Aromatic poly(ester carbonate)/poly-(ethylene terephthalate) alloys

When mixtures of poly(ester carbonate) (PEC) and poly(ethylene terephthalate) (PET) containing up to two-thirds of the latter are melt extruded, they produce a single-phase amorphous “alloy.” This alloy is characterized by a sharp, single, composition-dependent glass transition temperature, T_g. When annealed below T_g, the alloy remains unaltered, but when annealed above its T_g, the alloy separates into minute pure-PET crystallites and an amorphous PEC/PET phase. The thermal and dynamic mechanical behavior, crystallization kinetics, and SAXS patterns all strongly suggest the PEC-rich alloys to be solid solutions in which the PET molecules are dispersed individually or in small aggregates containing only a few PET molecules each. Calculations of the interaction parameter and assumed interfacial layer thickness tend to support this suggestion. Use of appropriate solvents allows one to selectively dissolve the PEC and recover from the alloys both PET and PEC in the original purity and molecular weights. Diffusion constants of PET molecules through the amorphous alloys were obtained from studies of PET crystallization above T_g of the alloys. The magnitude of the constants are in the range of expectation. The mechanical properties of the amorphous alloys in the glassy state do not deviate greatly from simple additivity of the respective properties of the parent polymers. However, the melt viscosity of the PEC-rich alloys and their plateau modulus above T show drastic decreases from straight additivity. A qualitative, but not quantitative, explanation of these observations is offered.     

A unified description of sum frequency generation,parametric down conversion and two-photon fluorescence

A superoperator non-equilibrium Green's function formalism is presented for computing nonlinear optical processes involving any combination of classical and quantum optical modes. Closed correlation-function expressions based on superoperator time-ordering are derived for the combined effects of causal response and non-causal spontaneous fluctuations. Coherent three wave mixing (sum frequency generation and parametric down conversion) involving one and two quantum optical modes, respectively, are compared with their incoherent counterparts: two-photon-induced fluorescence and two-photon-emitted fluorescence.     

Two‐dimensional stimulated ultraviolet resonance Raman spectra of tyrosine and tryptophan: a simulation study

We report an ab initio simulation study of the ultrafast broad bandwidth ultraviolet stimulated resonance Raman spectra (SRRS) of l ‐tyrosine, l ‐tryptophan, and trans‐l ‐tryptophan‐l ‐tyrosine (WY) dipeptide. Two‐pulse one‐dimensional SRRS and three‐pulse two‐dimensional SRRS that reveal inter‐residue and intra‐residue vibrational correlations are simulated using electronically resonant or pre‐resonant pulse configurations that select the Raman signal and discriminate against excited state pathways. Multimode effects are incorporated via the cumulant expansion. The two‐dimensional SRRS technique is more sensitive to residue couplings than spontaneous Raman. Copyright © 2013 John Wiley & Sons, Ltd.     

Structural and Functional Consequences of the Weak Binding of Chlorin e6 to Bovine Rhodopsin

The chlorophyll‐derivative chlorin e6 (Ce6) identified in the retinas of deep‐sea ocean fish is proposed to play a functional role in red bioluminescence detection. Fluorescence and ¹H NMR spectroscopy studies with the bovine dim‐light photoreceptor, rhodopsin, indicate that Ce6 weakly binds to it with μm affinity. Absorbance spectra prove that red light sensitivity enhancement is not brought about by a shift in the absorbance maximum of rhodopsin. ¹⁹F NMR experiments with samples where ¹⁹F labels are either placed at the cytoplasmic binding site or incorporated as fluorinated retinal indicate that the cytoplasmic domain is highly perturbed by binding, while little to no changes are detected near the retinal. Binding of Ce6 also inhibits G‐protein activation. Chemical shift changes in ¹H‐¹⁵N NMR spectroscopy of ¹⁵N‐Trp labeled bovine rhodopsin reveal that Ce6 binding perturbs the entire structure. These results provide experimental evidence that Ce6 is an allosteric modulator of rhodopsin.     

Two-dimensional correlation spectroscopy of two-exciton resonances in semiconductor quantum wells

We propose a three-pulse coherent ultrafast optical technique that is particularly sensitive to two-exciton correlations. Two Liouville-space pathways for the density matrix contribute to this signal which reveals double quantum coherences when displayed as a two-dimensional correlation plot. Two-exciton couplings spread the cross peaks along both axes, creating a characteristic highly resolved pattern. This level of detail is not available from conventional one-dimensional four-wave mixing or other two-dimensional correlation spectroscopy signals such as the photo echo, in which two-exciton couplings show up along a single axis and are highly congested.     

Moments of partitions and derivatives of higher order

Journal of Algebraic Combinatorics - We define moments of partitions of integers, and show that they appear in higher-order derivatives of certain combinations of functions.     

Tobramycin and Nebramine as Pseudo‐oligosaccharide Scaffolds for the Development of Antimicrobial Cationic Amphiphiles

Antimicrobial cationic amphiphiles derived from aminoglycoside pseudo‐oligosaccharide antibiotics interfere with the structure and function of bacterial membranes and offer a promising direction for the development of novel antibiotics. Herein, we report the design and synthesis of cationic amphiphiles derived from the pseudo‐trisaccharide aminoglycoside tobramycin and its pseudo‐disaccharide segment nebramine. Antimicrobial activity, membrane selectivity, mode of action, and structure–activity relationships were studied. Several cationic amphiphiles showed marked antimicrobial activity, and one amphiphilic nebramine derivative proved effective against all of the tested strains of bacteria; furthermore, against several of the tested strains, this compound was well over an order of magnitude more potent than the parent antibiotic tobramycin, the membrane‐targeting antimicrobial peptide mixture gramicidin D, and the cationic lipopeptide polymyxin B, which are in clinical use.     

Stochastic Liouville equations for coherent multidimensional spectroscopy of excitons

Signatures of chemical exchange and spectral diffusion in 2D photon-echo line shapes of molecular aggregates are studied using model calculations for a dimer whose Hamiltonian parameters are stochastically modulated. Cross peaks induced by chemical exchange and by exciton transport have different dynamics and distinguish two models which have the same absorption spectrum (a two-state jump bath modulation model of a dimer and a four-state jump bath model of a single chromophore). Slow Gaussian-Markovian spectral diffusion of a symmetric dimer induces new peaks which are damped as the dipole moment is equilibrated. These effects require an explicit treatment of the bath and may not be described by lower-level theories such as the Redfield equations, which eliminate the bath.     

A low molecular weight hydrogel which exhibits electroosmotic flow and its use as a bioreactor and for electrochromatography of neutral species

A low molecular weight hydrogel which exhibits electroosmotic flow is described, and its use for separation and biocatalytic applications that require passage of a solvent stream through the gel is demonstrated.     

Probing multiple core-hole interactions in the nitrogen K-edge of DNA base pairs by multidimensional attosecond X-ray spectroscopy. A simulation study

Two-dimensional X-ray correlation spectroscopy (2DXCS) signals of the isolated DNA bases and Watson-Crick base pairs which contain multiple absorbing nitrogen atoms are calculated. Core-hole excited states are calculated using density functional theory with the B3LYP functional and 6-311G** basis set. Sum over states calculations of the signals reveal changes in cross-peak intensities between hydrogen-bonded and stacked base pairs. Nucleobase analogues are proposed for investigating base-stacking and hydrogen-bonding interactions.