Low-temperature chromophore isomerization reveals the photoswitching mechanism of the fluorescent protein Padron

Photoactivatable fluorescent proteins are essential players in nanoscopy approaches based on the super-localization of single molecules. The subclass of reversibly photoswitchable fluorescent proteins typically activate through isomerization of the chromophore coupled with a change in its protonation state. However, the interplay between these two events, the details of photoswitching pathways, and the role of protein dynamics remain incompletely understood. Here, by using a combination of structural and spectroscopic approaches, we discovered two fluorescent intermediate states along the on-switching pathway of the fluorescent protein Padron. The first intermediate can be populated at temperatures as low as 100 K and results from a remarkable trans-cis isomerization of the anionic chromophore taking place within a protein matrix essentially deprived of conformational flexibility. This intermediate evolves in the dark at cryotemperatures to a second structurally similar but spectroscopically distinct anionic intermediate. The final fluorescent state, which consists of a mixture of anionic and neutral chromophores in the cis configuration, is only reached above the glass transition temperature, suggesting that chromophore protonation involves solvent interactions mediated by pronounced dynamical breathing of the protein scaffold. The possibility of efficiently and reversibly photoactivating Padron at cryotemperatures will facilitate the development of advanced super-resolution imaging modalities such as cryonanoscopy.     

Aspects of ground effect modeling

A recently published one-parameter ground model based on Darcy's law is here generalized into a two-parameter model which depends on an effective flow resistivity and an effective layer depth. Extensive field measurements of the acoustic impedance of various ground types have been carried out for frequencies in the range from 200 Hz to 2.5 kHz. The model based on Darcy's law gives an improved fit to the measurements compared to the Delany-Bazley model. It is, in addition, argued on purely theoretical grounds that the suggested model is preferable. In contrast to the Delany-Bazley model it corresponds to a proper causal time-domain model. This is particularly relevant for extrapolation of the models to lower frequencies and for the recently developed harmonized methods intended for use in the implementation of the European Union directive on the assessment and management of environmental noise. The harmonized methods include frequencies down to the 25 Hz third octave band and have the Delany-Bazley ground impedance model as the default choice. The arguments presented here suggest that this default choice should be replaced by the more physically based model from the law of Darcy.     

The Spectrum and Convergence Rates of Exclusion and Interchange Processes on the Complete Graph

We give a short and completely elementary method to find the full spectrum of the exclusion process and a nicely limited superset of the spectrum of the interchange process (a.k.a. random transpositions) on the complete graph. In the case of the exclusion process, this gives a simple closed-form expression for all the eigenvalues and their multiplicities. This result is then used to give an exact expression for the distance in \( L^2 \) from stationarity at any time and upper and lower bounds on the convergence rate for the exclusion process. In the case of the interchange process, upper and lower bounds are similarly found. Our results strengthen or reprove many known results about the mixing time for the two processes in a very simple way.     

Sound reduction by barriers on the ground

An existing theory on the propagation of spherical sound waves over ground with a finite acoustic impedance is to a large extent verified by field measurements over different types of ground surfaces. It is then shown that it is reasonable to combine this theory with ordinary diffraction theory to get a solution of the mixed problem with a barrier on the ground. Full-scale measurements have been carried out with a 3 m high wall and the results show good agreement with the predicted values. It is also shown that it is generally of minor importance whether a thin screen is sound absorbing or not.     

Reversible α‐Hydrogen and α‐Alkyl Elimination in PC(sp3)P Pincer Complexes of Iridium

Despite significant progress in recent years, the cleavage of unstrained C(sp³)? C(sp³) bonds remains challenging. A C? C coupling and cleavage reaction in a PC(sp³)P iridium pincer complex is mechanistically studied; the reaction proceeds via the formation of a carbene intermediate and can be described as a competition between α‐hydrogen and α‐alkyl elimination; the latter process was observed experimentally and is an unusual way of C(sp³)? C(sp³) bond scission, which has previously not been studied in detail. Mechanistic details that are based upon kinetic studies, activation parameters, and DFT calculations are also discussed. A full characterization of a C? C agostic intermediate is presented.     

Tetravalent metal complexation by Keggin and lacunary phosphomolybdate anions

We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between Ce (IV) and Th (IV) with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo 11O 39] (7-), results in the formation of compounds containing the anions [Ce(PMo 11O 39) 2] (10-) and [Th(PMo 11O 39) 2] (10-), respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs 10[Ce(PMo 11O 39) 2].20H 2O and (NH 4) 10[Th(PMo 11O 39) 2].22H 2O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo 11O 39] (7-) anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably wide pH range. Density functional analysis suggests that the Ce-O bond strength in [Ce(PMo 11O 39) 2] (10-) is greater than the Th-O bond strength in [Th(PMo 11O 39) 2] (10-), with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state Zr (IV) and Hf (IV) complexes formed contain the anions [Zr(PMo 12O 40)(PMo 11O 39)] (6-) and [Hf(PMo 12O 40)(PMo 11O 39)] (6-), respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo 11O 39] (7-) ligand and to four bridging oxygens of a plenary Keggin anion, [PMo 12O 40] (3-). In addition, (NH 4) 5{Hf[PMo 12O 40][(NH 4)PMo 11O 39]}.23.5H 2O can be crystallized as a minor product. The structure of the anion, {Hf[PMo 12O 40][(NH 4)PMo 11O 39]} (5-), reveals coordination of the central Hf (IV) cation via four bridging oxygens on both the coordinated [PMo 11O 39] (7-) and [PMo 12O 40] (3-) anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation. (31)P NMR indicates that complexation of the Keggin anion, [PMo 12O 40] (3-), to Hf (IV) and Zr (IV) will stabilize the Keggin anion to a much higher pH than usually observed.     

Synthesis and Characterization of a Family of POCOP Pincer Complexes with Nickel: Reactivity Towards CO2 and Phenylacetylene

A cyclohexyl‐based POCOP pincer ligand (POCOP=cis‐1,3‐bis(di‐tert‐butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POCOP‐supported Ni^II complexes, including the halide, hydride, methyl, and phenyl species. trans‐[NiCl{cis‐1,3‐bis(di‐tert‐butylphosphinito)cyclohexane}], [(POCOP)NiCl] ( 1 a ) and the analogous bromide complex ( 1 b ) were synthesized and fully characterized by NMR spectroscopy and X‐ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PCP)NiCl] ( 2 a ) and [(PCP)NiCl] ( 2 a ) (PCP=cis‐1,3‐bis(di‐tert‐butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron‐withdrawing oxygen atoms in the pincer arms. The methyl [(POCOP)NiMe] ( 3 ) and phenyl [(POCOP)NiPh] ( 4 ) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH₄ to give the hydride complex [(POCOP)NiH] ( 5 ). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POCOP)NiCCPh] ( 6 ). The reactivity of compounds 3 – 5 towards CO₂ was studied. The hydride complex 5 and the methyl complex 3 both underwent CO₂ insertion to form the formate species [(POCOP)NiOCOH] ( 7 ) and acetate species [(POCOP)NiOCOCH₃] ( 8 ), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO₂ even at elevated temperatures. Complexes 3 – 8 were all characterized by NMR spectroscopy and X‐ray crystallography.