Multiple‐Frequency and Variable‐Temperature EPR Study of Gadolinium(III) Complexes with Polyaminocarboxylates: Analysis and Comparison of the Magnetically Dilute Powder and the Frozen‐Solution Spectra

102

Nanosized gold particles were functionalised with two types of paramagnetic surface tags, one having a nitroxide radical and the other one carrying a DTPA complex loaded with Gd 3+ . Selective measurements of nitroxide-nitroxide, Gd 3+ -nitroxide and Gd 3+ -Gd 3+ distances were performed on this system and information on the distance distribution in the three types of spin pairs was obtained. A numerical analysis of the dipolar frequency distributions is presented for Gd 3+ centres with moderate magnitudes of zero-field splitting, in the range of detection frequencies and resonance fields where the high-field approximation is only roughly valid. The dipolar frequency analysis confirms the applicability of DEER for distance measurements in such complexes and gives an estimate for the magnitudes of possible systematic errors due to the non-ideality of the measurement of the dipole-dipole interaction.

Section: Analysis Of Dipolar Frequencies: Numerical Simulationssupporting

confidence: 84%

Section: Analysis Of Dipolar Frequencies: Numerical Simulationsmentioning

confidence: 93%

Distance measurements in Au nanoparticles functionalized with nitroxide radicals and Gd3+–DTPA chelate complexes

Yulikov

Lueders

Warsi

et al. 2012

102

“…The simplest model for the distributions of D and E in the ZFS term of the spin Hamiltonian (Equation 2) was tested by Benmelouka et al [24]. The authors assumed that the distributions of D and E for Gd(III) complexes in frozen glassy solutions can be described by two uncorrelated Gaussian distributions (drawn schematically in Figure 1(a)), which we write here in the standard form: Pfalse(Dfalse)=12πσD2⋅normalexp(−(D−〈D〉)22σD2)newlinePfalse(Efalse)=12πσE2⋅normalexp(−(E−〈E〉)22σE2)The authors reported reasonably good agreement between EPR spectra of Gd(III) complexes and their simulations with this model for spectra measured in G band, Q band, and X band [24, 35]. Since the D and E values are linear combinations of the eigenvalues of the ZFS tensor, this model essentially assumes Gaussian distributions for the D X , D Y and D Z values.…”

Section: Theoretical Backgroundmentioning

confidence: 78%

Quantitative analysis of zero-field splitting parameter distributions in Gd(iii) complexes

Clayton

Keller

et al. 2018

The magnetic properties of paramagnetic species with spin S > 1/2 are parameterized by the familiar g tensor as well as "zero-field splitting" (ZFS) terms that break the degeneracy between spin states even in the absence of a magnetic field. In this work, we determine the mean values and distributions of the ZFS parameters D and E for six Gd(iii) complexes (S = 7/2) and critically discuss the accuracy of such determination. EPR spectra of the Gd(iii) complexes were recorded in glassy frozen solutions at 10 K or below at Q-band (∼34 GHz), W-band (∼94 GHz) and G-band (240 GHz) frequencies, and simulated with two widely used models for the form of the distributions of the ZFS parameters D and E. We find that the form of the distribution of the ZFS parameter D is bimodal, consisting roughly of two Gaussians centered at D and -D with unequal amplitudes. The extracted values of D (σD) for the six complexes are, in MHz: Gd-NO3Pic, 485 ± 20 (155 ± 37); Gd-DOTA/Gd-maleimide-DOTA, -714 ± 43 (328 ± 99); iodo-(Gd-PyMTA)/MOMethynyl-(Gd-PyMTA), 1213 ± 60 (418 ± 141); Gd-TAHA, 1361 ± 69 (457 ± 178); iodo-Gd-PCTA-[12], 1861 ± 135 (467 ± 292); and Gd-PyDTTA, 1830 ± 105 (390 ± 242). The sign of D was adjusted based on the Gaussian component with larger amplitude. We relate the extracted P(D) distributions to the structure of the individual Gd(iii) complexes by fitting them to a model that superposes the contribution to the D tensor from each coordinating atom of the ligand. Using this model, we predict D, σD, and E values for several additional Gd(iii) complexes that were not measured in this work. The results of this paper may be useful as benchmarks for the verification of quantum chemical calculations of ZFS parameters, and point the way to designing Gd(iii) complexes for particular applications and estimating their magnetic properties a priori.

“…51 A unique feature of Gd 3+ chelates in frozen solutions is the large distributions of D and E and of the orientation of the principal axis system of the ZFS tensor with respect to the molecular frame. 52,53 This leads to smearing of the powder pattern singularities of the ''other'' (except central) D-dependent transitions. It is this feature that is responsible for the lack of orientation selection in Gd 3+ -Gd 3+ DEER.…”

Section: The Gd 3+ Epr Spectrummentioning

confidence: 99%

Gd3+ spin labeling for distance measurements by pulse EPR spectroscopy

Goldfarb

2014

172

195

Methods for measuring nanometer scale distances between specific sites in biomolecules (proteins and nucleic acids) and their complexes are essential for describing and analyzing their structure and function. In the last decade pulse EPR techniques were proven very effective for measuring distances between two spin labels attached to a biomolecule. The most commonly used spin labels for such measurements are nitroxide stable radicals. Recently, a new family of spin labels, based on Gd(3+) chelates, has been introduced to overcome some of the limitations of using nitroxides, particularly at high magnetic fields, which are attractive due to the increased sensitivity they offer. The benefits that such S = 7/2 spin labels offer for frequencies of 30 GHz and higher, particularly at 95 GHz, include (1) high sensitivity, only ∼0.15 nmol of doubly labeled biomolecule is needed, (2) the lack of orientation selection, which allows straightforward data analysis. Gd(3+)-Gd(3+) DEER (double electron-electron resonance) distance measurements on labeled peptides, proteins and DNA have already been demonstrated and the results show that they are very promising in terms of sensitivity. In this Perspective we review these new developments. We briefly introduce the characteristics of the DEER experiment on a pair of S = 1/2 spins and characterize the EPR spectroscopic properties of Gd(3+) ions. We then introduce some of the tags employed to attach Gd(3+) to biomolecules and provide a few experimental examples of Gd(3+)-Gd(3+) DEER measurements. This is followed by a discussion of the parameters that affect the sensitivity of such DEER measurements. Since an important term in the spin Hamiltonian of Gd(3+) is the zero-field splitting (ZFS), its effect on the DEER modulation frequencies must be considered and this is discussed next. Finally, another recently reported approach for using Gd(3+) in distance measurements will be presented: the use of Gd(3+)-nitroxide pairs.