Hydroxymethylated Dioxobilins in Senescent Arabidopsis thaliana Leaves: Sign of a Puzzling Biosynthetic Intermezzo of Chlorophyll Breakdown

1‐Formyl‐19‐oxobilin‐type tetrapyrroles are characteristic, abundant products of chlorophyll breakdown in senescent leaves. However, in some leaves, 1,19‐dioxobilin‐type chlorophyll catabolites (DCCs) lacking the formyl group accumulate instead. A P450 enzyme was identified in in vitro studies that removed the formyl group of a primary fluorescent chlorophyll catabolite (pFCC) and generated fluorescent DCCs. These DCCs are precursors of isomeric nonfluorescent DCCs (NDCCs). Here, we report a structural investigation of the NDCCs in senescent leaves of wild‐type Arabidopsis thaliana. Four new NDCCs were characterized, two of which carried a stereoselectively added hydroxymethyl group. Such formal DCC hydroxymethylations were previously found in DCCs in leaves of a mutant of A. thaliana. They are now indicated to be a feature of chlorophyll breakdown in A. thaliana, associated with the specific in vivo deformylation of pFCC en route to NDCCs.     

Hydroxymethylated Phyllobilins: A Puzzling New Feature of the Dioxobilin Branch of Chlorophyll Breakdown

Colorless nonfluorescent chlorophyll (Chl) catabolites (NCCs) are formyloxobilin‐type phyllobilins, which are considered the typical products of Chl breakdown in senescent leaves. However, in degreened leaves of some plants, dioxobilin‐type Chl catabolites (DCCs) predominate, which lack the formyl group of the NCCs, and which arise from Chl catabolites by oxidative removal of the formyl group by a P450 enzyme. Here a structural investigation of the DCCs in the methylesterase16 mutant of Arabidopsis thaliana is reported. Eight new DCCs were identified and characterized structurally. Strikingly, three of these DCCs carry stereospecifically added hydroxymethyl groups, and represent bilin‐type linear tetrapyrroles with an unprecedented modification. Indeed, DCCs show a remarkable structural parallel, otherwise, to the bilins from heme breakdown.     

A Nontemplated Route to Macrocyclic Dibridgehead Diphosphorus Compounds: Crystallographic Characterization of a “Crossed‐Chain” Variant of in/out Stereoisomers

Reactions of (O=)PH(OCH₂CH₃)₂ and BrMg(CH₂)_mCH=CH₂ (4.9–3.2 equiv; m=4 ( a ), 5 ( b ), 6 ( c )) give the dialkylphosphine oxides (O=)PH[(CH₂)_mCH=CH₂]₂ ( 2 a – c ; 77–81 % after workup), which are treated with NaH and then α,ω‐dibromides Br(CH₂)_nBr (0.49–0.32 equiv; n=8 ( a′ ), 10 ( b′ ), 12 ( c′ ), 14 ( d′ )) to yield the bis(trialkylphosphine oxides) [H₂C=CH(CH₂)_m]₂P(=O)(CH₂)_n(O=)P[(CH₂)_mCH=CH₂]₂ ( 3 ab′ , 3 bc′ , 3 cd′ , 3 ca′ ; 79–84 %). Reactions of 3 bc′ and 3 ca′ with Grubbs’ first‐generation catalyst and then H₂/PtO₂ afford the dibridgehead diphosphine dioxides ( 4 bc′ , 4 ca′ ; 14–19 %, n′=2m+2); ³¹P NMR spectra show two stereoisomeric species (ca. 70:30). Crystal structures of two isomers of the latter are obtained, out,out‐ 4 ca′ and a conformer of in,out‐ 4 ca′ that features crossed chains, such that the (O=)P vectors appear out,out. Whereas 4 bc′ resists crystallization, a byproduct derived from an alternative metathesis mode, (CH₂)₁₂P (=O)(CH₂)₁₂(O=)P(C H₂)₁₂, as well as 3 ab′ and 3 bc′ , are structurally characterized. The efficiencies of other routes to dibridgehead diphosphorus compounds are compared.     

A Precoordination Complex of 1,2,3‐Trimethyl‐1,3,5‐triazacyclohexane with tert‐Butyllithium as Key Intermediate in Its Methylene Group Deprotonation

α‐Lithiated tertiary methylamines are important building blocks in all fields of chemistry, such as for the synthesis of new ligand or catalyst systems. However, the access to these compounds is still limited and the reaction mechanism, in general, not fully understood. We present herein X‐ray diffraction analyses of organolithium compounds with 1,2,3‐trimethyl‐1,3,5‐triazacyclohexane ( 1 ), such as a precoordination adduct of tert‐butyllithium, [(tBuLi)₃?C₆H₁₅N₃], which represents a potential intermediate of the lithiation of the methylene group of this ligand. By means of molecular structures and computational studies, the regioselectivity of this deprotonation reaction can be understood. Furthermore, the tBuLi adduct gives a hint to an alternative deaggregation process of organolithium compounds.     

Absolute Stereochemistry of a 4 a‐Hydroxyriboflavin Analogue of the Key Intermediate of the FAD‐Monooxygenase Cycle

The biological action of flavoenzymes, such as flavin adenine dinucleotide (FAD)‐containing monooxygenase, involves the formation of oxygenated flavin derivatives, such as 4 a‐hydroperoxyflavin and 4 a‐hydroxyflavin, in which a new center of chirality is created at the 4 a position during the enzymatic reactions. So far, the absolute configuration of this center of chirality in natural 4 a‐oxygenated flavins has remained unknown in spite of its key importance for the diverse functions of flavoenzymes. Herein, we report the 4 a‐hydroxy adduct 3 of 3‐benzyl‐5‐ethyl‐10‐(tetraacetyl‐D ‐ribityl)flavinium ( 1 ), one of the key intermediates involved in the enantioselective organocatalytic oxidation of sulfides to sulfoxides. The 4 a‐hydroxyflavin diastereomers (+)‐ 3 and (?)‐ 3 , separated by HPLC, were characterized by electronic circular dichroism (CD) spectroscopy. Their absolute configurations at the 4 a position were, for the first time, determined by comparing experimental CD spectra with those calculated by means of time‐dependent density functional theory (TDDFT) on DFT‐optimized structures obtained after an extensive conformation analysis.     

Thiophene‐S,S‐dioxidized Indophenine: A Quinoid‐Type Building Block with High Electron Affinity for Constructing n‐Type Polymer Semiconductors with Narrow Band Gaps

Three thiophene‐S,S‐dioxidized indophenine (IDTO) isomers, 3 a (E,E,E), 3 b (Z,E,E), and 3 c (Z,E,Z), were synthesized by oxidation of an indophenine compound. 3 b and 3 c could be converted into the most‐stable 3 a by heating at 110 °C. An IDTO‐containing conjugated polymer, PIDTOTT, was prepared using 3 a as a comonomer through a Stille coupling reaction, and it possesses a narrow band gap and low energy levels. In organic field effect transistors (OFETs), PIDTOTT exhibited unipolar n‐type semiconductor characteristics with unexpectedly high electron mobility (up to 0.14 cm² V^?1 s^?1), despite its rather disordered chain packing.     

Small‐Molecule Anion Recognition by a Shape‐Responsive Bowl‐Type Dodecavanadate

A dodecavanadate, [V₁₂O₃₂]⁴⁻, is an inorganic bowl‐type host with a cavity entrance with a diameter of 4.4 Å in the optimized structure. Linear, bent, and trigonal planar anions are tested as guest anions and the formation of host–guest complexes, [V₁₂O₃₂(X)]⁵⁻ (X=CN⁻, OCN⁻, NO₂⁻, NO₃⁻, HCO₂⁻, and CH₃CO₂⁻), were confirmed by X‐ray crystallographic analyses and a ⁵¹V NMR spectroscopy study. The degree of distortion of the bowl from a regular to an oval shape depends on the type of guest anion. In ⁵¹V NMR spectroscopy, all chemical shifts of the host–guest complexes are clearly shifted after guest incorporation. The incorporation reaction rates for OCN⁻, NO₂⁻, HCO₂⁻, and CH₃CO₂⁻ are much larger than those of NO₃⁻ and halides. The incorporated nonspherical molecular anions in the dodecavanadate host are easily dissociated or exchanged for other anions, whereas spherical halides in the host are preserved without dissociation, even in the presence of the tested anions.     

Synthetic and Structural Studies on Copper 1 H‐[1,10]‐Phenanthrolin‐2‐one Coordination Complexes: Isolation of a Novel Intermediate During 1,10‐Phenanthroline Hydroxylation

The synthesis and crystal structure elucidation of a novel dinuclear heteroleptic copper(II) complex has led to an alternative mechanism in the formation of covalent hydrates. During further studies on the synthesis and properties of [Cu₂(ophen)₂] ( 1 ), a dinuclear complex of copper(I) with 1 H‐[1,10]‐phenanthrolin‐2‐one (Hophen), two intermediates/alternative products 2 and 3 were isolated. The dinuclear, antiferromagnetic complex [Cu₂(ophen)₂(phen)₂] ? (NO₃)₂ ? 9H₂O ( 3 , phen=1,10‐phenanthroline) contains two five‐coordinate copper(II) ions, both with trigonal‐bipyramidal coordination, which are bridged together through deprotonated hydroxyl groups with a Cu? Cu non‐bonding distance of 3.100 Å. Complex [Cu(phen)₂(H₂O)] ? (NO₃)₂ ( 2 ) is a polymorph of a previously reported material. The occurrence of 2 and 3 has led us to propose a variation to the Gillard mechanism for the formation of covalent hydrates in bidentate N‐heterocycles in which the attacking nucleophile may be the deprotonated form of 2 , [Cu(phen)₂(OH)]^?, rather than free OH^?.     

A EuII‐Containing Cryptate as a Redox Sensor in Magnetic Resonance Imaging of Living Tissue

The Eu^II ion rivals Gd^III in its ability to enhance contrast in magnetic resonance imaging. However, all reported Eu^II‐based complexes have been studied in vitro largely because the tendency of Eu^II to oxidize to Eu^III has been viewed as a major obstacle to in vivo imaging. Herein, we present solid‐ and solution‐phase characterization of a Eu^II‐containing cryptate and the first in vivo use of Eu^II to provide contrast enhancement. The results indicate that between one and two water molecules are coordinated to the Eu^II core upon dissolution. We also demonstrate that Eu^II‐based contrast enhancement can be observed for hours in a mouse.     

A Versatile Triphenylamine/Fluoranthene‐Based Derivative as a Nondoped Green‐Emitting,Hole‐Transporting Interlayer for Electroluminescent Devices

A new triphenylamine‐bridged fluoranthene derivative, 4‐(7,10‐diphenylfluoranthen‐8‐yl)‐N‐[4‐(7,10‐diphenylfluoranthen‐8‐yl)phenyl]‐N‐phenylaniline (BDPFPA), with a high glass transition temperature of 220 °C has been synthesized and characterized. BDPFPA is a highly fluorescent and versatile material that can be used as a nondoped green emitter and as a hole transporter. BDPFPA was used in a standard trilayer device as the emitting layer, which showed a low turn‐on voltage (<3 V) and a high efficiency of 11.6 cd A^?1. The device also shows little efficiency roll‐off at high brightness. For example, the efficiency can still be maintained at 11.4 cd A^?1 (5.4 lm W^?1) at a brightness of 10 000 cd m^?2. These results are among the best reported for nondoped fluorescent green organic light‐emitting diodes. A simple bilayer device, in which BDPFPA serves as a hole‐transporting layer, has a maximum power efficiency of 3.3 lm W^?1 and the performance is nearly 40 % higher than that of an N,N′‐bis(1‐naphthyl)‐N,N′‐ diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB)‐based standard device.     

Preparation of Selenoinsulin as a Long‐Lasting Insulin Analogue

Synthetic insulin analogues with a long lifetime are current drug targets for the therapy of diabetic patients. The replacement of the interchain disulfide with a diselenide bridge, which is more resistant to reduction and internal bond rotation, can enhance the lifetime of insulin in the presence of the insulin‐degrading enzyme (IDE) without impairing the hormonal function. The [C7U^A,C7U^B] variant of bovine pancreatic insulin (BPIns) was successfully prepared by using two selenocysteine peptides (i.e., the C7U analogues of A‐ and B‐chains, respectively). In a buffer solution at pH 10 they spontaneously assembled under thermodynamic control to the correct insulin fold. The selenoinsulin (Se‐Ins) exhibited a bioactivity comparable to that of BPIns. Interestingly, degradation of Se‐Ins with IDE was significantly decelerated (τ _1/2≈8 h vs. ≈1 h for BPIns). The lifetime enhancement could be due to both the intrinsic stability of the diselenide bond and local conformational changes induced by the substitution.