INITIAL TRAJECTORY OF CARBON-MONOXIDE AFTER PHOTODISSOCIATION FROM MYOGLOBIN AT CRYOGENIC TEMPERATURES

Migration of the CO ligand following photolysis of carbonmonoxy myoglo bin (MbCO) in single crystals has been investigated by time-resolved X -ray diffraction at 40K. After short illumination by weak visible ligh t at a photolysis rate of similar to 1 s(-1), the photodissociated CO molecule is found about 1 Angstrom from its bound location. After cont inuous illumination over several hours, the CO molecule migrates to a more distant site in the distal pocket, about 2.5 Angstrom from its bo und location. Migration of the ligand under continuous illumination ac counts for different locations of the photodissociated CO molecule pre viously reported in three cryocrystallographic studies [Teng, T.-Y., e t al. (1994) Nat. Struct. Biol. 1, 701-705; Schlichting, I., et al. (1 994) Nature 371, 808-812; Hartmann, H., et al. (1996) Proc. Natl. Acad . Sci. U.S.A. 93, 7013-7016]. Due to the different photolysis protocol s employed in these studies, each reveals a different part of the traj ectory of the photodissociated CO molecule. When the different experim ental parts of the trajectory at 40 K are pieced together and combined with our nanosecond time-resolved studies at room temperature [Srajer , V., et al. (1996) Science 274, 1726-1729], excellent agreement is ob tained with recent theoretical predictions of the CO probability distr ibution in the ligand pocket [Vitkup, D., et al. (1997) Not. Struct. B iol. 4, 202-208]. The heme relaxation that accompanies ligand photolys is is incomplete, about 30% of that associated with-the conversion of MbCO to deoxy-mb at room temperature, and independent of the duration of illumination. Other tertiary structural changes in the globin are a lso greatly diminished. The globin structure is therefore very rigid a t cryogenic temperatures, and structural relaxation is greatly hindere d, consistent with numerous spectroscopic measurements.