CIDNP

It detects the non-Boltzmann (non-thermal) nuclear spin state distribution produced in these reactions as enhanced absorption or emission signals.

In 1969 an alternative explanation which relies on the nuclear spins affecting the probability of a radical pair recombining or separating.

It is related to chemically induced dynamic electron polarization (CIDEP) insofar as the radical-pair mechanism explains both phenomena.

The effect arises when unpaired electrons (radicals) are generated during a chemical reaction involving heat or light within the NMR tube.

The principal application of this photo-CIDNP technique, as devised by Kaptein in 1978, has been to proteins in which the aromatic amino acid residues histidine, tryptophan and tyrosine can be polarized using flavins or other aza-aromatics as photosensitisers.

The key feature of the method is that only solvent accessible histidine, tryptophan and tyrosine residues can undergo the radical pair reactions that result in nuclear polarization.

Photo-CIDNP has thus been used to probe the surface structure of proteins, both in native and partially folded states, and their interactions with molecules that modify the accessibility of the reactive side chains.