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

Abstract

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 (TyrO2−) at alkaline pH in the presence of β-cyclodextrin (CD). It was found that protonated DP radical ($$DP{H^ \cdot }$$) generated in the photoreaction is encapsulated into CD with binding constant, which is close to that for excited triplet state (3DP) and ground state (DP) of 2,2′-bipyridine and is equal to approximately 100 M− 1. The obtained T1 relaxation time of 5,5′-protons of $$DP{H^ \cdot }$$ in complex with CD (8 µs) is less than in the case of free $$DP{H^ \cdot }$$ (45 µs), however, it is larger than that theoretically predicted for the rigid complex (= 1.7 µs) indicating the anisotropic rotation of $$DP{H^ \cdot }$$ inside CD cavity. The recombination rate constant between $$DP{H^ \cdot }$$ in complex with CD and tyrosine radical ($$Tyr{O^{ - \cdot }}$$) ($$1 \times {10^9}\,{\text{M}^{ - 1}}\,{\text{s}^{ - 1}}$$) is somewhat lower than the recombination rate constant between free $$DP{H^ \cdot }$$ and $$Tyr{O^{ - \cdot }}$$ ($$1.6 \times {10^9}\;{\text{M}^{ - 1}}\;{\text{s}^{ - 1}}$$).