Catabolism of Tetrapyrroles

The enzymatic degradation of naturally occurring tetrapyrrolic pigments (heme, chlorophylls, and vitamin B12) is shortly reviewed

Stereospecific Synthesis of Carbanucleotides Designed for Antisense Methodology

A short stereospecific synthesis of the carbocyclic 2'-deoxynucleoside analogues 36 and 37 (Schemes 2 and 5) and 45 and 46 (Schemes 2 and 6) starting from optically active 8,9,10-trinorborn-5-en-2-one (1) is described. As two functional groups capable to react with each other are present in the same molecule of the synthetic carbanucleosides, the latter can form polymers similar to oligonucleotides

Isolation of the Phytochrome Chromophore. The Cleavage Reaction with Hydrogen Bromide

The cleavage of the bilin chromophore from C-phycocyanin with hydrogen bromide yields 3E-configurated phycocyanobilin (4) as the major and 3 Z-configurated phycocyanobilin (5) as the minor reaction product. The reaction of synthetic 3E-configurated phytochromobilin (2) with hydrogen bromide and methanol leads only to a methanol adduct at the C-18 side chain (7) whereas the same reaction with the 3Z-configurated phytochromobilin (3) leads to 7 and 2. The bilin chromophore was cleaved also from phytochrome after preparation of phytochromobilin peptides. The detection of 2 and 7 suggested that 3Z-and 3E-configurated phytochromobilin were the primary products of cleavage from phytochrome. A reaction scheme is given which can explain the results of the reaction with hydrogen bromide and methanol

Synthesis of C 1- and C 3ν-symmetric porphyrin trimers based on triphenylmethane cores

This work describes the synthesis of a new class of tripodaphyrin derivatives with a triphenylmethane core. Both C₁- and C3ν-symmetric tetrahedral large molecules with covalently linked rigid elements were obtained

Ultrafast Excited State Dynamics of Tri- and Hexaporphyrin Arrays

An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Å, while it varies from 26 to 70 Å in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units: a broadening of the Soret band, a shortening of the S2 lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S1 fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the Bx,y transition dipoles. The time constants for S1 energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S1−S1 annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S2 states. S2 and S1 energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed

Ultrafast excited-state dynamics of phenyleneethynylene oligomers in solution

The excited-state dynamics of oligomeric phenyleneethynylenes (OPEs) of various length and substitution has been investigated by femtosecond time-resolved spectroscopy. The fluorescence lifetime of the OPEs decreases with the number of phenyleneethynylene units up to about 9. This effect is due to an increase of the oscillator strength for the S(1)-S(0) transition. Dynamic features occurring within a few tens of picoseconds and ascribed to structural relaxation directly after population of the S(1) state can be observed in non-viscous solvents. The effect of torsional disorder on the fluorescence intensity is shown to depend strongly on the nature of the substituent on the phenyl groups. All these effects are qualitatively discussed with a simple exciton model

Investigations of Electronic Energy Transfer Dynamics in Multiporphyrin Arrays

A study of the dynamics of electronic energy transfer (EET) in arrays containing three, four, and six tetraphenylporphine units connected with phenylethynyl spacers is reported. For arrays containing the same chromophores, the EET rate constant was determined from the reorientational dynamics of the transition dipole using the crossed grating technique. EET time constants ranging from 150 ps up to 33 ns were measured, depending on the distance between the chromophores and on the metal ion complexed in the porphyrins. For the trimeric planar arrays, the interchromophoric distance varies by a factor of 2, while the ratio of the through space to through bond distances is constant. By comparing the measured EET rate constants with those calculated using Förster theory, the contributions of the Coulombic, through space, mechanism and of the exchange, through bond, mechanism could be estimated. For the arrays with the shortest spacer (through space distance of 23 Å), EET occurs through both exchange and Coulombic interactions with a ratio of about 3:1. This ratio increases up to about 10 as the distance is increased to 34.5 Å. At 46.5 Å, the ratio decreases and it appears that the Coulombic interaction becomes the dominant mechanism at longer distances. In the tetrahedral compound, the presence of a central saturated carbon strongly alters the electronic conducting properties of the spacer and makes the exchange mechanism inoperative