2‑Bromo[6]helicene as a Key Intermediate for [6]Helicene Functionalization

The synthesis of 2-bromo[6]­helicene was revised and improved up to 51% yield. Its reactivity was thoroughly investigated, and a library of 17 different carbon, boron, nitrogen, phosphorus, oxygen and sulfur substituted derivatives was prepared. The racemization barrier for 2-bromo[6]­helicene was determined, and the usage of enantiomers in the synthesis of optically pure helicenes was rationalized. The three most energy-demanding reactions using enantiomerically pure 2-bromo[6]­helicene were tested in order to confirm the predicted enantiomeric excess

Carbosilane Metallodendrimers with Titanocene Dichloride End Groups

Carbosilane metallodendrimers containing substituted titanocene dichloride end groups were prepared using hydrosilylation as the capping reaction. Two complementary pathways were followed: hydrosilylation of ω-alkenyl-substituted titanocene dichloride complexes with Si–H bond terminated dendrimers and hydrosilylation of vinyl terminated dendritic materials with 3-(dimethylsilyl)­propyl-substituted titanocene dichloride. The former procedure provided dendrimers of the first generation with four end units and of the second generation with eight end units. The latter method gave dendrititic wedges and dendrimers up to the second generation with 16 peripheral titanocene dichloride units and molecular weight 7070 Da. Dendritic materials were purified by GPC and characterized by MALDI-TOF mass spectrometry and ESI-TOF mass spectrometry (metallodendrimers) and also by multinuclear NMR

Structural Control of ¹A_2u-to‑³A_2u Intersystem Crossing in Diplatinum(II,II) Complexes

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per­(difluoroboro)­tetrakis­(pyrophosphito)­diplatinate­(II) ([Pt₂(μ-P₂O₅(BF₂)₂)₄]^4–, abbreviated Pt­(pop-BF₂)), yields a radiative decay rate (<i>k</i>_r = 1.7 × 10⁸ s^–1) an order of magnitude greater than that of the parent complex, Pt­(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt­(pop) [ISC activation energies: 2230 cm^–1 for Pt­(pop-BF₂); 1190 cm^–1 for Pt­(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin–orbit coupling between the ¹A_2u state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt­(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E_a is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt₂(P-O-P)₄ core that would allow direct ¹A_2u–³A_2u spin–orbit coupling

Reactivity of a Titanocene Pendant Si–H Group toward Alcohols. Unexpected Formation of Siloxanes from the Reaction of Hydrosilanes and Ph₃COH Catalyzed by B(C₆F₅)₃

The reaction of [Cp­(η⁵-C₅H₄CH₂SiMe₂H)­TiCl₂] (<b>1</b>; Cp = η⁵-C₅H₅) and methanol in the presence of catalytic amounts of B­(C₆F₅)₃ afforded a complex with a pendant silyl ether group, [Cp­(η⁵-C₅H₄CH₂SiMe₂OMe)­TiCl₂] (<b>2</b>), in good yield. The analogous reaction of <b>1</b> and Ph₃COH resulted in the unexpected formation of [CpTiCl₂{μ-η⁵:η⁵-(C₅H₄)­CH₂SiMe₂OSiMe₂CH₂(C₅H₄)}­TiCl₂Cp] (<b>4</b>). The formation of siloxanes from the reaction of 2 equiv of hydrosilane with Ph₃COH mediated by B­(C₆F₅)₃ has a general applicability and proceeds in two consecutive steps: (i) transfer of the hydroxyl group from the trityl moiety to the silicon atom and (ii) silylation of the silanol formed in situ with the second equivalent of hydrosilane. The different hydrosilane reactivity toward Ph₃COH in comparison with other alcohols can be attributed to the easy generation of the borate salt [Ph₃C]⁺[(C₆F₅)₃B­(μ-OH)­B­(C₆F₅)₃]⁻ (<b>5</b>) under catalytic conditions. The intramolecular Si–H and Ti–Cl exchange in <b>1</b> is catalyzed by B­(C₆F₅)₃ in the presence of no alcohol. This process affords presumably a transient titanocene hydrido chloride, which is either chlorinated to give [Cp­(η⁵-C₅H₄CH₂SiMe₂Cl)­TiCl₂] (<b>3</b>) in CD₂Cl₂ or decomposes into several paramagnetic Ti­(III) species in toluene-<i>d</i>₈. Complex <b>3</b> was independently synthesized from <b>1</b> and Ph₃CCl in a good yield

Are Time-Dependent Fluorescence Shifts at the Tunnel Mouth of Haloalkane Dehalogenase Enzymes Dependent on the Choice of the Chromophore?

Time-dependent fluorescence shifts (TDFS) of chromophores selectively attached to proteins may give information on the dynamics of the probed protein moieties and their degree of hydration. Previously, we demonstrated that a coumarin dye selectively labeling the tunnel mouth of different haloalkane dehalogenases (HLDs) can distinguish between different widths of tunnel mouth openings. In order to generalize those findings analogous experiments were performed using a different chromophore probing the same region of these enzymes. To this end we synthesized and characterized three new fluorescent probes derived from dimethylaminonaphthalene bearing a linker almost identical to that of the coumarin dye used in our previous study. Labeling efficiencies, acrylamide quenching, fluorescence anisotropies, and TDFS for the examined fluorescent substrates confirm the picture gained from the coumarin studies: the different tunnel mouth opening, predicted by crystal structures, is reflected in the hydration and tunnel mouth dynamics of the investigated HLDs. Comparison of the TDFS reported by the coumarin dye with those obtained with the new dimethylaminonaphthalene dyes shows that the choice of chromophore may strongly influence the recorded TDFS characteristics. The intrinsic design of our labeling strategy and the variation of the linker length ensure that both dyes probe the identical enzyme region; moreover, the covalently fixed position of the chromophore does not allow for a major relocalization within the HLD structures. Our study shows, for the first time, that TDFS may strongly depend on the choice of the chromophore, even though the identical region of a protein is explored

Fluorescence Quenching of (Dimethylamino)naphthalene Dyes Badan and Prodan by Tryptophan in Cytochromes P450 and Micelles

Fluorescence of 2-(<i>N</i>,<i>N</i>-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700–800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 10⁸ s^–1 (CYP102) and 3.7 × 10⁸ s^–1 (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at −1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at −0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(<i>N</i>,<i>N</i>-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics

Photophysics of Singlet and Triplet Intraligand Excited States in [ReCl(CO)₃(1-(2-pyridyl)-imidazo[1,5-α]pyridine)] Complexes

Excited-state characters and dynamics of [ReCl­(CO)₃(3-R-1-(2-pyridyl)-imidazo­[1,5-α]­pyridine)] complexes (abbreviated <b>ReGV-R</b>, R = CH₃, Ph, PhBu^<i>t</i>, PhCF₃, PhNO₂, PhNMe₂) were investigated by pico- and nanosecond time-resolved infrared spectroscopy (TRIR) and excited-state DFT and TD-DFT calculations. Near UV excitation populates the lowest singlet state S₁ that undergoes picosecond intersystem crossing (ISC) to the lowest triplet T₁. Both states are initially formed hot and relax with ∼20 ps lifetime. TRIR together with quantum chemical calculations reveal that S₁ is predominantly a ππ* state localized at the 1-(2-pyridyl)-imidazo­[1,5-α]­pyridine (= impy) ligand core, with impy → PhNO₂ and PhNMe₂ → impy intraligand charge-transfer contributions in the case of <b>ReGV-PhNO</b>_<b>2</b> and <b>ReGV-PhNMe</b>_<b>2</b>, respectively. T₁ is predominantly ππ*­(impy) in all cases. It follows that excited singlet and corresponding triplet states have to some extent different characters and structures even if originating nominally from the same preponderant one-electron excitations. ISC occurs with a solvent-independent (CH₂Cl₂, MeCN) 20–30 ps lifetime, except for <b>ReGV-PhNMe</b>_<b>2</b> (10 ps in CH₂Cl₂, 100 ps in MeCN). ISC is 200–300 times slower than in analogous complexes with low-lying MLCT states. This difference is interpreted in terms of spin–orbit interaction and characters of orbitals involved in one-electron excitations that give rise to S₁ and T₁ states. <b>ReGV-R</b> present a unique case of octahedral heavy-metal complexes where the S₁ lifetime is long enough to allow for separate spectroscopic characterization of singlet and triplet excited states. This study provides an insight into dynamics and intersystem crossing pathways of low-lying singlet and triplet excited states localized at bidentate ligands bound directly to a heavy metal atom. Rather long ¹IL lifetimes indicate the possibility of photonic applications of singlet excited states

Unusual Reactivity of a C,N-Chelated Stannylene with Siloxanes and Silanols

The reactivity of stannylene (L^CN)₂Sn (<b>1</b>), where L^CN is the 2-(<i>N,N</i>-dimethylaminomethyl)­phenyl substituent, toward (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO), silicon grease, and triphenylsilanol was explored. The reaction of <b>1</b> with one equivalent of TEMPO and silicon grease yielded <i>cyclo</i>-[(L^CN)₂SnOSn­(L^CN)₂OSiMe₂O] (<b>6a</b>), whereas the addition of two equivalents of TEMPO afforded <i>cyclo</i>-[(L^CN)­(L^CNO)­SnOSiMe₂OSiMe₂O] (<b>6b</b>), in which the amine nitrogen atom of one of the L^CN ligands is oxidized to N-oxide. The reaction of <b>1</b> with one equivalent of TEMPO and subsequent addition of triphenylsilanol gave (L^CN)₂Sn­(OSiPh₃)₂ (<b>5</b>), which further reacted with air in a chloroform solution to provide [(L^CN)­(L^CNO)­Sn­(OSiPh₃)­Cl] (<b>7</b>), containing one of the chelating L^CN ligands in the corresponding N-oxide form. In contrast, the direct reactions of <b>1</b> with one or two equivalents of triphenylsilanol gave rise to the adduct [Sn­(OSiPh₃)₂]·C₆H₅CH₂NMe₂ (<b>8a</b>) and the salt [C₆H₅CH₂N­(H)­Me₂]⁺[Sn­(OSiPh₃)₃]⁻ (<b>8b</b>), respectively. All compounds were characterized by NMR spectroscopy and by X-ray diffraction. Analogous reactions of <b>1</b> with activated silica were shown to yield tin-doped silica, which was characterized by powder X-ray diffraction and various spectroscopic and microscopic techniques

Cytotoxic Lipopeptide Muscotoxin A, Isolated from Soil Cyanobacterium <i>Desmonostoc muscorum</i>, Permeabilizes Phospholipid Membranes by Reducing Their Fluidity

There is mounting evidence that cyanobacterial lipopeptides can kill mammalian cells, presenting a hazard to human health. Unfortunately, their mechanism of toxicity is poorly understood. We have isolated new cyclic undeca-lipopeptides muscotoxin A and B containing unique lipophilic residue 3-amino-2,5-dihydroxydecanoic acid (5-OH Ahdoa). Muscotoxin B was not used for biological studies due to its poor yield. Muscotoxin A was cytotoxic to YAC-1, Sp/2, and HeLa cancer cell lines (LC₅₀ ranged from 9.9 to 13.2 μM after 24 h of exposure), causing membrane damage and influx of calcium ions. Subsequently, we studied this lytic mechanism using synthetic liposomes with encapsulated fluorescent probes. Muscotoxin A permeabilized liposomes composed exclusively of phospholipids, demonstrating that no proteins or carbohydrates present in biomembranes are essential for its activity. Paradoxically, the permeabilization activity of muscotoxin A was mediated by a significant reduction in membrane surface fluidity (stiffening), the opposite of that caused by synthetic detergents and cytolytic lipopeptide puwainaphycin F. At 25 °C, muscotoxin A disrupted liposomes with and without cholesterol/sphingomyelin; however, at 37 °C, it was selective against liposomes with cholesterol/sphingomyelin. It appears that both membrane fluidity and organization can affect the lytic activity of muscotoxin A. Our findings strengthen the evidence that cyanobacterial lipopeptides specifically disrupt mammalian cell membranes and bring new insights into the mechanism of this effect

Site-Specific Analysis of Protein Hydration Based on Unnatural Amino Acid Fluorescence

Hydration of proteins profoundly affects their functions. We describe a simple and general method for site-specific analysis of protein hydration based on the in vivo incorporation of fluorescent unnatural amino acids and their analysis by steady-state fluorescence spectroscopy. Using this method, we investigate the hydration of functionally important regions of dehalogenases. The experimental results are compared to findings from molecular dynamics simulations