-Called S*-State Feature in Transient Absorption of Carotenoids in Light-Harvesting Complex 2 (LH2) from Purple Photosynthetic Bacteria

Carotenoids are a class of natural pigments present in all phototrophic organisms, mainly in their light-harvesting proteins in which they play roles of accessory light absorbers and photoprotectors. Extensive time-resolved spectroscopic studies of these pigments have revealed unexpectedly complex photophysical properties, particularly for carotenoids in light-harvesting LH2 complexes from purple bacteria. An ambiguous, optically forbidden electronic excited state designated as S* has been postulated to be involved in carotenoid excitation relaxation and in an alternative carotenoid-tobacteriochlorophyll energy transfer pathway, as well as being a precursor of the carotenoid triplet state. However, no definitive and satisfactory origin of the carotenoid S* state in these complexes has been established, despite a wide-ranging series of studies. Here, we resolve the ambiguous origin of the carotenoid S* state in LH2 complex from Rba. sphaeroides by showing that the S* feature can be seen as a combination of ground state absorption bleaching of the carotenoid pool converted to cations and the Stark spectrum of neighbor neutral carotenoids, induced by temporal electric field brought by the carotenoid cation− bacteriochlorophyll anion pair. These findings remove the need to assign an S* state, and thereby significantly simplify the photochemistry of carotenoids in these photosynthetic antenna complexes

Redox effects on the excited-state lifetime in chlorosomes and bacteriochlorophyll c oligomers

Oligomers of [E,E] BChl CF (8, 12-diethyl bacteriochlorophyll c esterified with farnesol (F)) and [Pr,E] BChl CF (analogously, M methyl, Pr propyl) in hexane and aqueous detergent or lipid micelles were studied by means of steady-state absorption, time-resolved fluorescence, and electron spin resonance spectroscopy. The maximum absorption wavelength, excited-state dynamics, and electron spin resonance (EPR) linewidths are similar to those of native and reconstituted chlorosomes of Chlorobium tepidum. The maximum absorption wavelength of oligomers of [E,E] BChl CF was consistently blue-shifted as compared to that of [Pr,E] BChl CF oligomers, which is ascribed to the formation of smaller oligomers with [E,E] BChl CF than [Pr,E] BChl CF. Time-resolved fluorescence measurements show an excited-state lifetime of 10 ps or less in nonreduced samples of native and reconstituted chlorosomes of Chlorobium tepidum. Under reduced conditions the excited-state lifetime increased to tens of picoseconds, and energy transfer to BChl a or long-wavelength absorbing BChl c was observed. Oligomers of [E,E] BChl CF and [Pr,E] BChl CF in aqueous detergent or lipid micelles show a similar short excited-state lifetime under nonreduced conditions and an increase up to several tens of picoseconds upon reduction. These results indicate rapid quenching of excitation energy in nonreduced samples of chlorosomes and aqueous BChl c oligomers. EPR spectroscopy shows that traces of oxidized BChl c radicals are present in nonreduced and absent in reduced samples of chlorosomes and BChl c oligomers. This suggests that the observed short excited-state lifetimes in nonreduced samples of chlorosomes and BChl c oligomers may be ascribed to excited-state quenching by BChl c radicals. The narrow EPR linewidth suggests that the BChl c are arranged in clusters of 16 and 6 molecules in chlorosomes of Chlorobium tepidum and Chloroflexus aurantiacus, respectively

Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works

The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet's atmosphere ($O_2$, $O_3$, $CO_2$ etc.) have been identified as possible global biomarkers, we review here the detectability of a signature of life from the planet's surface, i.e. the green vegetation. The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the "Vegetation Red Edge" (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant. Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum. The egetation spectral signature is detected as a small 'positive shift' of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earth's phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better. Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of 'red edge' universality in the Universe.Comment: Invited talk in "Strategies for Life Detection" (ISSI Bern, 24-28 April 2006) to appear in a hardcopy volume of the ISSI Space Science Series, Eds, J. Bada et al., and also in an issue of Space Science Reviews. 13 pages, 8 figures, 1 tabl

New Magnetic Anomaly Map of the Antarctic

The second generation Antarctic magnetic anomaly compilation for the region south of 60 degrees S includes some 3.5 million line-km of aeromagnetic and marine magnetic data that more than doubles the initial map's near-surface database. For the new compilation, the magnetic data sets were corrected for the International Geomagnetic Reference Field, diurnal effects, and high-frequency errors and leveled, gridded, and stitched together. The new magnetic data further constrain the crustal architecture and geological evolution of the Antarctic Peninsula and the West Antarctic Rift System in West Antarctica, as well as Dronning Maud Land, the Gamburtsev Subglacial Mountains, the Prince Charles Mountains, Princess Elizabeth Land, and Wilkes Land in East Antarctica and the circumjacent oceanic margins. Overall, the magnetic anomaly compilation helps unify disparate regional geologic and geophysical studies by providing new constraints on major tectonic and magmatic processes that affected the Antarctic from Precambrian to Cenozoic times.Korea Polar Research Institute (KOPRI) programs, PM15040 and PE17050Germany's AWI/Helmholtz Center for Polar and Marine ResearchFederal Institute for Geosciences and Natural ResourcesBritish Antarctic Survey/Natural Environmental Research CouncilItalian Antarctic Research ProgrammeRussian Ministry of Natural ResourcesU.S. National Science Foundation and National Space and Aeronautics AdministrationAustralian Antarctic Division and Antarctic Climate & Ecosystem Cooperative Research CentreFrench Polar InstituteGlobal geomagnetic observatories network (INTERMAGNET

Antenna organization in green photosynthetic bacteria

This project is concerned with the structure and function of the unique antenna system found in the green photosynthetic bacteria. The antenna system in these organisms is contained within a vesicle known as a chlorosome, which is attached to the cytoplasmic side of the cell membrane. Additional antenna pigments and reaction centers are contained in integral membrane proteins. Energy absorbed by the bacteriochlorophyll c (BChl c) pigments in the chlorosome is transferred via a baseplate'' array of BChl a antenna pigments into the membrane and to the reaction center. This system is similar in some respects to the phycobilisome antenna system found in cyanobacteria and some types of algae, in that a membrane-associated structure absorbs light and transfers it to the membrane where conversion to chemical energy takes place. However, the overall structure, the type of pigments utilized and the nature of the proteins in these two types of membrane-associated antenna bodies are entirely different, and they clearly represent two independent evolutionary solutions to the problem of light collection and excitation transfer

Antenna organization in green photosynthetic bacteria

This project is concerned with the structure and function of the unique antenna system found in the green photosynthetic bacteria. The antenna system in these organisms is contained within a vesicle known as a chlorosome, which is attached to the cytoplasmic side of the cell membrane. Additional antenna pigments and reaction centers are contained in integral membrane proteins. Energy absorbed by the bacteriochlorophyll c (BChl c) pigments in the chlorosome is transferred via a baseplate'' array of BChl a antenna pigments into the membrane and to the reaction center. A schematic model of chlorosome structure is shown. This project is aimed at increasing our understanding of the organization of the pigments in the chlorosome and how the antenna system functions