Ultrafast vibrational dynamics of the myoglobin amide I band

Mid-infrared transient absorption ("pump-probe") measurements on the amide I band of myoglobin in D2O and in a glass-forming D2O/glycer(ol-d(3)) solvent mixture reveal very rapid vibrational energy relaxation. At 300 K, the exponential decay time is 1.3 +/- 0.2 ps in D2O, The temperature dependence of the vibrational relaxation in the solvent mixture is slight, changing from 1.9 +/- 0.2 ps below 100 K to 1.2 +/- 0.2 ps at 310 K. The lack of a strong temperature dependence is indicative of a low-order relaxation process where energy transfers into high-energy modes of the system rather than directly to low-energy solvent or protein "bath" modes. The pump-probe signal is also strongly wavelength-dependent. As the laser is tuned to the low-energy side of the absorption band, transient absorption contributions to the signal increase, indicating an anharmonicity of 15 +/- 2 cm(-1) for the amide I mode. The time-resolved polarization anisotropies at 300 and 100 K show a decay of about 10 ps, independent of temperature, which is attributed to energy transfer within the amide I band.