Abstract: A photochemically induced dynamic nuclear spin polarization (photo-CIDNP) study was presented of the amino acids that are polarisable with vitamin B? In the amino acid-vitamin B?systems with light irradiation, strong emission for 3-H, 5-H and enhanced absorption for 2-H, 6-H at the benzene ring of N-acetyl-L-tyrosine and enhanced absorption for the protons at the imidazole ring of N-acetyl-L-histidine were observed. No polarization was observed for other protons or protons at other amino acids and deriveatives. The CIDNP evidence indicates that the hydrogen-atom abstraction is important in generating radical pairs in the vitamin B₂ sensitizing systems. Vitamin B₂ was considered to be an excellent sensitizer and probe for the surface amino acid residues of proteins and enzymes.

Key words: chemically induced dynam ic nuclear spin polarization, amino acid, vitamin B