William DeW. Horrocks

On the determination of the number of water molecules, q, coordinated to europium(III) ions in solution from luminescence decay lifetimes

Abstract A refined equation (in its simplest form: q=1.11[τ−1H2O−τ−1D2O−0.31]) which allows the prediction of the number of water molecules in the first coordination sphere of a europium(III) (Eu3+) complex, q, in aqueous solution is presented. It is recognized that in the long history of the determination of the q-values of Eu3+ complexes from luminescent data of the excited metal ions in H2O and D2O solutions, certain inconsistencies are present. In some cases the q-values determined have either been non-integral when they should be integral, or have been in conflict with q-values predicted by other means. The original q-value equation put forth by this laboratory correlated the luminescence lifetime data of crystalline Eu3+ complexes to the known q-values based on XRD data from single crystals. In the current report, the difference in the decay rate of the Eu3+ luminescence of metal complexes in H2O and D2O solution is linearly correlated to q-values predicted using the original equation as a guide. Our current interpretation of the luminescence data of 25 Eu3+ complexes taken from the literature, along with the extensive research of many labs involving the effect of oscillators other than water molecules in the first coordination sphere of the Eu3+ ion, has yielded the following equation: q=1.11[τ−1H2O−τ−1D2O−0.31+0.45nOH+0.99nNH+0.075nOCNH] where nOH is the number of alcoholic OH oscillators in the first coordination sphere of Eu3+, nNH is the number of amine NH oscillators in the first coordination sphere of Eu3+, and nOCNH is the number of amide NH oscillators in which the amide carboxylic oxygen is in the first coordination sphere of Eu3+. The coefficient of determination parameter of the linear least-squares fit to the data is 0.998 and the standard error of the fit is ±0.1 in q. The equation is used to account for the effect of water molecules in the second coordination sphere of the Eu3+ ion as well as to cast light on aqueous Eu3+ complexes that have known labile coordination spheres.

On correlating the frequency of the 7F0 → 5D0 transition in Eu3+ complexes with the sum of ‘nephelauxetic parameters’ for all of the coordinating atoms

Abstract The frequency of the 7F0→5D0 transition of Eu3+ is found to correlate with the sum of derived nephelauxetic parameters of the ligating atoms in Eu3+ complexes. Parameters for nine distinct coordinating atom types are obtained via a multiple linear regression method applied to experimental 7F0→5D0 transition frequencies of twenty-seven 9-coordinate Eu3+ complexes with known ligand constituents. The linear correlation coefficient is 0.99. Addition of data from 7-, 8-, 10-, and 11-coordinate complexes and use of a multiplicative constant, characteristic of each coordination number, produces a second linear correlation covering thirty-seven systems with a correlation coefficient of 0.98. The present approach seeks to understand the factors which influence the 7F0→5D0 transition frequency and to make predictions of this quantity for particular ligand environments. It is completely empirical in nature; however, the results may be understood in terms of an underlying theoretical cause. The parameters obtained here correlate roughly with the nephelauxetic h-values of Jorgensen for the few cases where appropriate ligand matches are available. The 7F0→5D0 term separation will decrease from its free-ion value of 17 374 cm−1 with increasing Eu3+ -ligand covalency, and the presently derived parameters are consistent with this idea. The correlation presented here is clearly superior to our earlier attempt to identify a relationship between the 7F0→5D0 transition frequency and the total formal charge on the ligands in a complex.

Lanthanide complexes as nuclear magnetic resonance structural probes: paramagnetic anisotropy of shift reagent adducts.

Magnetic anisotropy measurements on single crystals of a series of paramagnetic 8-coordinate lanthanide shift reagent adducts of the type Ln[(CH3)3CCOCHCOC(CH3)3(4-CH3C5H1N)2 have been made for the following lanthanides: praseodymium, neodymium, samarium, europium, terbium, dysprosium, holmium, erbium, thulium, and ytterbium. The susceptibility tensors are highly anisotropic and nonaxial. Dipolar nuclear magnetic resonance shifts evaluated from the solid-state date are in satisfactory agreement with the solution results.

LANTHANIDE IONS AS REDOX PROBES OF LONG-RANGE ELECTRON TRANSFER IN PROTEINS

Abstract Occupancy of the two calcium-binding sites of codfish parvalbumin by the redox-active probe ions Yb 3+ and Eu 3+ causes the average tryptophan (Trp) fluorescence lifetime in this protein to decrease and become non-exponential from the single exponential values found for Ca 2+ and La 3+ of 4.45 ns. These observations are interpreted in terms of an electron transfer (ET) deexcitation mechanism wherein excited singlet state Trp transfers an electron to the Ln 3+ ion, reducing it to the +2 oxidation state and producing a Trp cation radical. Back ET reestablishes the initial system. The driving forces, Δ G °, for the Eu 3+ and Yb 3+ ET systems are different, whereas the nuclear rearrangement factor, λ, and electron donor-acceptor coupling, H AB , of semiclassical ET theory should be nearly the same for both ions. This allows the λ -value to be determined from the measured rates (∼2.05 eV). Temperature dependence studies show that the rate constant for the Eu 3+ system is near the activationless maximum value. Yb 3+ and Eu 3+ are established as redox probes of long-range ET in proteins making Ln 3+ -substituted calcium-binding proteins convenient model systems for studying the distance-dependence of ET.

Evaluation of dipolar nuclear magnetic resonance shifts in low-spin hemin systems: ferricytochrome c and metmyoglobin cyanide.

Abstract A general method is presented for the evaluation of the principal magnetic susceptibilities and NMR dipolar shifts in d 5 , low-spin hemin systems. Experimental g values and the directions of the magnetic axes are necessary input. The method is applied to ferricytochrome c and metmyoglobin cyanide. The results demonstrate the importance of including the second order Zeeman effect and in-plane anisotropy when evaluating NMR dipolar shifts.

Probing the binding of Tb(III) and Eu(III) to the hammerhead ribozyme using luminescence spectroscopy

BACKGROUND Divalent metal ions serve as structural as well as catalytic cofactors in the hammerhead ribozyme reaction. The natural cofactor in these reactions is Mg(II), but its spectroscopic silence makes it difficult to study. We previously showed that a single Tb(III) ion inhibits the hammerhead ribozyme by site-specific competition for a Mg(II) ion and therefore can be used as a spectroscopic probe for the Mg(II) it replaces. RESULTS Lanthanide luminescence spectroscopy was used to study the coordination environment around Tb(III) and Eu(III) ions bound to the structurally well-characterized site on the hammerhead ribozyme. Sensitized emission and direct excitation experiments show that a single lanthanide ion binds to the ribozyme under these conditions and that three waters of hydration are displaced from the Tb(III) upon binding the RNA. Furthermore, we show that these techniques allow the comparison of binding affinities for a series of ions to this site. The binding affinities for ions at the G5 site correlates linearly with the function Z(2)/r of the aqua ion (where Z is the charge and r is the radius of the ion). CONCLUSIONS This study compares the crystallographic nature of the G5 metal-binding site with solution measurements and gives a clearer picture of the coordination environment of this ion. These results provide one of the best characterized metal-binding sites from a ribozyme, so we use this information to compare the RNA site with that of typical metalloproteins.

The Resolution of Laser-Induced Europium(III) Ion Excitation Spectra through the Use of the Marquardt Nonlinear Regression Method

The ability to resolve a 7F0 → 5D0 spectrum into individual peaks is important in the study of Eu(III) complexes where the wavelength of the transition is sensitive to the environment of the ion. This paper describes a computer program, based upon the Marquardt nonlinear regression algorithm, developed to resolve such spectra. Several different line shapes were examined, with the Lorentzian function providing the most accurate description for solid complexes and the Lorentzian-Gaussian product function being the most acceptable for species in solution. The program can resolve a spectrum into as many as five peaks with the option to hold any of the parameters constant. The best fit is determined by evaluation of the sum of the squares of the differences between the calculated and the experimental values.

Lanthanide ion probes of structure in biology. Environmentally sensitive fine structure in laser-induced terbium(III) luminescence.

The 488 nm line of the CW argon ion laser provides a convenient visible source for the direct excitation of the emissive 5D4 state of the Tb(III) ion. Room temperature emission spectra of Tb(III) in a variety of environments have been examined under relatively high resolution. The samples studied include structurally well-characterized crystalline solids, model chelate complexes in solution and Tb(III) bound to the enzyme thermolysin and the protein parvalbumin. The fine structure in the emissions is caused by ligand field splittings of both ground and excited state J manifolds. These spectra provide signatures sensitive to the immediate coordination environment of the Tb(III) ion. Solid state/solution state structural comparisons are made. The emission fine structure reveal differences between the EF side calcium-binding sites of parvalbumin and the calcium site 1 of thermolysin.

Characterization of the five-fold Ca2+ binding site of satellite tobacco necrosis virus using Eu3+ luminescence spectroscopy: A marked size-selectivity among rare earth ions

Satellite tobacco necrosis virus (STNV) is an icosahedral virus which contains three classes of Ca2+ binding site. One of these classes, a five-fold carbonyl site which is believed to exist in a Ca2+ channel, has been investigated using laser-induced Eu3+ luminescence spectroscopy. These twelve identical sites are rather rigid, as evidenced by the single narrow (full width at half-maximum is 6.5 cm-1) band observed at 579.58 nm in the 7F0----5D0 excitation spectrum of the Eu(3+)-STNV complex. Lifetimes of 270 microseconds in H2O and 1620 microseconds in D2O indicate that there are three water molecules bound to the Eu3+ at this site. Ligand field splitting of the 7F0----5D1 and 7F0----5D2 excitation spectra show that this site possesses fairly high symmetry (less than or equal to C5V). The Eu3+ complex of nitrilotriacetic acid was determined via titration to have a dissociation constant, Kd, of 20 +/- 2 nM; this value has been used in competition experiments to deduce that the virus site class binds Eu3+ with a Kd of 1.1 +/- 0.3 nM. This putative ion channel demonstrates remarkable size selectivity, with lanthanide affinities varying by more than one order of magnitude.

New metalloporphyrins. Thorium and yttrium complexes of tetraphenylporphin.

meso-Tetraphenyl-porphyrin (I) reagiert mit den Acetylacetonaten von Thorium(IV) und Yttrium(III) zu den entsprechenden Komplexen (II) bzw. (III).