On the photosensitivity of liganded hemoproteins and their metal-substituted analogues.

Abstract

We have examined the photosensitivity of low-spin liganded hemoglobin, myoglobin, and peroxidase, and their metal-substituted analogues, using three different metals (Fe, Mn, Co) in several oxidation states and employing a variety of diatomic or pseudo-diatomic ligands (L). We have discovered a number of photosensitive systems, and present an overall stereo-electronic classification scheme for these photodissociation reactions: Linear, formally d6, metal-ligand fragments [e.g., HbCO (4,5), and isocyanide-hemes show a high quantum yield for photodissociation (-I) (Q. H. Gibson, unpublished data) in contrast to a lowered value for isocyanidehemoproteins (3). This has compounded the difficulty in isolating those factors that control the quantum yield for ligand photorelease by a metalloporphyrin.In our studies of metal-substituted hemoglobins (MHb) we noted that MnHbNO and HbCO are isoelectronic, and we proceeded to confirm the resulting expectation that NO would be readily photodissociated from the manganese protein, despite the minimal photolability of HbNO (8,9). Encouraged by this observation, we speculated that the variation in photodissociation quantum yields might be general, and might represent a stereo-electronic discrimination in which MHb(L) systems with a linear, formally d6, metal-ligand fragment [e.g., Fe(II) + CO; Mn(II) + NO] would be highly photodissociable, but systems with a bent fragment and higher electron occupancy [e.g., Fe(II) + 02; Fe(ii) + NO] would be relatively photoinert. This speculation has prompted us to undertake a broader study of the axial-ligand photodissociation from lowspin metalloporphyrins. We have primarily employed liganded Hb and Mb, and their metal-substituted analogues as a chemically flexible experimental system in which nitrogen atoms of the porphyrin and proximal imidazole occupy five coordination sites of a first transition series metal ion, and the sixth site is occupied by a reversibly bound diatomic or pseudodiatomic ligand, L.We here discuss flash photolysis photodissociation measurements for a large number of such systems, many previously unexamined. The results so far confirm the occurrence of a dichotomy in the values of (0L, which we show to reflect the geometry and electronic structure of the metal-ligand linkage and which appears to correlate with features of the optical spectra. MATERIALS AND METHODSHemoglobin and its liganded derivatives were prepared by published procedures, as were metal-substituted hemoglobins and their derivatives (8,9). Ligand photodissociation was observed spectrophotometrically after flash photolysis. The samples were exposed to light from a Multiblitz III electronic flash with 100-J energy input, and flash duration, (l/e) -400 Asec. The optical absorption spectra of low-spin liganded MPor and MHb and MMb are typically dominated by porphyrincentered transitions, the Soret band in the vicinity of 400 nm (E of order 105), and the a-f bands in the vicinity of 550 nm (E of order 104) (see refs. 10 and 11). The photoflash w...

Enhancement of nitric oxide release from nitrosyl hemoglobin and nitrosyl myoglobin by red/near infrared radiation: Potential role in cardioprotection

“…It is worth noting that the relative rates of nitrite reduction are T-state Hb <<R-State Hb < Mb [ 6 ]. Consistent with other metalloporphyrin-ligand interactions, MbNO and HbNO were found to be photodissociable at wavelengths up to 608 nm, however their quantum yield is extremely low (ϕ=0.001)[ 40 , 41 ].…”

Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

“…However, it is widely recognized that the CO and NO serve as excellent probes of protein dynamics on time scales from picoseconds to seconds. Studies of heme photophysics (Cornelius et al;, Martin et al, 1983bPetrich et al, 1987) are key to understanding the nature of the barrier to ligand rebinding (Franzen et al, 1995b;Walda et al, 1994) and the mechanism of photolysis of diatomic -acid ligands (Hoffman and Gibson, 1978).…”

Increase in peripheral blood flow due to extraocular direct irradiation of visible light in rats

“…The question arises as to why the NO release from NO-Hb occurs in such a very low irradiation density, whereas flash photolysis results reveal that recombination of released NO to heme is very fast (10,19). At this issue, we speculate that NO binding to the R state heme in the sixfold coordination may have a tendency to release NO.…”