Long-Lived Amide I Vibrational Modes in Myoglobin

Pump-probe experiments in the infrared measure vibrational relaxation rates. Myoglobin, which is almost entirely $\alpha$ helix in secondary structure, has an unusually long, nonexponential excited state relaxation generated by optically pumping at the blue side ( $5.85\mu \mathrm{m}$) of the amide I band. The amino acid alanine and the predominantly $\beta$ sheet protein photoactive yellow protein do not have such a long-lived state, suggesting that the $\alpha$ helix in proteins can support nonlinear states of 15 ps characteristic times.