I₂BODIPY as a new photoswitchable spin label for light-induced pulsed EPR dipolar spectroscopy exploiting magnetophotoselection

Electron paramagnetic resonance (EPR) pulsed dipolar spectroscopy (PDS) using triplet states of organic molecules is a growing area of research due to the favourable properties that these transient states may afford over stable spin centers, such as switchability, increased signal intensity when the triplet is formed in a non-Boltzman distibution and the triplet signal used for detection, and high orientation selection, when the triplet signal is probed by microwave pulses, due to the large spectral width at low fields, a result of the large zero field splitting and limited bandwidth of microwave pulses used. Here we propose the triplet state of a substituted BODIPY moiety as a spin label in light induced PDS, coupled to a nitroxide, in a model peptide with rigid structure. Orientation selection allows to obtain information on the relative orientation of the two
labels. Additionally, magnetophotoselection effects are employed to introduce optical selection and additional constrains for the determination of the relative orientation of the spin labels.