Designer Reagents for Mass Spectrometry-Based Proteomics: Clickable Cross-Linkers for Elucidation of Protein Structures and Interactions

We present novel homobifunctional amine-reactive clickable cross-linkers (CXLs) for investigation of three-dimensional protein structures and protein–protein interactions (PPIs). CXLs afford consolidated advantages not previously available in a simple cross-linker, including (1) their small size and cationic nature at physiological pH, resulting in good water solubility and cell-permeability, (2) an alkyne group for bio-orthogonal conjugation to affinity tags via the click reaction for enrichment of cross-linked peptides, (3) a nucleophilic displacement reaction involving the 1,2,3-triazole ring formed in the click reaction, yielding a lock-mass reporter ion for only clicked peptides, and (4) higher charge states of cross-linked peptides in the gas-phase for augmented electron transfer dissociation (ETD) yields. Ubiquitin, a lysine-abundant protein, is used as a model system to demonstrate structural studies using CXLs. To validate the sensitivity of our approach, biotin-azide labeling and subsequent enrichment of cross-linked peptides are performed for cross-linked ubiquitin digests mixed with yeast cell lysates. Cross-linked peptides are detected and identified by collision induced dissociation (CID) and ETD with linear quadrupole ion trap (LTQ)-Fourier transform ion cyclotron resonance (FTICR) and LTQ-Orbitrap mass spectrometers. The application of CXLs to more complex systems (e.g., in vivo cross-linking) is illustrated by Western blot detection of Cul1 complexes including known binders, Cand1 and Skp2, in HEK 293 cells, confirming good water solubility and cell-permeability

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S1. In this situation, the resulting emission is an S1→S0 fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of CuI or AgI complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing CuI and AgI materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S1–T1) between the lowest excited singlet state S1 and the triplet state T1 should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S1–T1) value of 1000 cm−1 (120 meV) and, for comparison, a small one of 370 cm−1 (46 meV). From these studies—extended by investigations of many other CuI TADF compounds—we can conclude that just small ΔE(S1–T1) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S1 state to the electronic ground state S0, expressed by the radiative rate kr(S1→S0) or the oscillator strength f(S1→S0), is also very important. However, mostly small ΔE(S1–T1) is related to small kr(S1→S0). This relation results from an experimental investigation of a large number of CuI complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P2-nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P2-nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the AgI complex for a fast radiative S1→S0 rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 μs at 100 % emission quantum yield. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Luminescent gold–silver complexes derived from neutral bis(perfluoroaryl)diphosphine gold(I) precursors

Complex [Au{4-C6F4(4-C6BrF4)}(tht)] reacts with diphosphines (L–L) such as bis(diphenylphosphino)methane (dppm) or 1,2-bis(diphenylphosphino)benzene (dppb) in a 2 : 1 molar ratio in dichloromethane, leading to neutral products of stoichiometry [(Au{4-C6F4(4-C6BrF4)})2(μ-L–L)] (L–L = dppm (1), dppb (2)). In the crystal structure of complex 2 short Au⋯Au interactions of 2.9367(5) and 2.9521(5) Å appear. This complex displays an orange emission, which is assigned to arise from a charge transfer transition from a metal centered Au–Au orbital to an orbital located at the diphosphine ligand. Addition of silver trifluoroacetate to these complexes in a 1 : 1 or a 2 : 1 molar ratio generates polymeric heterometallic gold–silver compounds of stoichiometry [Ag2Au2{4-C6F4(4-C6BrF4)}2(CF3CO2)2(μ-L–L)]n (L–L = dppm (3), dppb (4)), which confirms the capability of the neutral [(Au{4-C6F4(4-C6BrF4)})2(μ-diphosphine)] units to act as electron density donors when treated with a Lewis acid substrate. These heterometallic derivatives show blue emissions indicating large HOMO–LUMO band gaps, due to the stabilization that the gold-based HOMO orbitals suffer when the electron withdrawing silver trifluoroacetate fragments interact with them

Correction to blue-light emission of Cu(I) complexes and singlet harvesting

Page 8293. The emission decay time of Cu(pop)(pz₂Bph₂) at ambient temperature is erroneously given as 13 s instead of 13 μs. This short decay time represents an essential and characteristic property of the discussed Cu(I) complex. Therefore, the corrected Abstract is reproduced again. Strongly luminescent neutral copper(I) complexes of the type Cu(pop)(NN), with pop = bis(2-(diphenylphosphanyl)-phenyl)ether and NN = bis(pyrazol-1-yl)borohydrate (pz₂BH₂), tetrakis(pyrazol-1-yl)borate (pz₄B), or bis(pyrazol-1-yl)-biphenylborate (pz₂Bph₂), are readily accessible in reactions of Cu(acetonitrile)⁴⁺ with equimolar amounts of the pop and NN ligands at ambient temperature. All products were charcterized by means of single crystal X-ray diffractometry. The compounds exhibit very strong blue/white luminescence with emission quantum yields of up to 90%. Investigations of spectroscopic properties and the emission decay behavior in the temperature range between 1.6 K and ambient temperature allow us to assign the emitting electronic states. Below 100 K, the emission decay times are in the order of many hundreds of microseconds. Therefore, it is concluded that the emission stems from the lowest triplet state. This state is assigned to a metal-to-ligand charge-transfer state (³MLCT) involving Cu-3d and pop-π* orbitals. With temperature increase, the emission decay time is drastically reduced, e.g. to 13 μs (Cu(pop)-(pz₂Bph₂)), at ambient temperature. At this temperature, the complexes exhibit high emission quantum yields, as neat material or doped into poly(methyl methacrylate) (PMMA). This behavior is assigned to an efficient thermal population of a singlet state (being classified as ¹MLCT), which lies only 800 to 1300 cm⁻¹ above the triplet state, depending on the individual complex. Thus, the resulting emission at ambient temperature largely represents a fluorescence. For applications in OLEDs and LEECs, for example, this type of thermally activated delayed fluorescence (TADF) creates a new mechanism allowing for harvesting both singlet and triplet excitations in the lowest singlet state. This effect of singlet harvesting leads to drastically higher radiative rates than obtainable for emissions from triplet states of Cu(I) complexes

The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Based on a very comprehensive set of experimental data and on theoretical models, an understanding of the triplet state properties of organo-transition metal compounds is worked out. Important trends and guidelines for controlling photophysical properties are revealed. In this respect, we focus on spin–orbit coupling (SOC) and its importance for radiative as well as for nonradiative transitions between the lowest triplet state and the electronic ground state. Moreover, as is discussed on the basis of an extensive data set, summarized for the first time, the efficiency of SOC also depends on the geometry of a complex. The investigations are exemplified and supported by instructive case studies, such as efficient blue and very efficient green and red emitters. Additionally, trends being important for applications of these compounds as emitters in OLEDs are worked out. In particular, the properties of the emitters are discussed with respect to the harvesting of singlet and triplet excitons that are generated in the course of the electroluminescence process. The well-known triplet harvesting effect is compared to the recently discovered singlet harvesting effect. This latter mechanism is illustrated by use of a blue light emitting Cu(I) complex, which represents an efficient fluorescent emitter at ambient temperature. By this mechanism, 100% of the generated singlet and triplet excitons can, at least in principle, be harvested by the emitting singlet state. Potentially, this new mechanism can successfully be applied in future OLED lighting with a distinctly reduced roll-off of the efficiency

Triplet state properties of a red emitting [Pt(s-thpy)(acac)] compound

A photophysical characterization based on optical high-resolution spectra and emission decay properties at low temperatures and at high magnetic fields is carried out for [Pt(s-thpy)(acac)] (s-thpy = 5,2-bis(2-thienyl)pyridinate and acac = acetylacetonate). The electronic 0-0 transition between the lowest triplet state and the ground state lies at 16150 cm⁻¹ (619 nm). The zero-field splitting (ZFS) of T₁ is smaller than 1 cm⁻¹. Thus, the emitting excited state is mainly of ligand-centered ³LC(³ππ*) character and experiences only weak spin-orbit couplings to higher lying singlet states. The compound does not fulfill important requirements for OLED applications, but strategies for improvements are pointed out

Photophysical properties of Re(pbt)(CO)₄ studied by high resolution spectroscopy

Photophysical properties of Re(pbt)(CO)₄ are investigated at cryogenic temperatures and high magnetic fields. The highly resolved spectra show that the zero field splitting of the lowest triplet T₁ into three substates is smaller than 2 cm⁻¹. With this result, the T₁ state can be classified as only slightly MLCT-perturbed ³LC (³ππ*) state. Consistently, spin-lattice relaxation times are slow at T = 1.2 K and emission decay times with τ(I) = 960 μs, τ(II) = 320 μs, and τ(III) = 24 μs are long due to only small singlet admixtures. The vibrational satellite structures observed reflect different vibrational deactivation mechanisms and reveal similar geometries of the emitting triplet and the ground state

Luminescent gold-silver complexes derived from neutral bis(perfluoroaryl) diphosphine gold(i) precursors

Complex [Au{4-C6F4(4-C6BrF 4)}(tht)] reacts with diphosphines (L-L) such as bis(diphenylphosphino)methane (dppm) or 1,2-bis(diphenylphosphino)benzene (dppb) in a 2:1 molar ratio in dichloromethane, leading to neutral products of stoichiometry [(Au{4-C6F4(4-C6BrF 4)}2(μ-L-L)] (L-L = dppm (1), dppb (2)). In the crystal structure of complex 2 short Au⋯Au interactions of 2.9367(5) and 2.9521(5) Å appear. This complex displays an orange emission, which is assigned to arise from a charge transfer transition from a metal centered Au-Au orbital to an orbital located at the diphosphine ligand. Addition of silver trifluoroacetate to these complexes in a 1:1 or a 2:1 molar ratio generates polymeric heterometallic gold-silver compounds of stoichiometry [Ag 2Au2{4-C6F4(4-C6BrF 4)}2(CF3CO2)2(μ-L-L)] n (L-L = dppm (3), dppb (4)), which confirms the capability of the neutral [(Au{4-C6F4(4-C6BrF4)} 2(μ-diphosphine)] units to act as electron density donors when treated with a Lewis acid substrate. These heterometallic derivatives show blue emissions indicating large HOMO-LUMO band gaps, due to the stabilization that the gold-based HOMO orbitals suffer when the electron withdrawing silver trifluoroacetate fragments interact with them. This journal is © 2013 The Royal Society of Chemistry.The German Federal Ministry of Education and Research (BMBF) and D.G.I.(MEC)/FEDER (project number CTQ2010-20500-C02-02) are acknowledged for the funding of our research. T. L. thanks MICINN for a grant. J. M. L-de-L and H. Y. acknowledge the receipt of an MICINN-DAAD joint project (HD2008-0022). The authors thank to the Centro de Supercomputación de Galicia (CESGA) for computational resources.Peer Reviewe