Helmut Huber

Rotational barriers in perylene fluorescent dyes

Rotational barriers in N-substituted perylene dyes have been determined. Phenyl substituents with tert-butyl groups in the o-position give rigid systems, whereas secondary alkyl groups cause low rotational barriers. In spite of that, fluorescent quantum yields are high in both cases. Conformations in solution are discussed.

Primary electron-transfer dynamics in modified bacterial reaction centers containing pheophytin-a instead of bacteriopheophytin-a

Abstract Primary electron transfer was studied in modified reaction centers of Rhodobacter sphaeroides R26.1, in which both bacteriopheophytin-a molecules were replaced by pheophytin-a to more than 90%. The absorption spectrum of the modified reaction centers shows a new Q y band at 674 nm, while the Q y absorption of bacteriopheophytin-a at 760 nm in native reaction centers is not present. The bands assigned to the monomeric bacteriochlorophylls show a slight blue shift (3 nm) and small but distinct changes in the circular dichroism spectra. Subpicosecond absorption spectroscopy reveals that the electron transfer kinetics in modified reaction centers are different from the kinetics in native samples. The accessory bacteriochlorophyll anion can be directly observed in the spectral region around 1000 nm. A detailed analysis of the data suggests that the modification raised the energy of the radical pair state P + H A − , leading to a long-lived P + B A − population of 30%. Relative to P 7 the free energy levels of the intermediates are −450 cm −1 (P + B A − and −630 cm −1 (P + H A − ) in the modified reaction centers. Based on these calculations, a simplified electron transfer model is presented.

Primary photosynthesis in reaction centers containing four different types of electron acceptors at site HA

Femtosecond spectroscopy is used to study the primary electron transfer reactions in photosynthetic reaction centers, where the chromophore at the secondary acceptor site is changed from bacteriopheophytin a via 3-vinyl-bacteriopheophytin a and 3-acetyl-pheophytin a to pheophytin a. Pronounced changes in the primary electron transfer reactions are observed which are directly related to the redox potential of the different chromophores and the corresponding changes in free energy of the first intermediates. The reaction dynamics do not primarily correlate with the changes in absorption spectra.

Electron transfer in bacterial reaction centers with an energetically raised primary acceptor: ultrafast spectroscopy and ENDOR/TRIPLE studies

Abstract Femtosecond time resolved absorbance changes in the near infrared spectral region were recorded in photosynthetic reaction centers (RCs) of Rhodobacter ( Rb .) sphaeroides and Chloroflexus aurantiacus . For Rb. sphaeroides , wild type and site directed mutant RCs, where the tyrosine at the M210 position was replaced by phenylalanine and leucine, were investigated. In the mutant RCs the decay of the stimulated emission signal was drastically slower and displayed strong multiexponentiality. In the spectral range between 1000 and 1100 nm, the existence of a fast kinetic component with a time constant of 1 ps could be demonstrated. Based on the spectral and dichroic characteristics of the data, we suggest a weakly populated P + B A - state as first electron transfer intermediate in these mutants. The results indicate that primary electron transfer follows a stepwise mechanism, even in the case of an energetically increased primary acceptor. Investigations of the electronic structure of the photooxidized primary donor using ENDOR/TRIPLE spectroscopy demonstrated that changes at the position M210 mainly affect B A and not P.

Thiocarbonyl ylides and electrophilic azo compounds; Sterically hindered cyclic hydrazodicarboxylic esters☆

The cycloadduct 5, prepared from adamantanethione S-methylide with dimethyl azodicarboxylate, exists in solution in two conformations P and Q (70:30 in CDCl3), which are separated by a barrier of ΔG± 18.3 ± 0.6 kcal mol−1; according to NMR spectra, both forms are chiral. Steric hindrance to 3′N-CO2CH3 rotation by the adamantane system generates Cue5f8O outside and Cue5f8O inside conformations. The barrier to enantiomerization (AB → A2 for 5′-H2) is higher (> 20.1 kcal mol−1). Three combinations of rate processes are discussed. Except for the 5′ -methyl compound 19a, the chiral cycloadducts of four other thiocarbonyl ylides to dimethyl azodicarboxylate do not show diastereoisomeric conformations in their NMR spectra at ambient temperature.

Isoquinolinium N-arylimides and electrophilic ethylenes: Structures and NMR spectra of cycloadducts ☆

Abstract The title compounds furnish high yields of substituted 3-aryl-1,2,3,10b-tetrahydropyrazolo[5,1-a]isoquinolines of type 2. The structural conclusions from 1H NMR spectra are confirmed by X-ray analyses of cycloadducts 3b and 4a. The lone pair repulsion of the two nitrogen atoms freezes the N-inversion in the bicyclic hydrazines and determines the conformation; the torsion angle of the n-orbitals is close to the optimum of 90°. An intramolecular hydrogen bond of the weakly acidic 2β-H to the pyridyl nitrogen of 3b was established, corroborating previous 1H NMR evidence. The 13C NMR spectra of 14 cycloadducts reveal the contributions of substituents to δc. Two-dimensional NMR techniques secure the assignments of all 1H and 13C signals of selected cycloadducts.

α-Diazocarbonyl compounds and enamines - a dichotomy of reaction paths

Abstract 2,5-Dimethyl-1-pyrrolidinocyclopentene produces with methyl diazoacetate and diazoketones 2-pyrazolines as cycloadducts whereas with dimethyl diazomalonate an azo coupling equilibrium is established. The rate constants of these two reactions respond differently to solvent polarity.

Isoquinolinium N-arylimides and acetylenic dipolarophiles; cycloadducts and their rearrangements

Abstract Dimethyl acetylenedicarboxylate, methyl propiolate, and ethyl phenylpropiolate surpass the corresponding ethylenic carboxylic esters in dipolarophilic activity vs . isoquinolinium N -arylimides, a class of azomethine imines. The cycloadducts contain a N s -vinylphenylhydrazine system and enter into a Fischer indole synthesis which stops one step short of the indole. The [3.3]-sigmatropic rearrangement involved is likewise faster for the cycloadducts of acetylenic dipolarophiles than for ethylenic ones and does not require acid catalysis; in some cases the initial adduct escapes 1 H NMR observation. The products 11–17 , obtained with ethyl phenylpropiolate, provide beautiful NMR models for steric interaction of benzo ring E and the 12-phenyl group. On treatment with strong acid, the pentacyclic rearrangement products suffer fragmentation; e.g. , 11 furnishes 4-(o-aminophenyl)-isoquinoline and methyl benzoylacetate in methanolic HCl.

The Primary Electron Transfer in Bacterial Reaction Centers with Altered Energetics of the Primary Acceptor

Recent femtosecond experiments on bacterial reaction centers (RCs) have shown that a bacteriochlorophyll anion is formed in the first step of the photosynthetic charge separation process [1]. A fast 0.9 ps reaction was explained with the transient reduction of the monomeric bacteriochlorophyll molecule BA, located on one of the two pigment branches, between the primary donor (P, a bacteriochlorophyll dimer) and the secondary acceptor (HA, a bacteriopheophytin molecule).

The First Femtoseconds of Primary Photosynthesis - The Processes of The Initial Electron Transfer Reaction

Infrared and visible spectroscopy is used to investigate the primary processes in bacterial photosynthesis. Experiments are presented showing convincingly that the first electron transfer step in primary photosynthesis leads to the radical pair state P+BA- where the electron has reached the accessory bacteriochlorophyll BA of the active branch. Ultrafast spectroscopy in the mid infrared indicates that the electron transfer to the bacteriochlorophyll is preceded by a very fast (200 fs) reaction in the excited electronic state of the special pair.