Orientation selection in high-field RIDME and PELDOR experiments involving low-spin Co^II ions

Abstract

Orientation selective (OS) RIDME and PELDOR were conducted on a low-spin Co^II complex coordinated by two nitroxide (NO) labelled 2,2':6',2''-terpyridine ligands. Co-NO RIDME at W- and Q-band gave insight into the relative orientation between the Co-NO interspin vector (r_Co-NO) and the NO moiety. This was further supported by W-band Co-NO PELDOR that also allowed elucidating the relative orientation of the Co^II and NO g-tensors. Differences to earlier predictions were confirmed by DFT calculations. Finally, NO-NO PELDOR allowed retrieving the mutual orientations between the NO-NO interspin vector (r_NO-NO) and the NO moieties. The results demonstrate that OS-RIDME and -PELDOR can provide geometric structure information on a system containing a Co^II ion and two nitroxides. Especially, the high sensitivity and ease of interpretation of RIDME at W-band opens avenues for new applications of Co^II as orthogonal spin label.