and B4 where the signs are reversed in 10 compared to the difference between the deprotonated and protonated nucleoside and nucleotide. This suggests that in addition to hydrogen bonding between B3 and an e side chain amide hydrogen another interaction may be occurring involving the top edge of the benzimidazole moiety. This additional interaction could be a hydrophobic interaction between the top edge of the benzimidazole moiety and the e side chain methylene groups, although we cannot rule out the possibility that dipolar shielding from the cobalt atom is responsible for this effect, as neither the exact geometry of the hydrogen-bonded species nor the magnetic anisotropy of the cobalt atom in any base-off cobalamin is known.Attempts are currently in progress to evaluate the equilibrium constant for formation of the hydrogen-bonded base-off complex and thus to reinterpret the thermodynamics of the base-on/base-off reaction. These results will be reported in a subsequent communication.Abstract: Resonance Raman (RR) spectra are reported for a series of nickel(I1) dihydroporphyrins with excitation in the B, Qx, and Qy absorption regions. The molecules include trans-octaethylchlorin, y,6-deuteriated trans-octaethylchlorin, 9-deoxomethylmesopyropheophorbide a, methylmesopyropheophorbide a, and methylpyropheophorbide a. These molecules represent a series in which the structural complexities of chlorophyll a (reduced pyrrole ring, isocyclic ring, 9-keto group, and 2-vinyl group) are systematically added to the basic tetrapyrrole structure. All of the observed in-plane chlorin skeletal modes and vibrations of the isocyclic ring and vinyl group are assigned. Assignments are then proposed for chlorophyll a based on analogy to those of the Ni(I1) complexes. In addition to the spectral assignments, normal coordinate calculations are performed on the various nickel(I1) dihydroporphyrins. These calculations indicate that the forms of the normal coordinates of the metallochlorins bear little resemblance to those of the parent metalloporphyrins. In the low-symmetry environment which characterizes the reduced pyrrole pigments, a number of the vibrations are localized in semicircles or quadrants of the macrocycle rather than being delocalized over the entire ring. This vibrational localization is due to geometrical changes in the n-bonded system which occur as a result of reduction of one of the pyrrole rings and addition of the isocyclic ring.
The first near-infrared (Q,)-excitation resonance Raman spectrum of photosynthetic reaction centers (Rb. sphaeroides wild type) is reported. This spectrum exhibits features which are not observed with excitation into either the Q, or Soret absorption bands. The spectral data indicate that the partial double-bond character is induced in the &Cl0 bond of the isocyclic ring of one of the pigments via interactions with the protein.It is proposed that this modified pigment is the L-subunit bacteriopheophytin and that the preference for electron transfer to this molecule could be in part due to the change in electronic structure induced by the site-specific pigment-protein interaction.
The ferrous form of native cytochrome c peroxidase (CCP) is known to undergo a reversible transition when titrated over the pH range of 7.00-9.70. This transition produces a conversion from a pentacoordinate high-spin to a hexacoordinate low-spin heme active site and is clearly apparent in the heme optical absorption spectra. Here, we report the characterization of this transition and its effect upon the local heme environment using various optical spectroscopies. The formation of hexacoordinate low-spin heme is interpreted to involve the binding of His-52 at the distal site after the perturbation of the extensive H-bonded network within and around the heme pocket of CCP(II) at alkaline pH. Interestingly, CD investigations of CCP(II) in the far-UV and Soret regions indicate the dissappearance of a single high-spin species and the existence of at least two low-spin species of CCP(II) as the pH is raised above 7.90. Furthermore, transient resonance Raman experiments demonstrate that the hexacoordinate low-spin species can be photolyzed within 10-ns laser pulses, producing a species similar to the low-pH (high-spin) form of CCP(II) at alkaline pH. However, the extent of photolysis is quite pH dependent, with a maximum photodissociation yield at pH = 8.50.
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