TRANSIENT AND TIME-RESOLVED OPTICAL STUDIES OF PHOTOLYZED CARBONMONOXY HEMOGLOBIN AND MYOGLOBIN

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).

Hydrophobic Distal Pocket Affects NO−Heme Geminate Recombination Dynamics in Dehaloperoxidase and H64V Myoglobin

The recombination dynamics of NO with dehaloperoxidase (DHP) from Amphitrite ornata following photolysis were measured by femtosecond time-resolved absorption spectroscopy. Singular value decomposition (SVD) analysis reveals two important basis spectra. The first SVD basis spectrum reports on the population of photolyzed NO molecules and has the appearance of the equilibrium difference spectrum between the deoxy and NO forms of DHP. The first basis time course has two kinetic components with time constants of tau(11) approximately 9 ps and tau(12) approximately 50 ps that correspond to geminate recombination. The fast geminate process tau(11) arises from a contact pair with the heme iron in a bound state with S = 3/2 spin. The slow geminate process tau(12) corresponds to the recombination from a more remote docking site >3 A from the heme iron with the greater barrier corresponding to a S = 5/2 spin state. The second SVD basis spectrum represents a time-dependent Soret band shift indicative of heme photophysical processes and protein relaxation with time constants of tau(21) approximately 3 ps and tau(22) approximately 17 ps, respectively. A comparison between the more rapid rate constant of the slow geminate phase in DHP-NO and horse heart myoglobin (HHMbNO) or sperm whale myoglobin (SWMbNO) suggests that protein interactions with photolyzed NO are weaker in DHP than in the wild-type MbNOs, consistent with the hydrophobic distal pocket of DHP. The slower protein relaxation rate tau(22) in DHP-NO relative to HHMbNO implies less effective trapping in the docking site of the distal pocket and is consistent with a greater yield for the fast geminate process. The trends observed for DHP-NO also hold for the H64V mutant of SWMb (H64V MbNO), consistent with a more hydrophobic distal pocket for that protein as well. We examine the influence of solution viscosity on NO recombination by varying the glycerol content in the range from 0% to 90% (v/v). The dominant effect of increasing viscosity is the increase of the rate of the slow geminate process, tau(12), coupled with a population decrease of the slow geminate component. Both phenomena are similar to the effect of viscosity on wild-type Mb due to slowing of protein relaxation resulting from an increased solution viscosity and protein surface dehydration.

Transient and time-resolved resonance Raman investigation of photoinitiated electron transfer in ruthenated cytochromes c

Ruthenation of exterior amino acid residues of heme proteins provides an effective means by which biological ET reactions can be studied within the context of highly complex protein environments. Resonance Raman spectroscopy can probe both ET kinetics and structural dynamics at the molecular level. Here we present the first comprehensive use of time-resolved and transient resonance Raman spectroscopies to examine photoinduced ET in cytochromes. Two ruthenated cytochromes c, Ru(lys72)-cyt.c and Ru(cyt102)cyt.c, were studied with TRRS using 10 ns laser pulses and with TRRRS on a 10 ns to 10 ms time scale. It was found that resonance Raman protocols can effectively trace ET kinetics and associated heme--protein structural dynamics. Care must be exercised, however, when drawing comparisons to measurements made by other methods (i.e., transient absorbance). The TRRS studies directly probe the heme and its local environment and reveal that the heme dynamics accompanying ET are very rapid relative to phenomenological ET kinetics. The heme and its local environment evolve to their equilibrium (ferrous) structure in less than 10 ns subsequent to ET, with no evidence for the existence of metastable heme pocket geometries analogous to those observed in the dynamic response of hemoglobins and oxidases. Finally, species-specific differences are observed in the photoinduced ET kinetics and heme structural dynamics. However, these differences are confined to nanosecond or faster time scales.