Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time

Photoexcited molecular trajectories on potential energy surfaces (PESs) prior to thermalization are intimately connected to the photochemical reaction outcome. The excited-state trajectories of a diplatinum complex featuring photo-activated metal–metal σ-bond formation and associated Pt−Pt stretching motions were detected in real time using femtosecond wide-angle X-ray solution scattering. The observed motions correspond well with coherent vibrational wavepacket motions detected by femtosecond optical transient absorption. Two key coordinates for intersystem crossing have been identified, the Pt−Pt bond length and the orientation of the ligands coordinated with the platinum centers, along which the excited-state trajectories can be projected onto the calculated PESs of the excited states. This investigation has gleaned novel insight into electronic transitions occurring on the time scales of vibrational motions measured in real time, revealing ultrafast nonadiabatic or non-equilibrium processes along excited-state trajectories involving multiple excited-state PESs.     

Multiplex digital PCR: breaking the one target per color barrier of quantitative PCR

Quantitative polymerase chain reactions (qPCR) based on real-time PCR constitute a powerful and sensitive method for the analysis of nucleic acids. However, in qPCR, the ability to multiplex targets using differently colored fluorescent probes is typically limited to 4-fold by the spectral overlap of the fluorophores. Furthermore, multiplexing qPCR assays requires expensive instrumentation and most often lengthy assay development cycles. Digital PCR (dPCR), which is based on the amplification of single target DNA molecules in many separate reactions, is an attractive alternative to qPCR. Here we report a novel and easy method for multiplexing dPCR in picolitre droplets within emulsions-generated and read out in microfluidic devices-that takes advantage of both the very high numbers of reactions possible within emulsions (>10(6)) as well as the high likelihood that the amplification of only a single target DNA molecule will initiate within each droplet. By varying the concentration of different fluorogenic probes of the same color, it is possible to identify the different probes on the basis of fluorescence intensity. Adding multiple colors increases the number of possible reactions geometrically, rather than linearly as with qPCR. Accurate and precise copy numbers of up to sixteen per cell were measured using a model system. A 5-plex assay for spinal muscular atrophy was demonstrated with just two fluorophores to simultaneously measure the copy number of two genes (SMN1 and SMN2) and to genotype a single nucleotide polymorphism (c.815A>G, SMN1). Results of a pilot study with SMA patients are presented.     

A double quantum (129)Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds

A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using (129)Xe double quantum NMR spectroscopy. Double quantum filtered (129)Xe NMR spectra, performed on the xenon clathrate of Dianin's compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak (129)Xe-(129)Xe dipole-dipole couplings that exist between xenon atoms in close spatial proximity. Because the (129)Xe-(129)Xe dipole-dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the (129)Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26 dipolar recoupling sequence proving appropriate. We have also attempted to measure the (129)Xe-(129)Xe dipole-dipole coupling constant between xenon atoms in the cavities of the xenon-Dianin's compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as (129)Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in (129)Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.     

Predicting class I major histocompatibility complex (MHC) binders using multivariate statistics: comparison of discriminant analysis and multiple linear regression

The accurate in silico identification of T-cell epitopes is a critical step in the development of peptide-based vaccines, reagents, and diagnostics. It has a direct impact on the success of subsequent experimental work. Epitopes arise as a consequence of complex proteolytic processing within the cell. Prior to being recognized by T cells, an epitope is presented on the cell surface as a complex with a major histocompatibility complex (MHC) protein. A prerequisite therefore for T-cell recognition is that an epitope is also a good MHC binder. Thus, T-cell epitope prediction overlaps strongly with the prediction of MHC binding. In the present study, we compare discriminant analysis and multiple linear regression as algorithmic engines for the definition of quantitative matrices for binding affinity prediction. We apply these methods to peptides which bind the well-studied human MHC allele HLA-A*0201. A matrix which results from combining results of the two methods proved powerfully predictive under cross-validation. The new matrix was also tested on an external set of 160 binders to HLA-A*0201; it was able to recognize 135 (84%) of them.     

Dinuclear Nickel(II) and Zinc(II) Complexes of 2,6-[N,N′-bis(2-hydroxyphenylmethyl)-N,N′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol

Abstract

The symmetric 'end-off' compartmental proligand 2,6-[N,N′-bis(2-hydroxy-phenylmethyl)-N,N′-bis(2-pyridylmethyl)aminomethyl]-4-methylphenol (H₃L) has been used to generate homodinuclear nickel(II) and zinc(II) complexes. The crystal structures of the complexes reveal that the di-nickel(II) complex is donor asymmetric and the di-zinc(II) complex is coordination number asymmetric. In both complexes non-coordinated acetic acid molecules are tightly hydrogen-bonded to the pendant phenols of the ligand generating a double acid salt of the type[CH₃COO…H…L…H…OOCCH₃]^3? in the dinickel complex and a single acid salt of the type [CH₃COO…H…L]^3? in the dizinc complex. In both cases the ligand periphery has been extended to provide a supraligand in which the donor potential of the original ligand has been enhanced.     

A Short and Efficient Stereoselective Synthesis of the Potent 5-Lipoxygenase Inhibitor CMI-977

A short and efficient synthesis of the potent 5-lipoxygenase inhibitor CMI-977 is described using as the key step a stereoselective anomeric oxygen to carbon rearrangement of an alkynyl stannane tetrahydrofuranyl ether derivative mediated by boron trifluoride etherate.