No abstract

One mechanism by which heme proteins control active site reactivity is through interaction of distal pocket residues with exogenous ligands.2 In myoglobin (Mb), it is believed that energetically unfavorable steric interactions with distal pocket residues3 reduce the binding affinity of CO, while bound O2 is stabilized by a hydrogen bond with the distal histidine.4 The distal pocket interactions in MbCO have particular interest, due to the existence of conformational substates that are functionally distinct5•6 and can be identified by vibrational frequencies of the Fe-C-O group.7•8 The relative populations of these substates can be controlled by experimental conditions including temperature, pressure, pH, and hydration.5•7•9 Spectroscopic10 and crystallographic118 evidence indicates that His64, which interacts with the bound ligand in the "closed" distal pocket states (Aj and A3), is displaced from the heme pocket toward solvent in the "open" pocket state (A0).The orientation of the bound CO in Mb remains an unresolved issue, despite several crystallographic studies.11•12 According to earlier structural models,12 bending of the Fe-C-O unit displaces the C-0 bond by an angle = 40-60°from the heme normal, but a recent structure116 shows the Fe-C-O moiety deviating from linearity by only 13°in MbCO. Photoselection measurements 13 based on the IR dichroism of samples partially photolyzed with polarized visible light lead to substate-specific values for that range from 15°( for Aq) to 33°( for A3). Here, we present the results of polarized IR measurements on single crystals of MbCO in order to characterize the CO orientation in the major (1) (a

No abstract

Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

This letter reports on the self-organized growth of nanoscale dot-like CdSe-based islands during molecular beam epitaxy of CdSe/ZnSe nanostructures with a CdSe thickness between 0.75 and 3.0 monolayers. An increase in the nominal CdSe thickness results in a higher density of islands (up to 2×1010 cm−2) and is accompanied by dramatic enhancement of the photoluminescence efficiency. The density of large relaxed islands appears to saturate at a value of (3–4)×109 cm−2. Room temperature (Zn, Mg)(S, Se)-based optically pumped lasers with an extremely low threshold (less than 4 kW/cm2), as well as (Be, Mg, Zn)Se-based injection laser diodes using a single (2.5–2.8) monolayer thick CdSe active region, both demonstrating significantly enhanced degradation stability, have been fabricated and studied.