Mediation of ultrafast light-harvesting by a central dimer in phycoerythrin 545 studied by transient absorption and global analysis

We report ultrafast femtosecond transient absorption measurements of energy-transfer dynamics for the antenna protein phycoerythrin 545, PE545, isolated from a unicellular cryptophyte Rhodomonas CS24. The phycoerythrobilins are excited at both 485 and 530 nm, and the spectral response is probed between 400 and 700 nm. Room-temperature measurements are contrasted with measurements at 77 K. An evolution-associated difference spectra (EADS) analysis is combined with estimations of bilin spectral positions and energy-transfer rates to obtain a detailed kinetic model for PE545. It is found that sub pulse-width dynamics include relaxation between the exciton states of a chromophore dimer (beta 50/60) located in the core of the protein. Energy transfer from the lowest exciton state of the phycoerythrobilin (PEB) dimer to one of the periphery 15,16-dihydrobiliverdin (DBV) bilins is found to occur on a time scale of 250 fs at room temperature and 960 fs at 77 K. A host of energy-transfer dynamics involving the beta 158, beta 82, and alpha 19 bilins occur on a time scale of 2 ps at room temperature and 3 ps at 77 K. A final energy transfer occurs between the red-most DBV bilins with a time scale estimated to be approximately 30 ps. The role of the centrally located phycoerythrobilin dimer is seen as crucial-spectrally as it expands the cross-section of absorption of the protein; structurally as it sits in the middle of the protein acting as an intermediary trap; and kinetically, as the internal conversion and subsequent red-shift of the excitation is extremely fast.     

Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: unification of different basicity scales

The earlier compiled self-consistent spectrophotometric basicity scale in acetonitrile (AN) was expanded to range from 3.8 to 32.0 pK(a) units, that is 28 orders of magnitude. Altogether 54 new relative basicity measurements (DeltapK(a) measurements) were carried out and 37 new compounds were introduced to the scale (it now includes altogether 89 bases). The relative basicity of any two bases in the scale can be obtained by combining at least two independent sets of measurements. Multiple overlapping measurements make the results more reliable. The overall consistency (as defined earlier) of the measurements is s = 0.03 pK(a) units. Thorough analysis of all of our experimental data (DeltapK(a) values of this and earlier works) and experimental pK(a) data in AN available in the literature (works from the groups of Coetzee and Padmanabhan, Kolthoff and Chantooni, Jr., the Schwesinger group, Bren' et al. and some others, altogether 19 papers) was carried out. On the basis of this analysis the anchor point of the scale-pyridine-was shifted upward by 0.20 pK(a) units thereby also revising the absolute pK(a) values of all the bases on the scale. This way very good agreement between our relative data and the absolute pK(a) values of the abovementioned authors was obtained. The revised basicity scale was interconnected with the earlier published self-consistent acidity scale by DeltapK(a) measurements between acids and bases. The rms deviation between the directly measured DeltapK(a) values and the absolute pK(a) values of the compounds was 0.10 pK(a) units.     

The Ideal Ionic Liquid Salt Bridge for the Direct Determination of Gibbs Energies of Transfer of Single Ions,Part I: The Concept

Described is a procedure for the thermodynamically rigorous, experimental determination of the Gibbs energy of transfer of single ions between solvents. The method is based on potential difference measurements between two electrochemical half cells with different solvents connected by an ideal ionic liquid salt bridge (ILSB). Discussed are the specific requirements for the IL with regard to the procedure, thus ensuring that the liquid junction potentials (LJP) at both ends of the ILSB are mostly canceled. The remaining parts of the LJPs can be determined by separate electromotive force measurements. No extra‐thermodynamic assumptions are necessary for this procedure. The accuracy of the measurements depends, amongst others, on the ideality of the IL used, as shown in our companion paper Part II.     

Basicity of Very Weak Bases in 1,2‐Dichloroethane

The basicity scale of very weak bases has been set up in 1,2‐dichloroethane to give, for the first time, reliable quantitative insights into the basic properties of weak bases in a low‐polarity solvent. The scale contains 30 compounds, including anilines; phosphanes, and carbonyl bases, such as esters and amides, linked by 53 relative basicity measurements. The scale spans more than 12 pK_ip units, expanding to as low pK_ip values as possible with our current experimental methodology.     

Band structure of even and ions of odd polyenes

The analysis of experimental data for singlet transitions (E _n) of even polyenes (I), cations (II) and anions (III) of odd polyenes show that for infinite chains E (I)/E (II)=E (I)/E (III) = 2:1. It is shown that the energy gap is equal for the three systems. In cases (II) and (III) there is a level (NBMO) in the gap which is vacant in (II) and occupied in (III). That is why the first optical transition in (II) and (III) depends on the semiwidth of the gap.     

tert‐Amino effect at a coumarin and a 2‐quinolone system: Synthesis of 1,2 fused 5H‐chromeno[4,3‐b]pyridin‐5‐ones and a 6H‐benzo[h][1,6]naphthyridin‐5‐one

Some novel 1,2‐fused 5H‐chromeno[4,3‐b]pyridin‐5‐ones ( 5a,b ) and a 6H‐benzo[h][1,6]naphthyridin‐5‐one ( 5c ) have been synthesized starting from the 4‐chlorocoumarin‐3‐carbaldehyde ( 1a ) or its N‐methyl‐2‐quinolone analogue ( 1b ) via subsequent Knoevenagel condensation and ring closure reaction known as the ‘tert‐amino effect’. These are rare examples of the tert‐amino effect occurring at 2‐pyrone and 2‐pyridone ring. An unusual intramolecular redox reaction of the iminium ion 6 , reported earlier, most probably follows analogous mechanism as the tert‐amino effect reactions leading to 5 .     

Polypyrrole modified solvent resistant nanofiltration membranes

New solvent resistant nanofiltration (SRNF) membranes with polypyrrole (PPy) modified toplayer were prepared on different types support by in situ pyrrole polymerization. The morphology of the membranes was studied by SEM. The PPy modified membranes were applied in the filtration of organic solvents. All the PPy modified membranes showed a very high retention of the negatively charged RB in different solvent systems, comparable to those of the MPF-50 and STARMEM 122 commercial membranes, but at much higher flux. The extended filtration experiment in strong aprotic DMF of PPy modified membranes showed a clearly stable permeability and retention over 30 h. In addition, the PPy modified membranes showed a much higher flux in THF systems than for earlier reported crosslinked poly(imide) membranes.     

Substituent effects on the basicity of 3,7-diazabicyclo[3.3.1]nonanes

Basicity constants for a series of 3,7-diazabicyclo[3.3.1]nonane derivatives in acetonitrile with a variation over 13 orders of magnitude have been determined using a spectrophotometric titration technique. An excellent correlation between basicity and calculated proton affinities obtained at PCM-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level was found. The results are discussed in terms of substituent effects and compared to (15)N NMR chemical shifts.