Probing the inclusion complexes of short-lived radicals with β-cyclodextrin by CIDNP

An extension of the CIDNP kinetics approach was proposed based on a particular example of the photoreaction between 2,2 '-bipyridine (DP) and N-acetyl l-tyrosine (TyrO(2-)) at alkaline pH in the presence of beta-cyclodextrin (CD). It was found that protonated DP radical (DPH) generated in the photoreaction is encapsulated into CD with binding constant, which is close to that for excited triplet state ((DP)-D-3) and ground state (DP) of 2,2 '-bipyridine and is equal to approximately 100 M-1. The obtained T-1 relaxation time of 5,5 '-protons of DPH in complex with CD (8 mu s) is less than in the case of free DPH (45 mu s), however, it is larger than that theoretically predicted for the rigid complex (=1.7 mu s) indicating the anisotropic rotation of DPH inside CD cavity. The recombination rate constant between DPH in complex with CD and tyrosine radical (TyrO-) (1x109 M-1 s-1) is somewhat lower than the recombination rate constant between free DPH and TyrO- (1.6x109 s-1).