The structure of alanine anionic-zwitterionic dimers on Pd(111); formation of salt bridges

The structure of alanine zwitterion-anion dimers previously proposed to form on Pd(111) is investigated using low-energy electron diffraction (LEED). Because of the propensity of amino acids on Pd(111) to undergo electron-beam-induced decomposition, LEED intensity versus beam voltage (I-V) curves were measured using a delay line detector LEED (DLD-LEED) system, which enables the complete LEED I-V curves to be obtained for an electron exposure of less than one electron per adsorbate. Since no long-range order is found following alanine adsorption on Pd(111), the adsorbate structure is determined from the substrate diffraction spots. The I-V data were analyzed using the CLEED: Automated Surface Structure package for an initial input structure from previous DFT calculations of the alanine zwitterion-anion dimer, the best fit structure yielded a satisfactory Pendry R-factor of 0.249, thus confirming the correctness of the originally proposed structure. Such anionic-zwitterionic dimers are a class of hydrogen-bonding intermolecular interactions in proteins that are dominated by direct electrostatic interactions, in particular for residues such a lysine and arginine, known as salt bridges. While such salt bridges interactions are relatively weak in biological systems, the attractive interaction energy between the anion and the zwitterion in the dimer on Pd(111) in vacuo is found to be ∼95 kJ/mol. It is proposed that the weaker binding biological systems occurs because the electrostatic screening in aqueous media weakens the electrostatic interaction between the anion and zwitterion, causing their structures to relax

Optical absorption and spectral photoconductivity in α-(Fe \u3csub\u3e1-x\u3c/sub\u3eCr\u3csub\u3ex\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e solid-solution thin films

Hematite, α-Fe2O3, is an attractive narrow gap oxide for consideration as an efficient visible light photocatalyst, with significant potential for band gap engineering via doping. We examine optical absorption in α-(Fe1-xCrx)2O3 epitaxial films and explain the observed excitations, and the nature of the band gap dependence on x, through first-principles calculations. The calculated and measured optical band gap becomes smaller than that of bulk α-Fe 2O3 and reaches a minimum as the Cr cation fraction increases to 50%. The lowest energy transitions in the mixed-metal alloys involve electron excitation from occupied Cr 3d orbitals to unoccupied Fe 3d orbitals, and they result in a measurable photocurrent. The onset of α-Fe2O3 photoconductivity can be reduced by nearly 0.5 eV (to 1.60 eV) through addition of Cr. © 2013 IOP Publishing Ltd

Identifying Breast Cancer Care Quality Measures for a Cancer Facility in Rural Sub-Saharan Africa: Results of a Systematic Literature Review and Modified Delphi Process

The burden of cancer is growing in low- and middle-income countries (LMICs), including sub-Saharan Africa. Ensuring the delivery of high-quality cancer care in such regions is a pressing concern. There is a need for strategies to identify meaningful and relevant quality measures that are applicable to and usable for quality measurement and improvement in resource-constrained settings. To identify quality measures for breast cancer care at Butaro Cancer Center of Excellence (BCCOE) in Rwanda, we used a modified Delphi process engaging two panels of experts, one with expertise in breast cancer evidence and measures used in high-income countries and one with expertise in cancer care delivery in Rwanda. Our systematic review of the literature yielded no publications describing breast cancer quality measures developed in a low-income country, but it did provide 40 quality measures, which we adapted for relevance to our setting. After two surveys, one conference call, and one in-person meeting, 17 measures were identified as relevant to pathology, staging and treatment planning, surgery, chemotherapy, endocrine therapy, palliative care, and retention in care. Successes of the process included participation by a diverse set of global experts and engagement of the BCCOE community in quality measurement and improvement. Anticipated challenges include the need to continually refine these measures as resources, protocols, and measurement capacity rapidly evolve in Rwanda. A modified Delphi process engaging both global and local expertise was a promising strategy to identify quality measures for breast cancer in Rwanda. The process and resulting measures may also be relevant for other LMIC cancer facilities. Next steps include validation of these measures in a retrospective cohort of patients with breast cancer