Multidomainic crystals

In a multidomain protein whose domains have fixed orientations relative to each other, a unique alignment tensor will represent the preferred orientation of all the domains in the anisotropic environment. Therefore, structure refinement with dipolar couplings is performed as in one-domain proteins (Sect. 8.4). Several examples are reported in the literature of cases with conformational ambiguity due to the lack of NOE contacts between the domains. One example is the determination of subdomain orientation of the riboso-mal protein S4 z)41 [97]. In this work the lack of NOE contacts between the domains produces an ambiguity in interdomain orientation. The authors use two different anisotropic media to obtain dipolar couplings (DMPC/DHPC bicelles and Pfl filamentous bacteriophages). They conclude that subdomain orientation in solution is similar to the one present in the crystal structure. 

Structural characterization of NRPSs has yielded significant insight into the enzymology of these complex biosynthetic machines and has provided a framework for engineering these systems toward novel function. As summarized in this section, X-ray crystal and NMR structures have been determined for both individual NRPS domains and multidomain constructs. Overall, these studies support a monomeric structure for NRPS assembly line units where significant domain motion is necessary to allow participation of the various active sites in the chemistry leading to peptide products. 

Conditions which promote multi-domainic goethites are high ionic strength (either [KOH] or salt) and also low synthesis temperature (<40°C). In alkaline solutions, multi-domainic character decreases and domain width increases as Al substitution increases to Al/(Fe-i-Al) of 0.15, whereas at Al/( Al-nFe) >0.15 single domain crystals result (Schulze Schwertmaim, 1984 Mann et al., 1985). Multidomainic goethites can recrystallize to single domain crystals as a result of hydrothermal treatment at 125-180 °C (Fig. 4.9) (Schwertmann et al., 1985). 

Fig. 4.14 Synthetic lepidocrocite produced by oxidation of a FeCl2 solution, a) Monodomainic, lath-shaped crystals, produced by oxidation with 100 ml air min " at 50°C and pH 7.5 shadowed with 5 nm chromium at 45° (Courtesy R.Ciova-noli). b) Multidomainic crystals obtained at pH 7-7.5 and room temperature (see Schwertmann Taylor, 1972a). c) Crystal aggregates produced in the presence of urotropin (courtesy R. Ciova-noli). d) Very small crystals showing (010) lattice fringes of 1 nm (Schwertmann. Taylor, 1979, with permission), e) Cubic crystals formed after ageing multidomainic crystals shown in (b) in M KOH containing 3.32 10 M Si at 80°C for 1749 h (Schwertmann Taylor, 1972, with per-...

Al-hematites formed slowly from Al-ferrihydrite at 25 °C over 20 years, varied between rhombohedra at low substitution and multidomainic ellipsoids ca. 100 nm across with a grainy interior at higher substitution (Al/(Al-rFe) = 0.15) (Fig. 4.20e f) (Schwertmann et al. 2000). Allophane as a source of A1 had the same effect (Schwert-mann et al. 2000a). Mn substituted hematites grown from ferrihydrite were ellipsoidal in the presence of oxalate and platy in the presence of NaHCOa buffer (Cornell Gio-vanoli, 1987 Cornell et al., 1990). Cu substituted (0.09 mol mol" ) hematite grows as large (0.2 pm) rhombohedral crystals the crystal faces are most probably 102 or 104 (Fig. 4.20d) (Cornell Giovanoli, 1988). 

Fig. 16.9 Electron micrographs of soil lepidocro-cite. a) Large multidomainic lath-like crystal viewed perpendicularto [001] with laminar pores from a re-doximorphic soil, Natal, South Africa, b) Poorly crystalline grassy lepidocrocite crystals mixed with tiny ferrihydrite particles and pseudo-hexagonal kaolinite platelets. Origin as before (a. b courtesy P. Self), c) Small lepidocrocite crystal from a hydromorphic soil (with ferrihydrite) viewed perpendicularto [001] and showing (020) lattice fringes (see also Schwert-mann. Taylor, 1989,with permission).

Single crystal and neutron diffraction measurements have recently revealed that K2CuF4 possesses a multidomain structure due to displacements of F atoms96, 975. The symmetry of the compound has been deduced to be orthorhombic (D -Bbcm, with a = b = 5.87 A, c = 12.75 A)975 rather than tetragonal. This result is not supported by recent EPR works2745. F displacement is similar to that found in KCuF3 and is attributed to the two possible orientations of Cuu eg orbitals. 

Schadt, M., Seiberle, and Schuster, A. Optical patterning of multidomain liquid-crystal displays with wide-viewing angles. Nature 381, 212 (1996). 

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