Deep-ultraviolet (DUV) optical materials with wide bandgaps and non-centrosymmetric structures are critical for advanced optoelectronic applications such as DUV lithography, high-resolution spectroscopy and nonlinear optics. In this work, we report the discovery of a noncentrosymmetric phase of KNaSiF6 (space group Pna21), synthesized via a hydrothermal cation-exchange method. Single crystals with dimensions up to 4.45 mm × 1 mm × 1 mm were grown. The crystal exhibits an absorption edge below 200 nm, corresponding to a bandgap greater than 6.2 eV. Vibrational spectroscopy (IR and Raman) confirms the integrity of the [SiF6]2- anionic groups, and energy-dispersive X-ray spectroscopy verifies the expected stoichiometry. First-principles calculations on the Pna21 phase yield a small second-harmonic generation (SHG) coefficient of ∼0.00227 pm V-1, consistent with the weak SHG response observed experimentally. The combination of a DUV transparency window, a wide bandgap and a noncentrosymmetric structure renders Pna21-KNaSiF6 a promising candidate for next-generation DUV optical windows and nonlinear optical crystals.
The matrix method of averaging material coefficients of crystallites deposited at an angle to a revolving substrate, disclosed in the prequel [Ballato & Ballato (2024). Acta Cryst. B80, 760-765], is applied to tensors in their indicial form. The results are used to determine averaged nonlinear dielectric, piezoelectric, and elastic constants, as well as linear pyroelectric coefficients, for triclinic crystals; these are further specialized for Laue classes of higher symmetry.
The crystal structure of Rb2[Si2O5], a phyllosilicate with previously questionable structural details, has been solved and refined as an incommensurately modulated phase in (3 + 1)-dimensional superspace. This paper describes the monoclinic structure in superspace group C2/c(0β0)s0 with unit-cell parameters a = 9.8662 (6), b = 8.3986 (5), c = 14.7641 (9) Å, β = 90.114 (5)°, V = 1223.4 (1) Å3 and with modulation wavevector q = 0.377 (1)b*, refined to an R1 value of 0.0419 for 2603 reflections with intensities greater 3σ. All [SiO4] tetrahedra within the structure adopt two distinct orientations, and the positions of their constituent silicon and oxygen atoms are modeled using a combination of crenel and positional modulation functions. For the rubidium atoms, harmonic modulation waves were used for the atomic coordinates and the anisotropic displacement parameters. Rubidium cations are predominantly coordinated by six oxygen atoms, forming linkages between adjacent layers built of [SiO4] tetrahedra. However, the ionic radius of Rb in comparison to the mesh-size of the silicate layer leads to deformation and modulation of the layers. The modulation results in more balanced bond valence sums for rubidium, acceptable Si-O distances and anisotropic displacement parameters, which is not the case for a three-dimensionally periodic structure model with split atom positions. A comparison with related phyllosilicates featuring the same 4.82 net layer topology, but with larger interlayer cations or higher cationic content, reveals that these structures exhibit more relaxed silicate layers. This highlights the role of cation size and content in influencing the structural modulation and strain within the crystal structure of phyllosilicates.
Peptidyl-tRNA hydrolase (Pth2) hydrolyzes peptidyl-tRNA, an immature product of aborted translation, into peptide and tRNA, thereby maintaining cellular protein synthesis through peptide release and tRNA recycling. Here, we present the crystal structure of Pth2 from Candidatus Lokiarchaeum sp. GC14_75 (LokiPth2) at 2.12 Å resolution. This is the first structure of Pth2 from a lineage within Promethearchaeati, a kingdom of archaea closely related to eukaryotes. The structure reveals that LokiPth2 forms a homodimer and closely resembles Pth2 structures from other species. However, LokiPth2 exhibits two prominent structural differences: a short helix around the catalytic center, which is absent in other Pth2s, and a distinct orientation of the C-terminal helix. Detailed comparative structural analysis suggests that these regions may regulate enzymatic activity and substrate binding, respectively. Furthermore, the corresponding regions in other Pth2s also exhibit high flexibility, suggesting that similar mechanisms may be conserved among Pth2s. To gain insights into the growth environment of Candidatus Lokiarchaeum sp. GC14_75, we assess the optimal temperature for the catalytic reaction of LokiPth2, which suggests that Candidatus Lokiarchaeum sp. GC14_75 inhabits moderately thermophilic environments.
The Anticalin CL31d, an engineered lipocalin protein previously designed to specifically bind rare-earth and related metal ions as chelate complexes with p-NH2-Bn-CHX-A″-DTPA (DTPA-NH2), was subjected to structural and binding studies with a series of 11 different MIII·DTPA-NH2 complexes. These complexes include various lanthanide and main-group metal(III) ions whose radioisotopes are useful in nuclear medicine, with ionic radii ranging from 0.62 to 1.03 Å. Binding activities of the Anticalin for the MIII·DTPA-NH2 complexes were quantified by fluorescence titration (probing the intrinsic Tyr/Trp emission), revealing Kd values of 0.8-2.4 nM for most of the lanthanide ions investigated (rion = 0.75-0.96 Å), but showing markedly reduced affinities towards the small and large main-group metal ions Ga3+ (Kd = 15.3 nM) and Bi3+ (Kd = 47.8 nM), respectively. The crystal structures of six representative MIII·DTPA-NH2 complexes bound to the Anticalin were solved at high resolution (1.5-1.8 Å) using synchrotron X-ray diffraction. Superposition onto the previously described Anticalin CL31 with bound Y3+·DTPA-SCN indicated an essentially invariant conformation both for the binding protein and its metal-chelate ligands, including conserved hydrogen bonds, and a surprisingly uniform ninefold metal coordination via five carboxylate groups, three N atoms and one water molecule. However, there were two exceptions: the small Sc3+ ion appeared to be coordinated only eightfold with the DTPA-NH2 chelator but lacking the water ligand, whereas no metal electron density was observed for the Ga3+ ion, in line with its known noncanonical DTPA complex geometry. Interestingly, in this case a water molecule was detected at the expected position of the central metal ion within the protein-bound DTPA-NH2 chelator. Our investigation of the influence of chelate geometry on complex stability establishes the Anticalin CL31d as a small and robust universal binding protein for medically relevant MIII·DTPA complexes with surprisingly broad tolerance towards varying ionic radii, thus enabling flexible radionuclide-targeting strategies in nuclear medicine.
