Bandlimiting in conflict with spatial bounding of radiative sources

The curl of the Poynting vector for the bandlimited electromagnetic field of a timeharmonic point-source dipole is expanded asymptotically for large axial and off-axial distances of a halfspace. Truncating the plane-wave spectrum of the dipole field at any finite value of spatial frequency produces whirls in the time-averaged energy flux which are at variance with the expansion theorem for the field of a bounded source; due to bandlimiting, the angular and radial coordinates of the Poynting vector are not for all directions separable and the curl of the Poynting vector has the wrong orderr ^−5/2, at least for some directions, when the radial distancer approaches infinity.     

Depolarized light-scattering spectroscopy and polymer characterization

Light scattering has long been established as a technique for measuring the molecular weight and radius of gyration of polymer molecules. Depolarized light scattering can be studied accurately with lassers. The theory of static and dynamic depolarized light-scattering spectroscopy is presented and illustrated with two examples of novel particles in solution. From the static intensity measured as a function of angle, the length of rod-like molecules can be evaluated. More complicated geometrical measures are obtained in the eneral case. From the dynamic light scattering, the translational self-diffusion coefficient and the rotational diffusion coefficient are obtained. These quantities can be interpreted in terms of a major semi-axis length and an axial ratio for the molecule or particle by using the Perrin equations.     

Molecular model of the neural dopamine transporter

The dopamine transporter (DAT) regulates the action of dopamine by reuptake of the neurotransmitter into presynaptic neurons, and is the main molecular target of amphetamines and cocaine. DAT and the Na⁺/H⁺ antiporter (NhaA) are secondary transporter proteins that carry small molecules across a cell membrane against a concentration gradient, using ion gradients as energy source. A 3-dimensional projection map of the E. coli NhaA has confirmed a topology of 12 membrane spanning domains, and was previously used to construct a 3-dimensional NhaA model with 12 trans-membrane -helices (TMHs). The NhaA model, and site directed mutagenesis data on DAT, were used to construct a detailed 3-dimensional DAT model using interactive molecular graphics and empiric force field calculations. The model proposes a dopamine transport mechanism involving TMHs 1, 3, 4, 5, 7 and 11. Asp79, Tyr252 and Tyr274 were the primary cocaine binding residues. Binding of cocaine or its analogue, (–)-2-carbomethoxy-3-(4-fluorophenyl)tropane (CFT), seemed to lock the transporter in an inactive state, and thus inhibit dopamine transport. The present model may be used to design further experimental studies of the molecular structure and mechanisms of DAT and other secondary transporter proteins.     

The role of the active site tyrosine in the mechanism of lytic polysaccharide monooxygenase

Catalytic breakdown of polysaccharides can be achieved more efficiently by means of the enzymes lytic polysaccharide monooxygenases (LPMOs). However, the LPMO mechanism has remained controversial, preventing full exploitation of their potential. One of the controversies has centered around an active site tyrosine, present in most LPMO classes. Recent investigations have for the first time obtained direct (spectroscopic) evidence for the possibility of chemical modification of this tyrosine. However, the spectroscopic features obtained in the different investigations are remarkably different, with absorption maximum at 420 and 490 nm, respectively. In this paper we use density functional theory (DFT) in a QM/MM formulation to reconcile these (apparently) conflicting results. By modeling the spectroscopy as well as the underlying reaction mechanism we can show how formation of two isomers (both involving deprotonation of tyrosine) explains the difference in the observed spectroscopic features. Both isomers have a [TyrO–Cu–OH]⁺ moiety with the OH in either the cis- or trans-position to a deprotonated tyrosine. Although the cis-[TyrO–Cu–OH]⁺ moiety is well positioned for oxidation of the substrate, preliminary calculations with the substrate reveal that the reactivity is at best moderate, making a protective role of tyrosine more likely.

With QM/MM, we investigate the mechanism of tyrosine deprotonation in lytic polysaccharide monooxygenases. Our results support deprotonation and our calculated UV-vis spectra show that two isomers must be formed to match recent experiments.     

Microwave-Optical Double Resonance and Intermodulated Fluorescence Studies of Thioformaldehyde

Studies of the 4¹₀ band of the à ¹A₂ X?¹A₁ transition of thioformaldehyde by means of intermodulated fluorescence and microwave-optical double resonance have revealed numerous small perturbations in the excited state levels caused by high rovibronic levels of the ground state. Singlet-triplet perturbations have been studied by the same techniques and also by anticrossings produced in the presence of a magnetic field. The use of microwave-optical double resonance for studying collisional processes is discussed.