Robert J. Kurland

Isotropic NMR shifts in transition metal complexes: The calculation of the fermi contact and pseudocontact terms☆

Abstract The Fermi contact and pseudocontact contribution to isotropic NMR shifts in paramagnetic complexes are considered for the following cases, for which formulae customarily used are not strictly applicable: (1) there is an appreciable contribution to the magnetic moment of the complex from unquenched orbital angular momentum; (2) there is appreciable orbital contribution (induced by spin-orbit effects) from spin density at the ligand nucleus; (3) there is appreciable mixing of the ground electronic state and thermally populated excited states by the applied magnetic field. An approximate density matrix treatment is developed to handle these situations. The following examples are discussed as illustrations: octahedral ( t 2 g ) 1 and ( t 2 g ) 5 complexes and complexes in which zero-field splitting occurs.

Surface potential of phosphatidylserine monolayers. II. Divalent and monovalent ion binding.

Ion binding constants for phosphatidylserine membranes have been derived from the variation of the surface potential of phosphatidylserine monolayers with divalent cation concentrations in the presence of various monovalent salts in the aqueous subphase. The observed surface potential data for the monolayers, analyzed by use of the Gouy-Chapman diffuse potential theory, together with a simple binding reaction formula, yield, for Ca2+, Mg2+, Na+ and (Me)4N+ binding constant values of 30 M-1, 10 M-1, 0.6 M-1 and 0.05 M-1, respectively. The effect of pH on surface potential of phosphatidylserine monolayers was found to be dependent upon ionic species other than H+ in the subphase solution. The distinction between apparent and intrinsic dissociation constants of H+ for biomolecules was made in terms of ion binding due to other ions at the same site as for H+ in biomolecules.

Specificity of Na+ binding to phosphatidylserine vesicles from a 23Na NMR relaxation rate study.

23Na NMR relaxation rate measurements show that Na+ binds specifically to phosphatidylserine vesicles and is displaced partially from the binding site by K+ and Ca2+ but to a considerably less extent by tetraethylammonium ion. The data indicate that tetraethylammonium ion affects the binding of Na+ only slightly, by affecting the surface potential through its presence in the double layer, without competing for a phosphatidylserine binding site. Values for the intrinsic binding constant for the Na+-phosphatidylserine complex that would be consistent with the competition experiments (and the dependence of the relaxation rate on concentration of free Na+) fall in the range 0.4--1.2 M-1 with a better fit towards the higher values. We conclude that in the absence of competing cations in solution an appreciable fraction of the phosphatidylserine sites could be associated with bound Na+ at 0.1 M Na+ concentration.

Interactions of La3+ with phosphatidylserine vesicles. Binding, phase transition, leakage and fusion.

The interaction of La3+ with phosphatidylserine vesicles is elucidated by binding studies, differential scanning calorimetry, X-ray diffraction, freeze fracture electron microscopy, and release of vesicle contents. La3+ effectively competes with Ca2+ for phosphatidylserine binding sites. The saturation level is close to a La/lipid ratio of 1:3. A concentration of 0.1 mM of La3+ is sufficient to induce fusion between sonicated vesicles.

Binding of Ca2+ and Mg2+ to phosphatidylserine vesicles: different effects on P-31 NMR shifts and relaxation times.

Abstract Phosphorus-31 NMR lines corresponding to inner and outer surfaces of sonicated phosphatidylserine vesicles can be distinguished by the effects of added Ca 2+ or Mg 2+ at low bulk concentrations (millimolar or less). The changes in chemical shift and relaxation times indicate that Ca 2+ binds directly to the PS phosphate, neutralizing at least a portion of the negative charge and restricting the motion of this group. Mg 2+ ion also binds to the head group, but apparently not as strongly as Ca 2+ , nor is the mobility of the headgroup affected as much.

Fluoride ion as a nmr relaxation probe of paramagnetic metalloenzymes: the binding of fluoride to galactose oxidase

Summary The use of fluoride ion, as a novel nuclear magnetic resonance relaxation probe of paramagnetic metalloenzymes, has been tested on galactose oxidase. The concentration dependence of the F − longitudinal relaxation rates, R 1 = 1/T 1 , and competition studies with CN demonstrate that F binds to the enzyme at or near the Cu 2+ site with a binding constant of the order of 1 M −1 . Competition studies with galactose indicate that a ternary or higher order complex between enzyme, galactose and F − is formed.

The half-wave triplet pulse sequence for determination of longitudinal relaxation rates of single line spectra

Abstract A modified pulse triplet sequence has been developed for the measurement of longitudinal relaxation rates, 1/ T 1 . The analysis of this “half-wave” triplet method takes account of transverse relaxation occurring during the triplet, so that the approximation employed in previous use of the triplet sequence, viz., that the interval τ between component pulses of the triplet be much less than the interval δ t -2τ between successive triplets, is no longer required. Values of 1/ T 1 derived from triplet measurements and a Carr-Purcell spin echo measurement of 1/ T 2 on test samples of aqueous polymethacrylate and copper sulfate are in good agreement with values of 1/ T 1 obtained from conventional Carr-Purcell 180°- t -90° measurements. The half-wave triplet sequence would be useful for those cases where a single line is present in the spectrum: two repetitions of the triplet sequence to obtain n points on a magnetization recovery curve and a determination of T 2 are required for the determination of 1/ T 1 n this method, compared to at least n repetitions (for n points) in the Carr-Purcell 180°- t -90° method.

Fluoride ion as an NMR relaxation probe of galactose oxidase—substrate binding☆

From the dependence on substrate concentration of fluoride ion spin-lattice and spin-spin paramagnetic relaxation rate enhancements, a value for the dissociation constant, Kd = 0.059 +/0 0.002 M, for the anaerobic binding of dihydroxyacetone (monomer) to the Cu(II) site of the enzyme galactose oxidase (D-galactose:oxygen 6-oxidoreductase, EC 1.1.3.9) has been obtained. This value for Kd lies between previously reported values for Km derived by use of classical Michaelis-Menten kinetics. An analogous calculation for the anaerobic binding of galactose to the enzyme yields Kd = 0.145 +/- 0.004 M, a value different from several reported Michaelis constants. F- NMR relaxation measurements on air-exposed samples of galactose and the enzyme yield a dissociation constant for the active site-oxidation product (presumed to be galactohexodialdose), Kd = 2.2 +/- 0.2 M, a value at least an order of magnitude larger than the Michaelis or dissociation constants calculated for the binding of galactose to the enzyme active site; no value for this constant had been reported previously. Some implications of the competition results for the type of substrate binding are discussed.

A Phosphorus-31 NMR Study of Monovalent Cation Interactions with the Negatively Charged Surface of Phosphatidylserine Vesicles

The variation of P-31 nmr shifts and spinlattice relaxation times with cation atmosphere has been investigated for phosphatidylserine, present as a model membrane system of sonicated unilamellar vesicles. Two theoretical approaches, the ion condensation theory and a Gouy-Chapman theory modified to include specific binding sites, are used to describe the interaction of cations with the negatively charged PS surface. An important feature of the ion condensation model is the following: the total fraction of negative PS surface charge “neutralized” by cations in a thin “condensation” layer at the interface is independent of cation concentration in bulk solution. A similar result holds approximately for the binding site model, over an experimentally relevant range of concentrations. However the two models differ in their predicted dependence on cation bulk concentrations for the case where one cation species competes weakly against the other for the PS-solution interface. The observed dependence of P-31 nmr shifts on the fraction of Na+ in a NaCl/TEA·Cl bulk solution mixture appears to conform more closely to the behavior predicted by the binding site model than to that given by the ion condensation theory.

Phosphorus-31 and Sodium-23 NMR Studies of Diamagnetic Metal Ion Binding to Phosphatidylserine Vesicles

Changes in P-31 chemical shifts and relaxation times demonstrate that Ca2+ binds to the phosphate group of phosphatidylserine (PS) and restricts the motion of this group more than does Mg2+. Smaller differences in P-31 shifts and relaxation occur if tetramethylammonium is substituted for Na+ or the bulk concentration of Na+ is decreased. These latter changes support the conclusion from a Na-23 relaxation rate study that Na+ binds specifically to the PS headgroup.