Photoisomerization in proteorhodopsin mutant D97N

The first steps of the photocycle of the D97N mutant of proteorhodopsin (PR) have been investigated by means of ultrafast transient absorption spectroscopy. A comparison with the primary dynamics of native PR and D85N mutant of bacteriorhodopsin is given. Upon photoexcitation of the covalently bound all-trans retinal the excited state decays biexponentially with time constants of 1.4 and 20 ps via a conical intersection, resulting in a 13-cis isomerized retinal. Neither of the two-deactivation channels is significantly preferred. The dynamics is slowed down in comparison with native PR at pH 9 and reaction rates are even lower than for native PR at pH 6, where the primary proton acceptor (Asp97) is protonated. Therefore, the ultrafast isomerization is not only controlled by the charge distribution within the retinal binding pocket. This study shows that in addition to direct electrostatics other effects have to be taken into account to explain the catalytic function of Asp97 in PR on the ultrafast isomerization reaction. This may include sterical interactions and/or bound water molecules within the retinal binding pocket.     

Difluorophosphoryl Nitrene F2P(O)N: Matrix Isolation and Unexpected Rearrangement to F2PNO

Triplet difluorophosphoryl nitrene F₂P(O)N (X³A′′) was generated on ArF excimer laser irradiation (λ=193 nm) of F₂P(O)N₃ in solid argon matrix at 16 K, and characterized by its matrix IR, UV/Vis, and EPR spectra, in combination with DFT and CBS‐QB3 calculations. On visible light irradiation (λ>420 nm) at 16 K F₂P(O)N reacts with molecular nitrogen and some of the azide is regenerated. UV irradiation (λ=255 nm) of F₂P(O)N (X³A′′) induced a Curtius‐type rearrangement, but instead of a 1,3‐fluorine shift, nitrogen migration to give F₂PON is proposed to be the first step of the photoisomerization of F₂P(O)N into F₂PNO (difluoronitrosophosphine). Formation of novel F₂PNO was confirmed with ¹⁵N‐ and ¹⁸O‐enriched isotopomers by IR spectroscopy and DFT calculations. Theoretical calculations predict a rather long P? N bond of 1.922 Å [B3LYP/6‐311+G(3df)] and low bond‐dissociation energy of 76.3 kJ mol^?1 (CBS‐QB3) for F₂PNO.     

以2-硝基-苯-1,4-二氧二乙酸为柔性配体的[Ca(L)(H2O)2] n的合成、晶体结构及表征

采用缓慢蒸发法以2-硝基-苯-1,4-二氧二乙酸为柔性配体合成新型多孔配位聚合物[Ca(nbdo)(H₂O)₂] _n,并对其进行元素、红外光谱、X-射线单晶衍射、DSC、TG-DTG和荧光光谱的分析测试。晶体结构显示为一维微孔结构,由于四种形式的氢键的存在使晶胞堆积形成了三维超分子网络结构。热分析表明该化合物在375K时失去了水分子,当温度升高到了550K时,配位聚合物的网状结构出现了破坏。荧光光谱分析测试表明该化合物在室温固体状态下320nm处具有较强的荧光性。     

Crystal structure and photoluminescence properties of a new monomeric copper(II) complex: bis(3‐{[(3‐hydroxypropyl)imino]methyl}‐4‐nitrophenolato‐κ3O,N,O′)copper(II)

Copper(II)–Schiff base complexes have attracted extensive interest due to their structural, electronic, magnetic and luminescence properties. The title novel monomeric Cu^II complex, [Cu(C₁₀H₁₁N₂O₄)₂], has been synthesized by the reaction of 3‐{[(3‐hydroxypropyl)imino]methyl}‐4‐nitrophenol (H₂L ) and copper(II) acetate monohydrate in methanol, and was characterized by elemental analysis, UV and IR spectroscopies, single‐crystal X‐ray diffraction analysis and a photoluminescence study. The Cu^II atom is located on a centre of inversion and is coordinated by two imine N atoms, two phenoxy O atoms in a mutual trans disposition and two hydroxy O atoms in axial positions, forming an elongated octahedral geometry. In the crystal, intermolecular O—H…O hydrogen bonds link the molecules to form a one‐dimensional chain structure and π–π contacts also connect the molecules to form a three‐dimensional structure. The solid‐state photoluminescence properties of the complex and free H₂L have been investigated at room temperature in the visible region. When the complex and H₂L are excited under UV light at 349 nm, the complex displays a strong green emission at 520 nm and H₂L displays a blue emission at 480 nm.     

Synthesis of Layered Sodium Manganese Phosphate via Low-heating Solid State Reaction and Its Properties

The layered nanocrystalline sodium manganese phosphate was synthesized by low‐heating solid state reaction using MnSO₄·H₂O and Na₃PO₄·12H₂O as raw materials. The resulting sodium manganese phosphate and its calcined products were characterized using element analysis, thermogravimetry and differential thermal analyses (TG/DTA), Fourier transform IR (FT‐IR), X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), ultraviolet‐visible (UV‐Vis) absorption spectroscopy, and magnetic susceptibility. The results showed that the product obtained at 70°C for 3 h, NaMnPO₄·3H₂O, was a layered compound, and its crystallite size and interlayer distance were 27 nm and 1.124 nm, respectively. The thermal process of NaMnPO₄·3H₂O between room temperature and 700°C experienced three steps, the dehydration of the one adsorption water at first, and then dehydration of the two crystal waters, at last crystallization of NaMnPO₄. Magnetic susceptibility measurements of NaMnPO₄· 3H₂O from room temperature to 2.5 K point to ferrimagnetic ordering at T_N‐35 K.     

Computational study of pH‐responsive di‐lanthanide complexes

Lanthanide complexes have found extensive use as luminescent probes for biological and medical investigations. Recently, a di‐europium complex that exhibits pH‐dependent luminescence‐decay was reported, and the ligand in that complex includes a large number of ionizable sites. To better understand the pH‐dependence of luminescence‐decay of this complex, the pK _a's of all tautomers of the di‐Lu³⁺ version of this complex were calculated computationally. The calculated Boltzmann‐averaged pK _a's of the complex are 5.85, ?0.21, and ?1.47 for the di‐Lu³⁺ complex in its first, second, and third protonation states, respectively. These pK _a values across protonation states indicate that changes in luminescence‐decay rate at physiologically relevant pH may be related to first protonation event of the complex exclusively.     

The Photocycle of Channelrhodopsin‐2: Ultrafast Reaction Dynamics and Subsequent Reaction Steps

The photocycle of channelrhodopsin‐2 is investigated in a comprehensive study by ultrafast absorption and fluorescence spectroscopy as well as flash photolysis in the visible spectral range. The ultrafast techniques reveal an excited‐state decay mechanism analogous to that of the archaeal bacteriorhodopsin and sensory rhodopsin II from Natronomonas pharaonis. After a fast vibrational relaxation of the excited‐state population with 150 fs its decay with mainly 400 fs is observed. Hereby, both the initial all‐trans retinal ground state and the 13‐cis‐retinal K photoproduct are populated. The reaction proceeds with a 2.7 ps component assigned to cooling processes. Small spectral shifts are observed on a 200 ps timescale. They are attributed to conformational rearrangements in the retinal binding pocket. The subsequent dynamics progresses with the formation of an M‐like intermediate (7 and 120 μs), which decays into red‐shifted states within 3 ms. Ground‐state recovery including channel closing and reisomerization of the retinal chromophore occurs in a triexponential manner (6 ms, 33 ms, 3.4 s). To learn more about the energy barriers between the different photocycle intermediates, temperature‐dependent flash photolysis measurements are performed between 10 and 30 °C. The first five time constants decrease with increasing temperature. The calculated thermodynamic parameters indicate that the closing mechanism is controlled by large negative entropy changes. The last time constant is temperature independent, which demonstrates that the photocycle is most likely completed by a series of individual steps recovering the initial structure.     

Characterizing the Structure and Photocycle of PR 2D Crystals with CD and FTIR Spectroscopy†

We present here a study on proteorhodopsin (PR) 2D crystals with analytical ultracentrifugation, circular dichroism and Fourier transform infrared (FTIR) spectroscopy. The aim of our experiments was to test the activity of 2D crystal sample preparations and to gain further insight in PR structure, stability and function with these techniques. Our results demonstrate higher stability compared to detergent‐solubilized or reconstituted samples. For different pH values, low pH 2D crystals tend to form bigger aggregates and are less stable than at basic pH. The pH 9 sample shows a sharp phase transition during heat denaturation and there is also evidence for protein–protein interaction due to the close proximity of the proteins in the 2D crystals. In the FTIR measurements at cryogenic temperatures (77 K), we characterized the first step in the PR photocycle. At pH 9, the K intermediate could be observed and the samples showed no orientation effects. At pH 5, we could trap the K/L intermediate, characterized by its negative IR signal at 1741 cm^?1. In rapid‐scan FTIR experiments, we could also identify the M intermediate of the photocycle at basic pH. We conclude that the PR 2D crystals exhibit a fully functional photocycle and are therefore well suited for further studies on the proton transport mechanism of PR.     

15N Chemically Induced Dynamic Nuclear Polarization (15N‐CIDNP) Investigations of the Peroxynitrite Decay and Nitration of N‐Acetyl‐L‐tyrosine

During the decay of (¹⁵N)peroxynitrite (O?¹⁵NOO ^? ) in the presence of N‐acetyl‐L ‐tyrosine (Tyrac) in neutral solution and at 268 K, the ¹⁵N‐NMR signals of ¹⁵NO and ¹⁵NO show emission (E) and enhanced absorption (A) as it has already been observed by Butler and co‐workers in the presence of L ‐tyrosine (Tyr). The effects are built up in radical pairs [CO , ¹⁵NO ]^S formed by O? O bond scission of the (¹⁵N)peroxynitrite? CO₂ adduct (O?¹⁵NO? OCO ). In the absence of Tyrac and Tyr, the peroxynitrite decay rate is enhanced, and ¹⁵N‐CIDNP does not occur. This is explained by a chain reaction during the peroxynitrite decay involving N₂O₃ and radicals NO ^. and NO . The interpretation is supported by ¹⁵N‐CIDNP observed with (¹⁵N)peroxynitrite generated in situ during reaction of H₂O₂ with N‐acetyl‐N‐(¹⁵N)nitroso‐dl ‐tryptophan ((¹⁵N)NANT) at 298 K and pH 7.5. In the presence of Na¹⁵NO₂ at pH 7.5 and in acidic solution, ¹⁵N‐CIDNP appears in the nitration products of Tyrac, 1‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (1‐¹⁵NO₂‐Tyrac) and 3‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (3‐¹⁵NO₂‐Tyrac). The effects are built up in radical pairs [Tyrac ^. , ¹⁵NO ]^F formed by encounters of independently generated radicals Tyrac ^. and ¹⁵NO . Quantitative ¹⁵N‐CIDNP studies show that nitrogen dioxide dependent reactions are the main if not the only pathways for yielding both nitrate and nitrated products.     

Steady‐State and Pseudo‐Steady‐State Photocrystallographic Studies on Linkage Isomers of [Ni(Et4dien)(η2‐O,ON)(η1‐NO2)]: Identification of a New Linkage Isomer

At temperatures below 150 K, the photoactivated metastable endo‐nitrito linkage isomer [Ni(Et₄dien)(η²‐O,ON)(η¹‐ONO)] (Et₄dien=N,N,N′,N′‐tetraethyldiethylenetriamine) can be generated with 100 % conversion from the ground state nitro‐(η¹‐NO₂) isomer on irradiation with 500 nm light, in the single crystal by steady‐state photocrystallographic techniques. Kinetic studies show the system is no longer metastable above 150 K, decaying back to the ground state nitro‐(η¹‐NO₂) arrangement over several hours at 150 K. Variable‐temperature kinetic measurements in the range of 150–160 K show that the rate of endo‐nitrito decay is highly dependent on temperature, and an activation energy of E_act=+48.6(4) kJ mol^?1 is calculated for the decay process. Pseudo‐steady‐state experiments, where the crystal is continually pumped by the light source for the duration of the X‐ray experiment, show the production of a previously unobserved, exo‐nitrito‐(η¹‐ONO) linkage isomer only at temperatures close to the metastable limit (ca. 140–190 K). This exo isomer is considered to be a transient excited‐state species, as it is only observed in data collected by pseudo‐steady‐state methods.     

Reduction of conformational mobility and aggregation in W60G β2‐microglobulin: assessment by 15N NMR relaxation

The amyloid pathology associated with long‐term haemodialysis is due to the deposition of β₂‐microglobulin, the non‐polymorphic light chain of class I major histocompatibility complex, that accumulates at bone joints into amyloid fibrils. Several lines of evidence show the relevance of the tryptophan residue at position 60 for the fibrillogenic transition of the protein. A comparative ¹⁵N NMR relaxation analysis is presented for wild‐type human β₂‐microglobulin and W60G β₂‐microglobulin, i.e. the mutant with a glycyne replacing the natural tryptophan residue at position 60. The experimental data, collected at 11.4 T and 310 K, were analyzed by means of the reduced spectral density approach. Molecular dynamics (MD) simulations and corresponding thermodynamic integration, together with hydrodynamic calculations were performed to support data interpretation. The analysis results for the mutant protein are consistent with a reduced aggregation with respect to the wild‐type counterpart, as a consequence of an increased conformational rigidity probed by either NMR relaxation and MD simulations. Although dynamics in solution is other than fibrillar competence, the assessed properties of the mutant protein can be related with its reduced ability of forming fibrils when seeded in 20% trifluoroethanol. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis,Crystal Structures,and Photoluminescence of Lanthanide Coordination Polymers with 4‐Acetamidobenzoate

The reactions of Ln(NO₃)₃ · 6H₂O and 4‐acetamidobenzoic acid (Haba) with 4,4′‐bipyridine (4,4′‐bpy) in ethanol solution resulted in three new lanthanide coordination polymers, namely {[Ln(aba)₃(H₂O)₂] · 0.5(4,4′‐bpy) · 2H₂O}_∞ [Ln = Sm ( 1 ), Gd ( 2 ), and Er ( 3 ), aba = 4‐acetamidobenzoate]. Compounds 1 – 3 are isomorphous and have one‐dimensional chains bridged by four aba anions. 4,4′‐Bipyridine molecules don’t take part in the coordination with Ln^III ions and occur in the lattice as guest molecules. Moreover, the adjacent 1D chains in the complex are further linked through numerous N–H ··· O and O–H ··· O hydrogen bonds to form a 3D supramolecular network. In addition, complex 1 in the solid state shows characteristic emission in the visible region at room temperature.     

Association between a photo-intermediate of a M-lacking mutant D75N of pharaonis phoborhodopsin and its cognate transducer

Pharaonis phoborhodopsin (ppR or pharaonis sensory rhodopsin II) is a receptor of the negative phototaxis of Natronobacterium pharaonis and forms a complex with its transducer pHtrII in membranes. Flash-photolyis of a D75N mutant did not yield the M-intermediate, but an O-like intermediate is observed in a ms time range. We examined the interaction between the D75N of ppR and t-Htr (truncated pHtrII). These formed a complex in the presence of 0.1% n-dodecyl-beta-maltoside, and the association accelerated the decay of the O of D75N from 15 to 56 s(-1). From the decay time constants under varying ratios of D75N and t-Htr, n, the molar ratio of D75N/t-Htr in the complex, and K(D), the dissociation constant, were estimated. The value of n was unity and K(D) was estimated to 146 nM. This K(D) value can be considered to be the association between the photo-intermediate and t-Htr, which is deduced by the method of estimation. Previously we (Photochem. Photobiol. 74 (2001) 489) reported a K(D) of 15 microM for the interaction between the wild-type and t-Htr by means of the change in M-decay rates. Therefore, this value should be the K(D) value for the interaction between M of the wild-type and t-Htr.     

Kinetic Analysis and Structural Interpretation of Competitive Ligand Binding for NO Dioxygenation in Truncated Hemoglobin N

The conversion of nitric oxide (NO) into nitrate (NO₃^?) by dioxygenation protects cells from lethal NO. Starting from NO‐bound heme, the first step in converting NO into benign NO₃^? is the ligand exchange reaction FeNO+O₂→FeO₂+NO, which is still poorly understood at a molecular level. For wild‐type (WT) truncated hemoglobin N (trHbN) and its Y33A mutant, the calculated barriers for the exchange reaction differ by 1.5 kcal mol^?1, compared with 1.7 kcal mol^?1 from experiment. It is directly confirmed that the ligand exchange reaction is rate‐limiting in trHbN and that entropic contributions account for 75 % of the difference between the WT and the mutant. Residues Tyr 33, Phe 46, Val 80, His 81, and Gln 82 surrounding the active site are expected to control the reaction path. By comparison with electronic structure calculations, the transition state separating the two ligand‐bound states was assigned to a ²A state.     

Synthesis,Characterization, and Photoluminescence Properties of Silver(I) Metal‐Organic Polymers with Nanochannels Based on 2‐Sulfoterephthalic Acid and Di(pyridin‐2‐yl)amine Ligands

Two new silver(I) 3D coordination polymers, namely [Ag₃(2‐stp)(dpa)]_n ( 1 ) and {[Ag₂(2‐stp)(H₂O)]?Hdpa}_n ( 2 ) (2‐NaH₂stp=sodium 2,5‐dicarboxysulfonate, dpa=di(pyridine‐2‐yl)amine) were synthesized. The complexes were characterized by elemental analysis, FT‐IR spectra, thermogravimetric analyses (TGA), and single‐crystal X‐ray diffraction. In complex 1 , three neighboring Ag ions are bridged by N‐ and O‐atom, forming a 3D coordination network. The molecular structure of 2 is cation? anion species, forming 3D host? guest supramolecular network with the [Hdpa]⁺ cations encapsulated in the nanochannels. The photoluminescence properties of the complexes were also investigated in the solid state at room temperature.     

A Novel Mixed‐Ligand Manganese(II) Complex Containing a V‐shape Water Trimer: Synthesis,Structure and Magnetic Properties of {[Mn(azpy)2(dca)(H2O)2](ClO4)(azpy)(H2O)2}n

A novel mixed‐ligand complex {[Mn(azpy)₂(dca)(H₂O)₂](ClO₄)(azpy)(H₂O)₂}_n ( 1 ) has been synthesized and characterized by single crystal X‐ray analysis, elemental analysis, IR spectroscopy and variable temperature magnetic measurement. The 4,4′‐azopyridine and dicyanamide ligands are abbreviated as azpy and dca, respectively. The crystal structure of 1 revealed that the 1D covalent bonding chains constructed by μ_1,5‐dca bridging the Mn^II ions are linked together via O–H···N and O–H···O hydrogen bonds and π‐π stacking interactions into a 3D supramolecular structure. V‐shape (bent) water trimers were also found in the structure. The water clusters play an important role in the formation of the 3D supramolecular structure. The determination of the variable temperature magnetic susceptibilities (2–300 K) shows the existence of a very weak antiferromagnetic interaction with a J value of ?0.16 cm^?1.     

Long‐Lived Room‐Temperature Near‐IR Phosphorescence of BODIPY in a Visible‐Light‐Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore

Room‐temperature long‐lived near‐IR phosphorescence of boron‐dipyrromethene (BODIPY) was observed (λ_em=770 nm, Φ_P=3.5 %, τ_P=128.4 μs). Our molecular‐design strategy is to attach Pt^II coordination centers directly onto the BODIPY π‐core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy‐atom effect of Pt^II. In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T₁ excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε=53800 M ^?1 cm^?1 at 574 nm), which is rare for Pt^II–acetylide complexes. The complex is dual emissive with ³M LCT emission at 660 nm and the ³IL emission at 770 nm. The T₁ excited state of the complex is mainly localized on the BODIPY moiety (i.e. ³IL state, as determined by steady‐state and time‐resolved spectroscopy, 77 K emission spectra, and spin‐density analysis). The strong visible‐light‐harvesting ability and long‐lived T₁ excite state of the complex were used for triplet‐triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η=ε×Φ_UC) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition‐metal complexes that show strong absorption of visible light and long‐lived ³IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.     

Direct Observation of Carbamoylnitrenes

As the prototype Curtius rearrangement reaction, carbamoyl azide decomposes into aminoisocyanate and molecular nitrogen. However, the key intermediate carbamoylnitrene was previously undetected, even though the decomposition of carbamoyl azides has been studied frequently since its discovery in the 1890s. Upon ArF laser (λ=193 nm) photolysis, the stepwise decomposition of the two simplest carbamoyl azides H₂NC(O)N₃ and Me₂NC(O)N_3, isolated in solid noble gas matrices, occurs with the formation of the corresponding carbamoylnitrenes H₂NC(O)N and Me₂NC(O)N. Both triplet species are characterized for the first time by combining matrix‐isolation IR spectroscopy and quantum‐chemical calculations. Subsequent visible‐light irradiations cause efficient rearrangement of these nitrenes into the respective aminoisocyanates.     

Spin‐State‐Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex

The generation of iron(V) nitride complexes, which are targets of biomimetic chemistry, is reported. Temperature‐dependent ion spectroscopy shows that this reaction is governed by the spin‐state population of their iron(III) azide precursors and can be tuned by temperature. The complex [(MePy₂TACN)Fe(N₃)]²⁺ (MePy₂TACN=N ‐methyl‐N ,N ‐bis(2‐picolyl)‐1,4,7‐triazacyclononane) exists as a mixture of sextet and doublet spin states at 300 K, whereas only the doublet state is populated at 3 K. Photofragmentation of the sextet state complex leads to the reduction of the iron center. The doublet state complex photodissociates to the desired iron(V) nitride complex. To generalize these findings, we show results for complexes with cyclam‐based ligands.     

Synthesis of [MnCl<Subscript>3</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>(Hatz)]·H<Subscript>2</Subscript>O and investigation of its structure via X-ray diffraction,spectroscopic measurements and DFT calculations

The 3-amino-1,2,4-triazole (atz)-based manganese complex was prepared and characterized through single-crystal X-ray diffraction, IR, EPR, and UV–visible spectroscopy. In the crystal structure, individual complex are interconnected through N(O)–H…Cl hydrogen bonds into 1D undulating chains running parallel to the [110] direction of the unit cell. Chains further grow into 2D supramolecular layers by way of the lattice water molecules of coordination and the chloride anions (O–H…Cl). Layers pack along the b-axis of the unit cell mediated by O–H…Cl(N) and N–H…O(Cl) hydrogen bonds forming a 3D supramolecular architecture. The theoretical calculations were also performed to optimize the structure of the complexes in the gas phase to confirm the structures proposed by X-ray crystallography. In addition, IR and UV–visible spectra of complex were calculated and compared with the corresponding experimental spectra to complete the experimental structural identification. The three-dimensional Hirshfeld surface (3D-HS) and their relative two-dimensional fingerprint plots (2D-FP) reveal that the structure is dominated by H…Cl/Cl…H (50.5%), H…O/O…H (11.3%) and N…O/O…N (10.2%) contacts.