The low-frequency vibrations of riboflavin and related compounds (alloxazine, lumichrome, lumiflavin as the ring system and D-mannitol as the side-chain system were observed by far-infrared (terahertz) spectroscopy. Vibrational mode assignments in this spectrally congested range were made using high precision quantum chemical calculations. These resonance frequencies located below 200 cm(-1) indicate the existence of motions important for biological reactions. The observed absorption bands in the low-frequency region of riboflavin are assigned to the in-plane and out-of-plane-ring deformations of pyrimidine and isoalloxazine. and to the torsion modes of the ribityl chain. (C) 2004 Elsevier B.V. All rights reserved.