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

Area of Science:

• Coordination chemistry
• Biophysical chemistry
• Spectroscopy

Background:

• High-spin gadolinium(III) and manganese(II) complexes are emerging as alternatives to nitroxide spin labels for nanometric distance measurements using pulsed electron-electron double resonance (PELDOR/DEER).
• The pseudosecular term in dipolar coupling can be significant for certain complexes with small zero-field splitting (ZFS) and short metal-metal distances, but is often masked by molecular flexibility in conventional analyses.

Purpose of the Study:

• To synthesize and characterize a model system with two manganese(II) complexes directly linked by a rigid oligo(phenylene-ethynylene) (OPE) spacer.
• To evaluate the utility of Mn(II) complexes as spin labels for measuring short nanometer distances using high-field/frequency PELDOR.
• To investigate the influence of the pseudosecular term in dipolar coupling for Mn(II)-Mn(II) systems.

Main Methods:

• Efficient synthesis of a model system: two [Mn(dota)](2-) (MnDOTA) complexes linked by a rodlike OPE spacer.
• Q-band PELDOR measurements on a bis-nitroxide analogue to confirm the rigidity of the OPE spacer.
• W-band PELDOR measurements on the Mn(II)-Mn(II) system to determine the distance distribution profile.

Main Results:

• The rigidity of the OPE spacer was confirmed using a bis-nitroxide analogue.
• W-band PELDOR measurements provided a Mn(II)-Mn(II) distance distribution profile that agreed reasonably with simulations.
• The flexibility of the linking MnDOTA arm was found to be more influential than the pseudosecular term at the measured interspin distance.

Conclusions:

• MnDOTA-based spin labels show potential for measuring short nanometer distances.
• This study provides a valuable model system for in-depth investigations of pulsed dipolar spectroscopy methods involving Mn(II)-Mn(II) systems.