Geminate rebinding dynamics of nitric oxide to ferric hemoglobin in D2O solution

Femtosecond mid-infrared (mid-IR) spectroscopy was used to probe geminate rebinding (GR) dynamics of photo-released nitric oxide (NO) to ferric hemoglobin (Hb(III)) in D2O solution at room temperature. Time-resolved vibrational spectra exhibit two overlapping NO bands for NO-bound Hb(III) (Hb(III)NO), a major band at 1925 cm(-1) (89%) and a minor one at 1905 cm(-1) (11%), suggesting that Hb(III)NO has at least two conformational substates. Both bands decay nonexponentially, each with a different time scale, and the decays are described by a stretched exponential function; the major band's decay is described by 0.96 exp(-t/40 ps)(0.86) + 0.04 and the minor band's decay is described by exp(-t/85 ps)(0.75). These decays arise mainly from the GR of the photo-released NO to Hb(III), indicating that the bound state's conformer influences the NO binding. In particular, the His64 residue, known to have inward conformation in the major band and outward conformation in the minor band, plays a significant role in controlling the binding of NO to Hb(III). The GR of NO to ferric Hb is slower than that to ferrous Hb, which shows fast and efficient GR due to the high reactivity of NO to the heme Fe(ii). The slower GR of NO to Hb(III) may be caused by the lower reactivity of NO to the heme Fe(iii).

ps-TRIR covers all the bases--recent advances in the use of transient IR for the detection of short-lived species in nucleic acids

Recent developments of the picosecond transient absorption infrared technique and its ability to elucidate the nature and kinetic behaviour of transient species formed upon pulsed laser excitation of nucleic acids are described.

Potential Energy Surface and Molecular Dynamics of MbNO: Existence of an Unsuspected FeON Minimum

Ligands such as CO, O(2), or NO are involved in the biological function of myoglobin. Here we investigate the energetics and dynamics of NO interacting with the Fe(II) heme group in native myoglobin using ab initio and molecular dynamics simulations. At the global minimum of the ab initio potential energy surface (PES), the binding energy of 23.4 kcal/mol and the Fe-NO structure compare well with the experimental results. Interestingly, the PES is found to exhibit two minima: There exists a metastable, linear Fe-O-N minimum in addition to the known, bent Fe-N-O global minimum conformation. Moreover, the T-shaped configuration is found to be a saddle point, in contrast to the corresponding minimum for NO interacting with Fe(III). To use the ab initio results for finite temperature molecular dynamics simulations, an analytical function was fitted to represent the Fe-NO interaction. The simulations show that the secondary minimum is dynamically stable up to 250 K and has a lifetime of several hundred picoseconds at 300 K. The difference in the topology of the heme-NO PES from that assumed previously (one deep, single Fe-NO minimum) suggests that it is important to use the full PES for a quantitative understanding of this system. Why the metastable state has not been observed in the many spectroscopic studies of myoglobin interacting with NO is discussed, and possible approaches to finding it are outlined.

Ultrafast time-resolved IR studies of protein-ligand interactions

Time-resolved mid-IR spectroscopy combines molecular sensitivity with ultrafast capability to incisively probe protein-ligand interactions in model heme proteins. Highly conserved residues near the heme binding site fashion a ligand-docking site that mediates the transport of ligands to and from the binding site. We employ polarization anisotropy measurements to probe the orientation and orientational distribution of CO when bound to and docked near the active binding site, as well as the dynamics of ligand trapping in the primary docking site. In addition, we use more conventional transient absorption methods to probe the dynamics of ligand escape from this site, as well as the ultrafast dynamics of NO geminate recombination with the active binding site. The systems investigated include myoglobin, hemoglobin, and microperoxidase.