A rhodamine−cyclen conjugate (1) behaves as a highly sensitive and selective fluorescent chemosensor for Hg2+. The high emission selectivity is due to the formation of 1−Hg2+ 1:2 complex leading to spirocycle opening of 1.

A rhodamine-cyclen conjugate as a highly sensitive and selective fluorescent chemosensor for Hg(II).

Proton Coordination by Polyamine Compounds in Aqueous Solution

Structure-stability correlations for imine formation in aqueous solution

Iron has been implicated in Alzheimer's disease, but until now no direct proof of Fe(II) binding to the amyloid-β peptide (Aβ) has been reported. We used NMR to evidence Fe(II) coordination to full-length Aβ40 and truncated Aβ16 peptides at physiological pH and to show that the Fe(II) binding site is located in the first 16 amino-acid residues. Fe(II) caused selective broadening of some NMR peaks that was dependent on the Fe:Aβ stoichiometry and temperature. Analysis of Fe(II) broadening effect in the (1)H, (13)C, and 2D NMR data established that Asp1, Glu3, the three His, but not Tyr10 nor Met35 are the residues mainly involved in Fe(II) coordination.

Iron(II) binding to amyloid-β, the Alzheimer's peptide.

The theory and practice of proton microspeciation based on NMR-pH titrations are surveyed. Principles of bi-, tri-, tetra-, and n-protic microequilibrium systems are discussed. Evaluation methods are exemplified by case studies on bi- and tetraprotic biomolecules. Selection criteria and properties of 'reporter' NMR nuclei are described. Literature data on complete microspeciations of small ligands and site-specific basicity characterizations of peptides and proteins are critically reviewed.

Determination of microscopic acid–base parameters from NMR–pH titrations

Dissociation constants of Br∅nsted acids in D2O and H2O: studies on polyaza and polyoxa-polyaza macrocycles and a general correlation

Some 12- to 14-membered oxatriaza macrocyclic ligands and their methylated derivatives have been synthesised and their protonation studied by potentiometric and 1H NMR techniques. The stability constants of the complexes formed by these ligands with several first-series transition-metal ions, ZnII, CdII and PbII, were determined by automated potentiometry or by an ‘out-of-cell’ technique in the cases of slow equilibration of the reactions. All the ligands have two very basic nitrogen atoms. The third nitrogen atom is protonated at lower pH, the lower the value the shorter the distance between two protonated nitrogen atoms. Stability constants for complexes of the oxatriaza ligands are lower than those for tetraaza ligands of the same size, the differences being more pronounced for the 14-membered ligands. Larger ions, such as CdII and PbII, seem to prefer smaller rings, contrary to what might be expected if the metals were inserted in the ring. Curiously, the behaviour of the copper(II) and especially nickel(II) complexes is opposite in the two series. Stability constants of complexes of the methylated amines are lower than those of the complexes with the parent amines and the same metal ions, but the differences are smaller for the oxatriaza than for the tetraaza series. This suggests that in the former series both the amines and their methylated derivatives adopt similar conformations in their complexes.

Oxatriaza macrocyclic ligands: studies of protonation and metal complexation

The sequences of protonation of a series of oxatriaza and dioxadiaza macrocyclic compounds have been studied by 1H and 13C n.m.r. spectroscopy. It is shown that Sudmeier and Reilley's quantitative approach to derive sequences of protonation suffers from several limitations when applied to these (and other) polyaza macrocycles due to the lack of suitable models, but the indications directly provided by the corresponding potentiometric and n.m.r. titration curves may complement the information required for this purpose. The results obtained suggest that the 12-membered oxatriaza and dioxadiaza macrocycles adopt different and more compact conformations when the nitrogen atoms are derivatized (e.g. with methyl or acetate groups); this results in stronger electrostatic repulsions of consecutive protonated nitrogen atoms of the ring, leading to changeable base strength and to protonation–deprotonation steps that produce ‘anomalous’ n.m.r. titration curves. In the case of the N-acetate derivatives, the interpretation of the trends is further complicated by the possibility of formation of CO2–⋯ HN+ intramolecular hydrogen bonds. The data now available make it possible to predict approximate values for the protonation constants of the various basic centres for the 12-membered polyoxapolyaza macrocycles and, consequently, the corresponding percentages of protonation at different pH values.

M. T. S. Amorim

Papers

Dissociation constants of Br∅nsted acids in D2O and H2O: studies on polyaza and polyoxa-polyaza macrocycles and a general correlation

Oxatriaza macrocyclic ligands: studies of protonation and metal complexation

Nuclear magnetic resonance studies of the protonation sequence of some oxaaza macrocyclic compounds

Oxatriaza Macrocyclic Ligands: Studies of Protonation and Metal Complexation.