Teresa S. Ortner

Balance between hydration enthalpy and entropy is important for ice binding surfaces in Antifreeze Proteins

Antifreeze Proteins (AFPs) inhibit the growth of an ice crystal by binding to it. The detailed binding mechanism is, however, still not fully understood. We investigated three AFPs using Molecular Dynamics simulations in combination with Grid Inhomogeneous Solvation Theory, exploring their hydration thermodynamics. The observed enthalpic and entropic differences between the ice-binding sites and the inactive surface reveal key properties essential for proteins in order to bind ice: While entropic contributions are similar for all sites, the enthalpic gain for all ice-binding sites is lower than for the rest of the protein surface. In contrast to most of the recently published studies, our analyses show that enthalpic interactions are as important as an ice-like pre-ordering. Based on these observations, we propose a new, thermodynamically more refined mechanism of the ice recognition process showing that the appropriate balance between entropy and enthalpy facilitates ice-binding of proteins. Especially, high enthalpic interactions between the protein surface and water can hinder the ice-binding activity.

Synthesis and characterization of a disordered variant of KB5O7(OH)2

Abstract The potassium pentaborate KB5O7(OH)2 reported herein crystallizes in the monoclinic space group P21/c (no. 14) with the lattice parameters a = 904.8(2), b = 753.2(2), c = 1214.9(5) pm, β = 117.16(2)°, and V = 0.73663(5) nm3 (Z = 4). It is a disordered structural variant of an already known compound with the same composition (a = 766.90(3), b = 904.45(3), c = 1223.04(4) pm, β = 119.132(2)°, and V = 0.74101(5) nm3 (Z = 4)) reported by Wu in 2011. The disorder of the potassium cation in the single crystal structure determination of KB5O7(OH)2 presented here leads to a remarkably elongated a axis and a corresponding reduction of the length of the b axis in comparison to the ordered compound. The disordered variant was obtained through a hydrothermal synthesis from KNO3, B2O3, and Ce(NO3)3·6H2O with a molar ratio of 1:1:0.07.

Synthesis and characterization of the alkali borate-nitrates Na3–x Kx[B6O10]NO3 (x=0.5, 0.6, 0.7)

Abstract Three novel mixed alkali borate-nitrates Na3−x Kx[B6O10]NO3 (x=0.5, 0.6, 0.7) were synthesized hydrothermally; their crystal structures were determined through Rietveld analyses, and supported through EDX as well as vibrational spectroscopy. The phases represent solid solutions of the alkali borate-nitrate Na3(NO3)[B6O10], which was reported in 2002 as a “New type of boron-oxygen framework in the Na3(NO3)[B6O10] crystal structure” (O. V. Yakubovich, I. V. Perevoznikova, O. V. Dimitrova, V. S. Urusov, Dokl. Phys. 2002, 47, 791). Only two of the three crystallographically independent Na+ positions in the new structures are partially substituted by K+; a pure potassium borate-nitrate was not formed until now. The cell parameters of the novel phases vary from a=1261.72(5)–1267.12(5), b=1004.19(5)–1007.96(4), c=770.55(3)–774.38(3) pm, and V=0.97630(6)–0.98905(6) nm3 in the orthorhombic space group Pnma (no. 62), in alignment with increasing K+ content.

Hydrothermal synthesis and characterization of the praseodymium borate-nitrate Pr[B5O8(OH)(H2O)0.87]NO3·2H2O

Abstract The praseodymium borate-nitrate Pr[B5O8(OH)(H2O)0.87]NO3·2H2O was obtained in a hydrothermal synthesis. It crystallizes monoclinically in the space group P21/n (no. 14) with four formula units (Z=4) and unit cell parameters of a=641.9(3), b=1551.8(7), c=1068.4(5) pm, with β=90.54(2)° yielding V=1.0643(8) nm3. The defect variant constitutes the missing member in the series of isostructural, early rare earth borate-nitrates of the composition RE[B5O8(OH)(H2O)x]NO3·2H2O [RE=La (x=0; 1), Ce (x=1), Nd (x=0.85), Sm (x=0)]. In addition to powder and single-crystal X-ray diffraction data, the novel borate-nitrate was characterized through IR and Raman spectroscopy.

Synthesis and characterization of the lead borate Pb6B12O21(OH)6

Abstract A lead borate with the composition Pb6B12O21(OH)6 was synthesized through a hydrothermal synthesis, using lead metaborate in combination with sodium nitrate and potassium nitrate. The compound crystallizes in the trigonal, non-centrosymmetric space group P32 (no. 145) with the lattice parameters a = 1176.0(4), c = 1333.0(4) pm, and V = 0.1596(2) nm3. Interestingly, the data of Pb6B12O21(OH)6 correct the structure of a literature known lead borate with the composition “Pb6B11O18(OH)9”. For the latter compound, nearly identical lattice parameters of a = 1176.91(7) and c = 1333.62(12) pm were reported, possessing a crystal structure, in which the localization and refinement of one boron atom was obviously overlooked. The structure of Pb6B12O21(OH)6 is built up from trigonal planar BO3 and tetrahedral BO4 groups forming complex chains. The Pb2+ cations are located between neighboring polyborate chains. The here reported compound Pb6B12O21(OH)6 and “Pb6B11O18(OH)9” were, however, produced under different synthesis conditions. While “Pb6B11O18(OH)9” was synthesized via a hydrothermal synthesis including ethylenediamine and acetic acid, the here reported lead borate Pb6B12O21(OH)6 could be obtained under moderate hydrothermal conditions (240°C) without the addition of organic reagents.