Pulsed electron-electron double resonance spectroscopy between a high-spin Mn(2+) ion and a nitroxide spin label

Area of Science:

• Biophysical Chemistry
• Electron Paramagnetic Resonance Spectroscopy

Background:

• Pulsed Electron-Electron Double Resonance (PELDOR) is a powerful technique for measuring distances between spin labels in biomolecules, typically within 1.5-8 nm.
• Conventional PELDOR often employs nitroxide spin labels attached to proteins.
• Exploring alternative paramagnetic species, such as metal ions and cofactors, is expanding PELDOR's applicability.

Purpose of the Study:

• To evaluate the performance of PELDOR using a high-spin Manganese (Mn(2+)) ion and a nitroxide spin label in a synthetic model system.
• To assess the accuracy of distance measurements obtained via high-frequency PELDOR (Q-band and G-band).
• To investigate the potential of Mn(2+) as a spin probe for biomolecular distance measurements.

Main Methods:

• Pulsed Electron-Electron Double Resonance (PELDOR) experiments were conducted at Q-band (34 GHz) and G-band (180 GHz) frequencies.
• A synthetic model compound incorporating a nitroxide spin label and a Mn(2+) ion was used.
• Experiments involved probing either the Mn(2+) ion or the nitroxide label, and analyzing changes in dipolar oscillation frequency related to the g-tensor.

Main Results:

• High-frequency PELDOR measurements yielded distances that closely matched structural predictions.
• At G-band frequencies, changes in dipolar oscillation frequency were observed, correlating with the pump-probe positions and indicating restricted nitroxide mobility.
• The study successfully demonstrated the utility of Mn(2+) for precise distance determination.

Conclusions:

• High-spin Mn(2+) ions are effective paramagnetic probes for accurate distance measurements using PELDOR.
• The findings support the use of Mn(2+) in biomolecular applications, particularly given its ability to substitute for naturally occurring Mg(2+) sites.
• High-frequency PELDOR with Mn(2+) broadens the scope of distance measurements in complex biological systems.