ULTRAFAST INFRARED-SPECTROSCOPY IN BIOMOLECULES - ACTIVE-SITE DYNAMICS OF HEME-PROTEINS

Rapid advances in the generation of intense tunable ultrashort mid-inf rared (IR) laser pulses allow the use of ultrafast IR pump-probe and v ibrational echo experiments to investigate the dynamics of the fundame ntal vibrational transition of CO bound to the active site of heme pro teins. The studies were performed using a free-electron laser (FEL) an d an experimental set up at the Stanford University FEL Center. These novel techniques are discussed in some detail. Pump-probe experiments on myoglobin-CO (Mb-CO) measure CO vibrational relaxation (VR). The VR process involves loss of vibrational excitation from CO to the protei n and solvent. Infrared vibrational echoes measure CO vibrational deph asing. The quantum mechanical treatment of the force-correlation funct ion description of vibrational dynamics in condensed phases is describ ed briefly. A quantum mechanical treatment is needed to explain the te mperature dependence of VR in Mb-CO from 10 to 300 K. A molecular-leve l description including elements of heme protein structure in the trea tment of vibrational dynamics is also discussed. Vibrational relaxatio n of CO in Mb occurs on the 10(-11)-s time scale. VR was studied in pr oteins with single-site mutations, proteins from different species, an d model heme compounds. A roughly linear relationship between carbonyl stretching frequency and VR rate has been observed. The dominant VR p athway is shown to involve anharmonic coupling from CO through the pi- bonded network of the porphyrin, to porphyrin vibrations with frequenc ies > 400 cm(-1). The heme protein influences VR of bound ligands at t he active site primarily via altering the through pi-bond coupling bet ween CO and heme. Preliminary vibrational echo studies of the effects of protein conformational relaxation dynamics on ligand dephasing are also reported. (C) 1996 John Wiley & Sons, Inc.