Controlling Biological Activity with Light: Diarylethene‐Containing Cyclic Peptidomimetics

Photobiological processes in nature are usually triggered by nonpeptidic chromophores or by modified side chains. A system is presented in which the polypeptide backbone itself can be conformationally switched by light. An amino acid analogue was designed and synthesized based on a reversibly photoisomerizable diarylethene scaffold. This analogue was incorporated into the cyclic backbone of the antimicrobial peptide gramicidin S at several sites. The biological activity of the resulting peptidomimetics could then be effectively controlled by ultraviolet/visible light within strictly defined spatial and temporal limits.     

Isolation of an Imino‐N‐heterocyclic Carbene/Germanium(0) Adduct: A Mesoionic Germylene Equivalent

An autoionization of germanium dichloride/dioxane complex with an imino‐N‐heterocyclic carbene ligand ( L ) afforded a novel germyliumylidene ion, [( L )GeCl]⁺[GeCl₃]^?, which was fully characterized. Reduction of the germyliumylidene ion with potassium graphite produced a cyclic species [( L )Ge], which can be viewed as both a Ge⁰ species and a mesoionic germylene. X‐ray diffraction analysis and computational studies revealed one of the lone pairs on the Ge atom is involved in the π system on the GeC₂N₂ five‐membered ring. It was also confirmed that the nucleophilic behavior of [( L )Ge] as a two lone‐pair donor.     

Probing local effects in silica sol-gel media by fluorescence spectroscopy of p-DASPMI

Stillbazolium salts present remarkable potential for application in several scientific areas. Their versatile behavior is explained by invoking the "twisted intramolecular charge-transfer" (TICT) mechanism, a model that describes the multiple fluorescence of DASPMI (4-(4-(dimethylamino)styryl)- N-methylpyridiniumiodine). One feature of their behavior is the sensitivity of the fluorescence lifetime to viscosity, thus identifying them as suitable probes for microheterogeneous systems, such as cells and sol-gel derived media. Because of their optical transparency, sol-gel matrices are light addressable and therefore appropriate for performing spectroscopic studies. The sol-gel process has been successfully used to produce hosts to biomolecules like proteins, for biosensor applications; however, these systems have to be optimized. Therefore, in this study modification of the matrices was performed through the incorporation of either silanes or polymers. (Aminopropyl)triethoxysilane, trimethoxypropylsilane, or (glycidyloxypropyl)triethoxysilane were added. The modification was also extended to the incorporation of the polymers poly(ethylene glycol) (molecular weight 300 and 20000) and Gelrite. The effect of these modifiers upon the gelation and aging processes was examined via the study of the photophysics of p-DASPMI by using both steady-state and time-resolved fluorescence. It was possible to discriminate the dominant dye-host interactions in each of the main steps of the preparation of modified sol-gel matrices.     

Efficient algorithm of the transcorrelated method for periodic systems

The transcorrelated (TC) method is one of the promising wave-function-based approaches for the first-principles electronic structure calculations. In this method, the many-body wave function is approximated as the Jastrow-Slater type and one-electron orbitals in the Slater determinant are optimized with a one-body self-consistent-field equation such as that in the Hartree-Fock (HF) method. Although the TC method has yielded good results for both molecules and solids, its computational cost in solid-state calculations, being of order O(N(k)(3)N(b)(3)) with N(k) and N(b) the respective numbers of k-points and bands, has for some years hindered its wide application in condensed matter physics. Although an efficient algorithm was proposed for a Gaussian basis set, that algorithm is not applicable to a plane-wave basis that is suited to and widely used in solid-state calculations. In this paper, we present a new efficient algorithm of the TC method for the plane-wave basis or an arbitrary basis function set expanded in terms of plane waves, with which the computational cost of the TC method scales as O(N(k)(2)N(b) (2)). This is the same as that of the HF method. We applied the TC method with the new algorithm to obtain converged band structure and cell parameters of some semiconductors.     

A theoretical study of FeNC in the Δ electronic ground state

We report an ab initio calculation, at the MR-SDCI + Q + E_rel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] level of theory, of the potential energy surface for ⁶Δ_i FeNC. From the ab initio results, we have computed values for the standard spectroscopic parameters of FeN¹²C and FeN¹³C. Analytical representations of the potential energy surfaces have been fitted through the ab initio points, and the resulting functions have been used for directly solving the rotation-vibration Schrödinger equation by means of the MORBID program and by means of an adiabatic-separation method. For ⁶Δ_i FeNC, our ab initio calculations show that the equilibrium structure is linear with r_e (Fe-N) = 1.9354 Å and r_e (N-C) = 1.1823 Å. We find that the bending potential is very shallow, and the MORBID calculations show that the zero-point averaged structure is bent with the expectation values 〈r (Fe-N)〉 = 1.9672 Å, 〈r(N-C)〉 = 1.1866 Å, and . The experimentally derived bond length r₀ (N-C) = 1.03(8) Å reported for ⁶Δ_i FeNC by Lie and Dagdigian [J. Chem. Phys. 114 (2001) 2137-2143] is much shorter than the corresponding ab initio r_e-value and the averaged value from MORBID. Our calculations suggest that this discrepancy is caused by the inadequate treatment of the large-amplitude bending motion of ⁶Δ_i FeNC. It would appear that for floppy triatomic molecules such as FeNC, r₀-values have little physical meaning, at least when they are determined with the effects of the large-amplitude bending motion being ignored, i.e., under the assumption that the r₀ structure is linear.     

Three putative photosensory light, oxygen or voltage (LOV) domains with distinct biochemical properties from the filamentous cyanobacterium Anabaena sp. PCC 7120

Light, oxygen or voltage (LOV) domains function as blue-light sensors in the phototropin family of photoreceptors found in plants, algae and bacteria. We detected putative LOV domains (Alr3170-LOV, All2875-LOV and Alr1229-LOV) in the genome of a filamentous cyanobacterium, Anabaena sp. PCC 7120. These cyanobacterial LOV domains are closely clustered with the known LOV domains. Alr3170-LOV and A112875-LOV carry the conserved cysteine residue unique to the photoactive LOV, whereas Alr1229-LOV does not. We expressed these three LOV domains in Escherichia coli and purified them. In fact, Alr3170-LOV and A112875-LOV that are conserved in Nostoc punctiforme, a related species, bound flavin mononucleotide and showed spectral changes unique to known LOV domains on illumination with blue light. Alr3170-LOV was completely photoreduced and dark reversion was slow, whereas A112875-LOV was slowly photoreduced and dark reversion was rapid. For comparison, AvA112875-LOV in a closely related A. variabilis was also studied as a homolog of A112875-LOV. Finally, we observed that Alr1229-LOV that is not conserved in N. punctiforme showed no flavin binding.     

Cover Picture: On the Function and Structure of Synthetically Modified Porins (Angew. Chem. Int. Ed. 26/2009)

Synthetic Modulators are introduced into the trimeric porin OmpF by protein semisynthesis/click chemistry or by S‐alkylation, as U. Koert, L.‐O. Essen et al. report in their Communication on page 4853 ff. The choice of synthetic modulator and the type of covalent attachment significantly affect the blockage efficiency. A model for the functional properties is postulated on the basis of an X‐ray structure analysis of a crown ether/OmpF hybrid.

     

Single-Nitrogen Atom Incorporation into B=B Bond via the N=N Bond Splitting of Diazo Compound and Diazirine

Triboraazabutenyne 3 is synthesized by the reaction of diboraazabutenyne 1 with aryl boron dibromide followed by the reduction. The ligand exchange to replace phosphine on the terminal sp² B atom with carbene furnishes 4 . ¹¹B NMR, solid-state structures, and computational studies disclose that 3 and 4 feature the extremely polarized B=B bond. 4 readily splits the N=N bond of both diazo compound and diazirine under ambient conditions, whereby one nitrogen atom is incorporated into the B=B moiety leading to a neutral diboraazaallene 6 . The mechanism of the reaction between 4 and diazo compound is extensively investigated by density functional theory (DFT) calculations, as well as the isolation of an intermediate.