Structure factor measurement by 3-beam convergent beam electron diffraction

It has been shown mathematically that both the magnitudes and 3-phase invariants of the structure factors of a centrosymmetric crystal can be expressed explicitly in terms of the distances to specific features in the 3-beam convergent beam electron diffraction (CBED) pattern [1].This theoretical inversion can be implemented experimentally, enabling direct observations of 3-phase invariants and the approximate measurement of structure factor magnitudes. This method then enables a different approach to crystal structure determination, which is based on the observation of phases, rather than the measurement of amplitudes. It has been shown that by inspection of just a few phases using 3-beam CBED patterns, centrosymmetric crystal structures can be determined directly to picometre precision without the need to measure magnitudes [2]. Here, we will explore a different approach for measuring structure factor magnitudes from 3-beam CBED patterns. It has been demonstrated that the relative structure factor magnitudes can be determined directly from the ratio of the intensity distributions along specific lines within the CBED discs [3]. We will investigate the potential of using this approach for the relatively fast measurement of approximate structure factor magnitudes from nano-scale volumes of crystals.

Comparison of the Structures and Activities of Cg10062 with other 4-OT Enzymes

The tautomerase superfamily is a broad family of proteins represented by 4-oxalocrotonate tautomerase (4-OT), 5-(carboxymethyl)-2-hydroxymuconate isomerase (CHMI), cis-3-chloroacrylic acid dehalogenase (cis-CaaD), malonate semialdehyde decarboxylase (MSAD), and macrophage migration inhibitory factor (MIF). 4-OT and many of its homologues are homo- or heterohexamers composed of small (60-75 a.a. residue) subunits while CHMI, MSAD, cis-CaaD and MIF are nearly twice that size and form trimers. The subunits of this family share two distinguishing features - one or two beta-alpha-beta structural motifs and a catalytically important N-terminal Pro residue. Several different catalytic activities are known to utilize this same structural motif - tautomerase, isomerase, decarboxylase, dehalogenase, etc. Cg10062 is a homologue of cis-CaaD, however, it differs from cis-CaaD in several respects. In addition to being able to process cis-3-chloroacrylic acid, it also displays multiple other functions such as the capability to process trans-3-chloroacrylic acid (like CaaD), to process phenylpyruvate (like PPT/MIF) and 2-oxo-3-pentynoate (like CaaD, cis-CaaD and MSAD). Furthermore, Cg10062 is inactivated by both (R) and (S)-oxirane-2-carboxylate. However, cis-CaaD is only inactivated by the (R)- isomer. The X-ray structures of native Cg10062, and its inactivated complexes have been determined and the results of comparing these structures and activities with CaaD, cis-CaaD, PPT/MIF and MSAD will be reported. This work was supported in part by The Welch Foundation ( F1334).