Angular Forces Around Transition Metals in Biomolecules

Quantum-mechanical analysis based on an exact sum rule is used to extract an semiclassical angle-dependent energy function for transition metal ions in biomolecules. The angular dependence is simple but different from existing classical potentials. Comparison of predicted energies with a computer-generated database shows that the semiclassical energy function is remarkably accurate, and that its angular dependence is optimal.Comment: Tex file plus 4 postscript figure

Different Mi-2 Complexes for Various Developmental Functions in Caenorhabditis elegans

Biochemical purifications from mammalian cells and Xenopus oocytes revealed that vertebrate Mi-2 proteins reside in multisubunit NuRD (Nucleosome Remodeling and Deacetylase) complexes. Since all NuRD subunits are highly conserved in the genomes of C. elegans and Drosophila, it was suggested that NuRD complexes also exist in invertebrates. Recently, a novel dMec complex, composed of dMi-2 and dMEP-1 was identified in Drosophila. The genome of C. elegans encodes two highly homologous Mi-2 orthologues, LET-418 and CHD-3. Here we demonstrate that these proteins define at least three different protein complexes, two distinct NuRD complexes and one MEC complex. The two canonical NuRD complexes share the same core subunits HDA-1/HDAC, LIN-53/RbAp and LIN-40/MTA, but differ in their Mi-2 orthologues LET-418 or CHD-3. LET-418 but not CHD-3, interacts with the Krüppel-like protein MEP-1 in a distinct complex, the MEC complex. Based on microarrays analyses, we propose that MEC constitutes an important LET-418 containing regulatory complex during C. elegans embryonic and early larval development. It is required for the repression of germline potential in somatic cells and acts when blastomeres are still dividing and differentiating. The two NuRD complexes may not be important for the early development, but may act later during postembryonic development. Altogether, our data suggest a considerable complexity in the composition, the developmental function and the tissue-specificity of the different C. elegans Mi-2 complexes

Matrix deuteration effects and spin-lattice relaxation in the lowest triplet of the palladium(II)-complex Pd(2-thpy)₂

Pd(2-thpy)₂ isolated in protonated or deuterated frozen n-octane (Shpol'skii matrices) exhibits highly resolved triplet emission and excitation spectra. One observes interesting differences for the two matrices: (i) The protonated matrix shows only one dominant guest site while the deuterated matrix exhibits two dominant sites. (ii) Low-energy satellites corresponding to lattice modes are distinctly shifted to lower energy due to deuteration of the matrix. (iii) At 1.3 K the triplet sublevels emit indepently with lifetimes being nearly equal for both matrices. However, for 1.3 < T < 5 K one observes obvious differences in the decay behavior. This is explained by substantially smaller rates of spin-lattice relaxation in the deuterated host. Different mechanisms of spin-lattice relaxation are discussed

Characterization of triplet sublevels by highly resolved vibrational satellite structures. Application to Pt(2-thpy)₂

The cyclometalated Pt(2-thpy)₂ complex with thpy⁻ as the deprotonated form of 2-(2-thienyl)pyridine shows highly resolved phosphorescence and triplet excitation spectra at low temperatures when the complex is isolated in Shpol’skii matrices, as is shown for the first time. Sharp-line Shpol’skii spectra were obtained by dissolving Pt(2-thpy)₂ in n-hexane, n-heptane, n-octane, n-nonane, and n-decane matrices. The highest resolution was reached using n-octane. In this matrix only one dominant site governs the spectra. The lowest electronic origins lie at 17156 (I), 17163 (II), and 17172 cm⁻¹ (III) ( ±1 cm⁻¹). They represent triplet sublevels that are split by the relatively large zero-field splitting of 16 cm⁻¹. These sublevels are assigned as π-π* ligand-centered (LC) with an appreciable metal-to-ligand charge transfer (MLCT) admixture. The emission from the lowest triplet sublevel |I) to the ground state |0) (origin line I) is strongly forbidden (emission lifetime at T = 1.3 K: 110 μs), but due to vibronic (Herzberg-Teller) coupling, additional radiative deactivation paths are opened and thus a large number of “false origins” occur. The emission and excitation spectra corresponding to the sublevels |II) and |III) show relatively strong origin lines due to direct spin-orbit coupling. Thus, one observes a large number of vibrational satellites of the Franck-Condon type and combinations. A comparison of the highly resolved vibrational satellite structures allows one to conclude that the emitting triplet state (all three sublevels) and the singlet ground state exhibit very similar force constants and nuclear equilibrium positions. Interestingly, a comparison to the properties of the homologous Pd(2-thpy)₂ (with triplets exhibiting only a very small MLCT or d-d* contribution) indicates that with increasing MLCT admixture the discussed distortions become less pronounced. Thus, an increase of MLCT character leads to a more pronounced covalency in the involved states

Time-resolved vibrational structures of the triplet sublevel emission of Pd(2-thpy)₂

Time-resolved phosphorescence spectra from the lowest electronic triplet of Pd(2-thpy)₂ (with 2-thpy⁻ = ortho-C-deprotonated form of 2-(2-thienyl)pyridine) (see the inset of Figure 2) are presented. The complex was isolated in a Shpol'skii matrix to obtain high resolution. The emitting triplet lies at 18418 ± 1 cm⁻¹ (electronic origin). Its zero-field splitting is less than 1 cm⁻¹ and could not be resolved optically. However, at 1.3 K, when the spin-lattice relaxation is slow compared to the emission lifetimes of the sublevels (130, 235, 1200 μs), the individual sublevels emit independently. Thus, by time-resolved spectroscopy it is possible to separate a fast-decaying emission spectrum from a slow-decaying one. A highlight of this investigation is that these spectra exhibit different vibrational satellite structures. This shows that different spin-orbit coupling mechanisms (direct spin-orbit coupling and Herzberg-Teller coupling) govern the radiative deactivation of the sublevels. In particular, it is found that specific vibrational modes couple very selectively to individual sublevels. For example, the 528 cm⁻¹ mode couples only to the slow-decaying sublevel. Thus, these optically well resolvable vibrational satellites display directly properties of the individual sublevels, which are unresolvable by conventional optical spectroscopy. This effect is observed for the first time for transition metal complexes

Highly resolved optical spectra of Pd(thpy)₂ in a Shpol'skii matrix

Pd(2-thpy)₂ is a representative of the interesting new class of ortho-metalated compounds. For the first time, we present highly resolved emission and excitation spectra. This could be achieved by using the Shpol’skii matrix isolation technique. From the intensity distribution of the highly resolved vibrational satellite structures and the corresponding vibrational energies it is concluded that the excited triplet lying at 18418 ± 1 cm⁻¹ and the singlet ground state exhibit nearly the same force constants and equilibrium positions of the potential hypersurfaces. The type of the electronic transition is assigned as being ligand centered with a relatively small MLCT admixture. The zero-field splitting of the triplet could not be resolved (experimental resolution 1 cm⁻¹), but at T = 1.3 K the three sublevels emit independently with τ(1) = 155 ± 20 μs, τ(2) ca. 200 μs, and τ(3) = 1200 ± 100 μs, respectively. With increasing temperature and thus increasing spin-lattice relaxation the emission lifetime becomes monoexponential with τ(4 .2 K) = 235 ± 10 μs

Arene-arene stacking in cis-bis[2-(2-thienyl)pyridine]platinum(II)

In the crystal structure of the title complex, [Pt(C₉H₆NS)₂], although the aromatic ligands are coordinated to a central heavy metal atom, T-shaped and shifted π-stacked arrangements of the aromatic moieties are preferred, leading to a sandwich herring-bone type of crystal-packing motif. The crystal structure is therefore consistent with the view that the arene-arene interactions are determined by electrostatics (multipole-multipole)