A Bis-Manganese(II)–DOTA Complex for Pulsed Dipolar Spectroscopy

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A Bis-Manganese(II)–DOTA Complex for Pulsed Dipolar Spectroscopy, ChemPhysChem 2016, 17, 2066 – 2078


Measuring nanometer-scale distances in biomacromolecules, such as proteins or nucleic acids, is an efficient way to unravel useful information about their structure and dynamics. One of the ways this can be achieved is by using pulsed electron paramagnetic resonance (EPR) techniques, through which distances can be measured by determining the magnitude of magnetic dipolar coupling between pairs of paramagnetic centers. The most commonly used method is pulsed electron–electron double resonance (PELDOR, also known as DEER). PELDOR measurements with nitroxide radicals (S=1/2), which can be incorporated into biomacromolecules through site-directed mutagenesis and spin labeling, have been extensively studied; most measurements are performed at X (9.5 GHz) and Q-band (34 GHz) frequencies. Theoretically, the sensitivity of the measurement can be further improved by using higher fields/frequencies. However, the spectral width for organic radicals increases with field as their g anisotropy becomes resolved, which potentially provides information on orientations, but can also reduce the expected sensitivity gain and complicate data analysis. Furthermore, commonly used nitroxide radicals, such as (S)-(1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-yl)methyl methanesulfonothioate (MTSL) and (2,2,6,6-tetramethyl piperidin-1-yl)oxyl (TEMPO) derivatives, can be readily converted into EPR-silent N-hydroxylamines in the reducing environment of a cell; this complicates PELDOR measurements in complex cellular environments.



A model system incorporating two DOTA ligands on a short OPE spacer was designed, synthesized, and fully characterized. PELDOR measurements were performed on the corresponding MnII complex and on a bis-nitroxide analogue. The experimental Mn-Mn distance distribution obtained by Tikhonov regularization was in good agreement with that simulated by using a simple rotamer analysis. However, there were small deviations, which might have arisen from a combination of shortcomings in the modeling and data analysis. Overall, our results suggested that PELDOR measurements with MnDOTA spin labels directly grafted on an object of interest could be used to determine nanometer distances with relatively narrow distributions predictably. Currently, we are studying the potential use of other MnDOTA derivatives as spin labels for PELDOR, as well as other dipolar EPR spectroscopic methods for MnII-MnII nanometer distance measurements, and these results will be reported in due course.


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ChemPhysChem 2016, 17, 2066 – 2078


High-spin gadolinium(III) and manganese(II) complexes have emerged as alternatives to standard nitroxide radical spin labels for measuring nanometric distances by using pulsed electron–electron double resonance (PELDOR or DEER) at high fields/frequencies. For certain complexes, particularly those with relatively small zero-field splitting (ZFS) and short distances between the two metal centers, the pseudosecular term of the dipolar coupling Hamiltonian is non-negligible. However, in general, the contribution from this term during conventional data analysis is masked by the flexibility of the molecule of interest and/or the long tethers connecting them to the spin labels. The efficient synthesis of a model system consisting of two [Mn(dota)]2- (MnDOTA; DOTA4-=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) directly connected to the ends of a central rodlike oligo(phenylene–ethynylene) (OPE) spacer is reported. The rigidity of the OPE is confirmed by Q-band PELDOR measurements on a bis-nitroxide analogue. The MnII-MnII distance distribution profile determined by W-band PELDOR is in reasonable agreement with one simulated by using a simple rotamer analysis. The small degree of flexibility arising from the linking MnDOTA arm appears to outweigh the contribution from the pseudosecular term at this interspin distance. This study illustrates the potential of MnDOTA-based spin labels for measuring fairly short nanometer distances, and also presents an interesting candidate for in-depth studies of pulsed dipolar spectroscopy methods on MnII-MnII systems.

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A Bis-Manganese(II)–DOTA Complex for Pulsed Dipolar Spectroscopy


Paul Demay-Drouhard, H. Y. Vincent Ching, Dmitry Akhmetzyanov, Régis Guillot, Leandro C. Tabares, Hélène C. Bertrand, and Clotilde Policar


ChemPhysChem 2016, 17, 2066 – 2078


doi: 10.1002/cphc.201600234