Observation of a bacteriochlorophyll anion radical during the primary charge separation in a reaction center

The primary light-induced charge separation in reaction centers of Rhodobacter sphaeroides was investigated with femtosecond time resolution. The absorption changes in the time range 100 fs to 1 ns observed after direct excitation of the primary donor P at 860 nm could only be explained by a kinetic model which uses three time constants. This finding supports the following reaction scheme: (i) the electronically excited primary donor P* decays with a time constant of 3.5 ps and populates a very short-lived intermediate involving a reduced accessory bacteriochlorophyll molecule; (ii) with a time constant of 0.9 ps the electron is transferred to the neighboring bacteriopheophytin molecule; and (iii) from there within 200 ps to the quinone

Initial electron-transfer in the reaction center from Rhodobacter sphaeroides.

The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond time-resolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the electron is transferred within 3.5 +/- 0.4 ps to the accessory bacteriochlorophyll B. State P+B- decays with a time constant of 0.9 +/- 0.3 ps passing the electron to the bacteriopheophytin H. Finally, the electron is transferred from H- to the quinone QA within 220 +/- 40 ps

What a Plant Sounds Like: The Statistics of Vegetation Echoes as Received by Echolocating Bats

A critical step on the way to understanding a sensory system is the analysis of the input it receives. In this work we examine the statistics of natural complex echoes, focusing on vegetation echoes. Vegetation echoes constitute a major part of the sensory world of more than 800 species of echolocating bats and play an important role in several of their daily tasks. Our statistical analysis is based on a large collection of plant echoes acquired by a biomimetic sonar system. We explore the relation between the physical world (the structure of the plant) and the characteristics of its echo. Finally, we complete the story by analyzing the effect of the sensory processing of both the echolocation and the auditory systems on the echoes and interpret them in the light of information maximization. The echoes of all different plant species we examined share a surprisingly robust pattern that was also reproduced by a simple Poisson model of the spatial reflector arrangement. The fine differences observed between the echoes of different plant species can be explained by the spatial characteristics of the plants. The bat's emitted signal enhances the most informative spatial frequency range where the species-specific information is large. The auditory system filtering affects the echoes in a similar way, thus enhancing the most informative spatial frequency range even more. These findings suggest how the bat's sensory system could have evolved to deal with complex natural echoes

Specific recognition of CG base pairs by 2-deoxynebularine within the purine•purine•pyrimidine triple-helix motif

The sequence-specific recognition of double-helical DNA by oligodeoxyribonucleotide-directed triple-helix formation is limited mostly to purine tracts. Within the geometric constraints of the phosphate-deoxyribose position of a purine•purine•pyrimidine triple-helical structure, model building studies suggested that the deoxyribonucleoside 2'-deoxynebularine (dN) might form one specific hydrogen bond with cytosine (C) or adenine (A) of Watson-Crick cytosine-guanine (CG) or adenine-thymine (AT) base pairs. 2-Deoxynebularine (dN) was incorporated by automated methods into purine-rich oligodeoxyribonucleotides. From affinity cleavage analysis, the stabilities of base triplets within a purine.purine.pyrimidine (Pu•Pu•Py) triple helix were found to decrease in the order N.CG approximately N•AT > N•GC approximately N•TA (pH 7.4, 37 °C). Oligodeoxyribonucleotides containing two N residues were shown to bind specifically within plasmid DNA a single 15 base pair site of the human immunodeficiency virus genome containing two CG base pairs within a purine tract. This binding event occurs under physiologically relevant pH and temperature (pH 7.4, 37 °C) and demonstrates the utility of the new base. Quantitative affinity cleavage titration reveals that, in the particular sequence studied, an N•CG base triplet interaction results in a stabilization of the local triple-helical structure by 1 kcal•mol^(-1) (10 mM NaCl, 1 mM spermine tetrahydrochloride, 50 mM Tris-acetate, pH 7.4, 4 °C) compared to an A•CG base triplet mismatch