Theoretical studies by computer simulation have shown that vibrational modes, which depend on the subdomain motions of proteins, are located in the submillimeter wave region (i.e., 10–100 cm−1, 0.3–3 THz). We have successfully observed, by measuring the difference spectrum between squid rhodopsin and metarhodopsin to avoid water absorption, that squid rhodopsin shows its absorption features in this region. Our experimental results show that a native protein in solution indicates not only an absorbent property in this region but also an actual change of absorption with changes in protein conformation.
