Femtosecond pump-probe experiments on trapped flavin: optical control of dissociation

Femtosecond pump-probe experiments are performed on flavin biomolecules isolated in an ion trap. Mass spectra of the photoinduced fragments show that the fragmentation pathways can be modified using two-color two-photon excitation. In particular, when an infrared probe pulse (810 nm) is added subsequent to the first excitation step (excitation of the S(1) state of flavin mononucleotide at 405 nm), branching ratios between lumichrome and lumiflavin production are inverted relative to the single excitation case.     

Deciphering second harmonic generation signals

Second harmonic generation (SHG) has emerged as one of the most powerful techniques used to selectively monitor surface dynamics and reactions for all types of interfaces as well as for imaging non-centrosymmetric structures, although the molecular origin of the SHG signal is still poorly understood. Here, we present a breakthrough approach to predict and interpret the SHG signal at the atomic level, which is freed from the hyperpolarisability concept and self-consistently considers the non-locality and the coupling with the environment. The direct ab initio method developed here shows that a bulk quadrupole contribution significantly overwhelms the interface dipole term in the purely interfacial induced second-order polarisation for water/air interfaces. The obtained simulated SHG responses are in unprecedented agreement with the experimental signal. This work not only paves the road for the prediction of SHG response from more complex interfaces of all types, but also suggests new insights in the interpretation of the SHG signal at a molecular level. In particular, it highlights the modest influence of the molecular orientation and the high significance of the bulk quadrupole contribution, which does not depend on the interface, in the total experimental response.

Second harmonic generation is one of the most powerful techniques used to selectively probe interfaces of all types. The direct ab initio method developed here allows predicting the signal and highlights the importance of local and non-local effects.     

Self-assembly of hybrid organic-inorganic polyoxovanadates: functionalised mixed-valent clusters and molecular cages

Herein we report the intra- and inter-molecular assembly of a {V(5)O(9)} subunit. This mixed-valent structural motif can be stabilised as [V(5)O(9)(L(1-3))(4)](5-/9-) (1-3) by a range of organoarsonate ligands (L(1)-L(3)) whose secondary functionalities influence its packing arrangement within the crystal structures. Variation of the reaction conditions results in the dodecanuclear cage structure [V(12)O(14)(OH)(4)(L(1))(10)](4-) (4) where two modified convex building units are linked via two dimeric {O(4)V(IV)(OH)(2)V(IV)O(4)} moieties. Bi-functional phosphonate ligands, L(4)-L(6) allow the intramolecular connectivity of the {V(5)O(9)} subunit to give hybrid capsules [V(10)O(18)(L(4-6))(4)](10-) (5-7). The dimensions of the electrophilic cavities of the capsular entities are determined by the incorporated ligand type. Mass spectrometry experiments confirm the stability of the complexes in solution. We investigate and model the temperature-dependent magnetic properties of representative complexes 1, 4, 6 and 7 and provide preliminary cell-viability studies of three different cancer cell lines with respect to Na(8)H(2)[6]·36H(2)O and Na(8)H(2)[7]·2DMF·29H(2)O.     

Prompt and Slow Electron‐Detachment‐Dissociation/Electron‐Photodetachment‐Dissociation of a 21‐Mer Peptide

Electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) are relatively new dissociation methods that involve electron detachment followed by radical‐driven dissociation from multiply deprotonated species. EDD yields prompt dissociation whereas only electron detachment is obtained by EPD; subsequent vibrational activation of the charge‐reduced radical anion is required to obtain the product ions. Herein, the fragmentation patterns that were obtained by EDD and by vibrational activation of the charge‐reduced radical anions that were produced through EDD or EPD (activated‐EDD and activated‐EPD) were compared. The observed differences were related to the dissociation kinetics and/or the contribution of electron‐induced dissociation (EID). Time‐resolved double‐resonance experiments were performed to measure the dissociation rate constants of the EDD product ions. Differences in the formation kinetics were revealed between the classical EDD/EPD ′a^._i/′′x_j complementary ions and some ′a^._i/c_i/′′′z^._j product ions, which were produced with slower dissociation rate constants, owing to the presence of specific neighbouring side chains. A new fragmentation pathway is proposed for the formation of the slow‐kinetics ′a^._i ions.     

Development and trends of biosurfactant analysis and purification using rhamnolipids as an example

During the last few decades, increasing interest in biological surfactants led to an intensification of research for the cost-efficient production of biosurfactants compared with traditional petrochemical surface-active components. The quest for alternative production strains also is associated with new demands on biosurfactant analysis. The present paper gives an overview of existing analytical methods, based on the example of rhamnolipids. The methods reviewed range from simple colorimetric testing to sophisticated chromatographic separation coupled with detection systems like mass spectrometry, by means of which detailed structural information is obtained. High-performance liquid chromatography (HPLC) coupled with mass spectrometry currently presents the most precise method for rhamnolipid identification and quantification. Suitable approaches to accelerate rhamnolipid quantification for better control of biosurfactant production are HPLC analysis directly from culture broth by adding an internal standard or Fourier transform infrared attenuated total reflectance spectroscopy measurements of culture broth as a possible quasi-online quantification method in the future. The search for alternative rhamnolipid-producing strains makes a structure analysis and constant adaptation of the existing quantification methods necessary. Therefore, simple colorimetric tests based on whole rhamnolipid content can be useful for strain and medium screening. Furthermore, rhamnolipid purification from a fermentation broth will be considered depending on the following application.     

An Experimental and Theoretical Investigation on Pentacoordinated Cobalt(III) Complexes with an Intermediate S=1 Spin State: How Halide Ligands Affect their Magnetic Anisotropy

Understanding the factors that control the magnitude and symmetry of magnetic anisotropy should facilitate the rational design of mononuclear metal complexes in the quest for single‐molecule magnets (SMMs), based on a single metal ion, with high blocking temperatures and large energy barriers. The best strategy is to define magnetostructural correlations through the investigation of a series of metal complexes. It has been demonstrated that the main contribution to the magnetic anisotropy arises from the spin‐orbit coupling (SOC) effect in metal‐ion‐based systems, so current studies focus particularly on the use of both ligands and metal ions possessing a large SOC. In this context, we report a unique series of halide Co^III complexes, [CoL(X)], with X=Cl, Br, I ( CoX ) and L=2,2′‐(2,2′‐bipyridine‐6,6′‐diyl)bis(1,1‐diphenylethanethiolate), which possess a rare intermediate S=1 spin ground state. The S=1 Co^III complexes are attractive species because they possess a remarkably large axial zero‐field splitting (defined by D from the following Hamiltonian: H =DS_z²), as well as the halide ligands inducing large SOC constants. The single‐crystal X‐ray structures reveal that the CoBr and CoI complexes are isostructural with the previously described CoCl complex. Their coordination sphere displays a distorted pentacoordinated square pyramidal geometry, with the halide located in the Co^III axial position. Large positive D values of 35, 26, and 18 cm^?1 are found for CoCl , CoBr , and CoI , respectively, through analysis of the magnetic susceptibility data as a function of temperature. To rationalize this trend, theoretical calculations based on both density functional theory (DFT) and complete active space self‐consistent field (CASSCF) methods are performed successfully. Both the sign and magnitude of D are predicted remarkably well by these theoretical approaches. The DFT calculations also show that the resulting D values originate from a balance of several contributions, and that many factors, including differences in their structural properties and in the contribution of the halide, should be taken into account to explain the trend of D in this series of complexes.     

Pd3Sn8Bi6]4-: a 14-vertex Sn/Bi cluster embedding a Pd3 triangle

The endohedral cluster anion [Pd(3)Sn(8)Bi(6)](4-) crystallizes as its K([2.2.2]crypt)(+) salt 1 upon reaction of [K([2.2.2]crypt)](2)[Sn(2)Bi(2)]·en and Pd(dppe)(2) in 1,2-diaminoethane (en)/toluene and incorporates a complete Pd(3) triangular cluster within a medium-size 14-vertex cage of Sn and Bi atoms. 1 was characterized by a combination of single crystal diffraction, ESI mass spectrometry, elemental analysis, and magnetic measurements. According to quantum chemical investigations, the Pd(3) triangle interacts only weakly with the Sn/Bi cluster shell despite the relatively small cavity inside the cage.     

Slow relaxation in a one-dimensional rational assembly of antiferromagnetically coupled [Mn4] single-molecule magnets

Four discrete Mn(III)/Mn(II) tetranuclear complexes with a double-cuboidal core, [Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)](ClO(4))(4).2CH(3)CN (1), [Mn(4)(hmp)(6)(H(2)O)(4)](ClO(4))(4).2H(2)O (2), [Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)](ClO(4))(2).4H(2)O (3), and [Mn(4)(hmp)(6)(Hhmp)(2)](ClO(4))(4).2CH(3)CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO(4))(2).6H(2)O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO(3) (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn(2+)-Mn(3+) and Mn(3+)-Mn(3+) magnetic interactions are ferromagnetic in 1-3 leading to an S(T) = 9 ground state for the Mn(4) unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn(4)] core geometry result in an S(T) = 1 ground state in 4. A one-dimensional assembly of [Mn(4)] units, catena-{[Mn(4)(hmp)(6)(N(3))(2)](ClO(4))(2)} (5), was obtained in the same synthetic conditions with the subsequent addition of NaN(3). Double chairlike N(3)(-) bridges connect identical [Mn(4)] units into a chain arrangement. This material behaves as an Ising assembly of S(T) = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Delta(xi) and Delta(tau) were estimated and successfully compared to the experimental values.