Nickel(II)-reconstituted hemoglobin (NiHb) and myoglobin (NiMb) and model Ni porphyrins have been investigated by Soret-resonance Raman difference spectroscopy. Two sets of frequencies for the oxidation-state and core-size marker lines in the region from 1300 to 1700 cm-1 indicate two distinct sites in NiHb. Only one of these sites is evident in the Raman spectra of NiMb. This result is consistent with the UV-visible absorption spectrum of NiHb, which shows two Soret bands at 397 and 420 nm and one Soret at 424 nm for NiMb. Excitation at the blue Soret component of NiHb with 406.7-nm laser radiation preferentially enhances the set of Raman marker lines typical of Ni-protoporphyrin IX [Ni(ProtoP )] in noncoordinating solvents. The wavelength of the blue Soret component and the Raman spectrum indicate four-coordination for this site in NiHb. Laser excitation in the red Soret band enhances a set of lines whose frequencies are compatible with neither four- nor six-coordinate frequencies but are intermediate between the two. The red Soret band of the proteins is also considerably less red shifted than six-coordinate Ni-porphyrin models. These results suggest that Ni in the second site possesses a single axial ligand. Raman spectra of 64Ni-reconstituted and natural abundance Ni-reconstituted hemoglobins, obtained simultaneously in a Raman difference spectrometer, have identified the Ni-ligand stretch at 236 cm-1. The line shifts to 229 cm-1 for the 64Ni-reconstituted Hb. For a pure Ni-ligand stretch a 10-cm-1 shift would be predicted.(ABSTRACT TRUNCATED AT 250 WORDS)

Copper(II) protoporphyrin IX has been introduced into apomyoglobin, and its utility as a reporter group of the heme environment has been examined. The Soret and visible absorption bands and electron spin resonance spectrum show that the Cu(II) is five coordinate, probably through coordination to the F-8 proximal histidine. The resonance Raman spectrum does not indicate any appreciable distortion from the solution conformation of copper(II) protoporphyrin IX dimethyl ester in CS2. The ultraviolet circular dichroism shows no alteration of the helical content of the globin from that of metmyoglobin. The circular dichroism of the porphyrin transitions suggests that the packing of the amino acid side chains around the porphyrin is different than that in the native metmyoglobin.

4009required for agreement between their observed and calculated spectra. Our conclusion that this Gaussian feature is an integral part of the Sz state multiline EPR spectrum, and not due to background interference, is further supported by our observation that it exhibits the same temperature dependence and microwave power saturation behavior as the sharper features of the spectrum (data not shown).It is important to point out that a Mn1"-MntV dimer model can only be successful in simulating the EPR spectrum of the S2 state produced from the untreated resting state by 200 K illumination. The results of Table XI show that the other S2 state multiline EPR signals have considerable tetramer character and, consequently, the Mnlll-MnlV dimer approximation of the ?Mn nuclear hyperfine coupling becomes invalid. ConclusionsIn this contribution, we advance a model for the S2 state of the OEC where two antiferromagnetically exchange coupled Mn dimers are ferromagnetically exchange coupled. Both the magnetic properties and the 55Mn nuclear hyperfine couplings of five distinct S2 state EPR signals can be satisfactorily explained by this model. Table I indicate that both large antiferromagnetic and ferromagnetic exchange couplings are needed to explain the temperature dependence and microwave power saturation behavior of the S2 state EPR spectra. Even though large ferromagnetic exchange couplings are not commonly found in synthetic complexes, a few examples do exist.'* There is also ample precedent in the literature of tetrameric complexes where both large antiferromagnetic and large ferromagnetic exchange interactions occur simultaneously, such as cud04 cubane-like clusters13 and the Fe4S4 centers of ferredoxins.I2 The analogies which can be made with the CU404 and Fe4S4 cubane-like complexes prompt us to speculate that the Mn site of the OEC exists as a Mn404 cubane-like structure in the S2 state. Abstract: The transient photoligation behavior of nickel protoporphyrin IX (Ni(PP)) in a variety of local environments has been studied by time-resolved resonance Raman scattering. In coordinating basic solvents the ligation changes engendered by the photoexcitation of a net d2g -d ' + z , d'g (IAlg -3B1,) transition are easily monitored by changes in Ni(PP) Raman modes that are sensitive to metalloporphyrin spin-state and ligation changes. The Raman results for those systems corroborate the photochemical cycle and excited state lifetimes proposed by Holten and co-workers (Chem. Phys. 1983, 75, 305). However, the photodynamics exhibited by Ni(PP) incorporated into Mb or Hb apoproteins or into detergent micelles are distinct from those of Ni(PP) in coordinating solvents and from each other. The equilibrium 4-coordinate Ni(PP) sites in NiHb display a transient photoassociation of a single ligand (most likely the proximal histidine) similar to the behavior of Ni(PP) in solution. The simulation parameters inThe photodynamics of the equilibrium 5-coordinate Ni(PP) sites in NiHb and NiMb are, however, greatly modulated by the surroundin...

No abstract

No abstract