ABSTRACT:A theoretical electronic structure analysis was performed on two crystalline phases of the homometallic Fe ludwigite, appearing in a structural transition recently found experimentally to occur at 283 K. The extended Hü ckel method was used to study different subunits of the compound, which is known to present low dimensionality in magnetic properties. Total energy calculations show an electronic stability of the low-temperature (distorted) phase, in agreement with experiment. A longitudinal Fe 2ϩ -Fe 2ϩ interaction (perpendicular to the flat planes of the orthorhombic unit cell) appears to be mainly responsible for the stabilization. A contribution of another Fe 2ϩ ion was seen to be related to an enhancement of Fe-O interaction. A remarkable result is the reversal of these results when high spin FeO 6 monomers are considered. In that case, it is the Fe-O interaction in an Fe 3ϩ -Fe 2ϩ -Fe 3ϩ triad which seems to drive the stabilization of the distorted phase. It is suggested that spin effects might be important to the structural transition in Fe 3 O 2 BO 3 .
Recebido em 16/10/11; aceito em 26/3/11; publicado na web em 26/6/12 UDP-N-ACETYLGLUCOSAMINE-ENOLPYRUVYL TRANSFERASE: DETERMINATION OF PROTONATION STATE OF ACTIVE SITE AMINOACID RESIDUES BY PM6 METHOD. UDP-N-acetylglucosamine-enolpyruvyl transferase (MurA) catalyzes the reaction between phosphoenol pyruvate and UDP-N-acetylglucosamine. We present a theoretical approach using the semiempirical PM6 method for defining protonation state of three active site residues, K22, H125, and K160. Prior comparison with neutron diffraction data showed that PM6 accurately predicted protonation states of active site residues of b-trypsin and D-xylose isomerase. Using the same methodology with MurA crystallographic data, we conclude that when reaction intermediate is located at the active site, H125 and K22 are in protonated form and K160 in neutral form.
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