Ultrafast Vibrational Dynamics of the Myoglobin Amide I Band

Abstract

Mid-infrared transient absorption (“pump−probe”) measurements on the amide I band of myoglobin in D 2 O and in a glass-forming D 2 O/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 D 2 O. 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.