Gd Derivative Research Articles

Monoclinic guanidinoacetate methyltransferase and gadolinium ion-binding characteristics.

Guanidinoacetate methyltransferase (GAMT) is the enzyme that catalyzes the last step of creatine biosynthesis. The enzyme is found in abundance in the livers of all vertebrates. Recombinant rat liver GAMT truncated at amino acid 37 from the N-terminus has been crystallized with S-adenosylhomocysteine (SAH) in a monoclinic modification and the crystal structure has been determined at 2.8 A resolution. There are two dimers in the crystallographic asymmetric unit. Each dimer has non-crystallographic twofold symmetry and is related to the other dimer by pseudo-4(3) symmetry along the crystallographic b axis. The overall structure of GAMT crystallized in the monoclinic modification is quite similar to the structure observed in the tetragonal modification [Komoto et al. (2002), J. Mol. Biol. 320, 223-235], with the exception of the loop containing Tyr136. In the monoclinic modification, the loops in three of the four subunits have a catalytically unfavorable conformation and the loop of the fourth subunit has a catalytically favorable conformation as observed in the crystals of the tetragonal modification. From the structures in the monoclinic and tetragonal modifications, we can explain why the Y136F mutant enzyme retains considerable catalytic activity while the Y136V mutant enzyme loses the catalytic activity. The crystal structure of a Gd derivative of the tetragonal modification has also been determined. By comparing the Gd-derivative structure with the native structures in the tetragonal and the monoclinic modifications, useful characteristic features of Gd-ion binding for application in protein crystallography have been observed. Gd ions can bind to proteins without changing the native protein structures and Gd atoms produce strong anomalous dispersion signals from Cu Kalpha radiation; however, Gd-ion binding to protein requires a relatively specific geometry.

Crystal Structures and Magnetic Properties of the Two Misfit Layer Compounds: [SrGd 0.5S 1.5] 1.16 NbS 2 and [Sr(Fe,Nb) 0.5S 1.5] 1.13 NbS 2

Crystal structures of two new misfit compounds, [SrGd 0.5S 1.5] 1.16NbS 2 and [Sr(Fe,Nb) 0.5S 1.5] 1.13NbS 2, were determined through the composite approach, i.e., by refining each subpart ( Q, H-parts, and the common part) of these composite materials, separately. The Q-part is a three-atom-thick layer, with the NaCl-type structure, where external SrS planes enclose the inner GdS or (Fe,Nb)S plane; the structural difference between these two compounds lies in the central layer within the Q-part: Gd and S atoms are in special positions (octahedral coordination), while Fe and S atoms are statistically distributed on split (×4) positions (tetrahedral coordination) around a central unique site (=special position occupied by Nb). The H-part is a sandwich of sulfur planes enclosing the inner Nb plane as observed for the structure of the binary compound NbS 2 itself. The Sr–Gd derivative shows a paramagnetic behavior in the whole studied temperature range (2–300 K). On the other hand, antiferromagnetic interactions occur in the Sr–Fe derivative; the complex magnetic behavior of this compound is related to the statistical distribution of Fe atoms which leads to frustration of the magnetic interactions. At room temperature, experimental values obtained from Mössbauer spectrum correspond to Fe 3+ in tetrahedral sulfur environment: isomer shift δ=0.32 mm s −1, and quadrupole splitting Δ E=0.48 mm s −1.

A stereoselective approach for the synthesis of alpha-sialosides.

A highly efficient synthesis of the human melanoma associated antigen GD(3) derivative has been described. A key feature of the synthetic approach was the use of sialyl donors that were protected with a C-5 trifluoroacetamide moiety. These sialyl donors gave high yields and excellent alpha-anomeric selectivities in direct glycosylations with a wide variety of glycosyl acceptors ranging from C-8 hydroxyls of sialic acids and C-3 hydroxyls of galactosides to reactive primary alcohols.

Effect of Cd substitution of superconductivity of Bi-2222 cuprates

Substitution of bismuth by cadmium was investigated in the compounds Bi 2− x Cd x Sr 2(Ln 0.82Ce 0.18) 2Cu 2O 10+δ (Ln=Gd, Eu, Dy and Ho) and the effect on superconducting properties was studied. Bi 2Sr 2(LnCe) 2Cu 2O 10+δ treated under high oxygen pressure up to 50 atm displayed semiconducting behavior instead of superconductivity. Superconductivity occurs in Cd-substituted compounds with a maximum T c of 36 K for the Gd derivative under the same treatment conditions.

Extended family of new Bi-2222 superconducting cuprates Bi2Sr2(RE1−Ce )2Cu2O10+, RE=rare earth

Abstract We report the synthesis of a new class of layered cuprates with BiO bilayers: Bi 2 Sr 2 (RE 1− x Ce x ) 2 Cu 2 O 10+ y (Bi-2222 phase) with RE=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm (at x =0.18). Superconductivity is observed for RE=Y, Nd, Sm, Eu, Gd, Dy and Ho, but not for Pr, Tb, Er and Tm. The transition temperature ( T c ) is highest for the Gd derivative (34K) and progressively decreases for larger or smaller RE ions. The origin of this effect is discussed.