Gd(III) and Yb(III) Complexes Derived from a New Water-Soluble Dioxopolyazacyclohexane Macrocycle

A new macrocyclic ligand was synthesized by a reaction between diethylenetriaminepentaacetic (DTPA) dianhydride and trans-1,4-diaminocyclohexane, and the Gd­(III) and Yb­(III) complexes were prepared. The compounds were characterized by spectroscopic methods. Structural calculation by DFT shows that the amide linkages are arranged in such a way that a conformational strain is minimized in the macrocyclic frame. The coordination modes of the ligand and water in the metal complexes were also determined by DFT. The longitudinal relaxation time T1 was measured for aqueous solutions of the Gd­(III) complex. The T1 relaxivity arises from the structural feature that a water molecule coordinated to the paramagnetic metal is surrounded by a large open space, through which the exchange of water occurs readily to shorten the relaxation time of water in the entire region, as a result of the chelate conformation defined strictly by the amide groups and the cyclohexane ring

Complexation of neurotransmitters ‒ dopamine, serotonin and melatonin ‒ with a DTPA-based cyclophane of high rigidity: ¹H NMR shift and line-broadening

<p>The ¹H NMR signals of the titled neurotransmitters undergo up-field shift accompanied by line-broadening in NMR titration with the DTPA-based cyclophane at pD 7.3; the cyclophane consists of a 4,4′-bis(1,1′-biphenyl-4,4′-dihydroxy)dianiline unit cyclised by a DTPA (diethylenetriaminepentaacetate) group through two amide linkages. Changes in chemical shifts of dopamine indicate the formation of a 1:1 complex with the formation constant <i>K</i>₁ 400 M⁻¹; the complex of serotonin is likely to form a 2:1 host‒guest complex with <i>β</i>₂ ≈ 10⁵ M⁻²; melatonin does not form a complex with definite stoichiometry. The primary binding forces in the dopamine and serotonin complexes are electrostatic interaction between cationic neurotransmitter and anionic cyclophane molecules, and the resulting ionic pairs are stabilised by encapsulation. The electrostatic interaction is weakened by electrolytes; in 0.1 M Trizma buffer, dopamine does not yield a definite complex, and serotonin forms a 1:1 complex with <i>K</i>₁ 80 M⁻¹. Extreme line-broadening of neurotransmitter signals suggests that the molecular motion of the guest molecule is slowed in the complex by interactions with the receptor molecule whose internal molecular motion is quenched partially. The high rigidity of the cyclophane enhances intermolecular interaction in the hydrophobic regions to prolong the lifetime of the complex.</p

d-Glutamic acid hydrochloride

The absolute structure of d-glutamic acid hydrochloride [systematic name: (R)-1,3-dicarboxypropan-1-aminium chloride], C5H10NO4+·Cl−, has been determined by single-crystal X-ray diffraction at room temperature using Cu Kα radiation