Magnetic circular dichroism

Fine-tuning of radiative properties by "mild" substituents: searching for a perfectly soft chromophore

Controlling spectral properties to achieve desired characteristics is an attractive goal in application-oriented research, e.g., in the design of fluorescence sensors. "Soft" chromophores, molecules with strong spectral responses to internal or external perturbations are good candidates for such studies. In this work, absorption, fluorescence, and magnetic circular dichroism (MCD) spectra were obtained for a series of porphyrins, substituted at the meso-positions with n-hexyl groups. As the number of substituents increases from 1 to 4, significant changes are observed. The intensity of the S0-S1 transition (Qx) in the 0-0 region strongly decreases in mono-substituted porphyrin, but upon additional substitutions it increases to values larger than in the parent, unsubstituted molecule. Such behavior can be explained, using the perimeter model, by changes in the energy splittings between the two highest (HOMO) and two lowest (LUMO) frontier molecular orbitals. Single substitution makes porphyrin a nearly perfect soft chromophore, but upon introduction of a larger number of n-hexyl groups it is transformed into a hard one. DFT simulations incorrectly predict a continuous transition from a soft to hard chromophore, because the calculated ordering of two HOMO orbitals is opposite to that obtained by experiment. On the other hand, for those porphyrins that can be classified as hard chromophores, the calculations nicely reproduce contributions of Franck-Condon and Herzberg-Teller terms to absorption and fluorescence spectra.

De novo design, synthesis and characterisation of MP3, a new catalytic four-helix bundle hemeprotein

A new artificial metalloenzyme, MP3 (MiniPeroxidase 3), designed by combining the excellent structural properties of four-helix bundle protein scaffolds with the activity of natural peroxidases, was synthesised and characterised. This new hemeprotein model was developed by covalently linking the deuteroporphyrin to two peptide chains of different compositions to obtain an asymmetric helix-loop-helix/heme/helix-loop-helix sandwich arrangement, characterised by 1) a His residue on one chain that acts as an axial ligand to the iron ion; 2) a vacant distal site that is able to accommodate exogenous ligands or substrates; and 3) an Arg residue in the distal site that should assist in hydrogen peroxide activation to give an HRP-like catalytic process. MP3 was synthesised and characterised as its iron complex. CD measurements revealed the high helix-forming propensity of the peptide, confirming the appropriateness of the model procedure; UV/Vis, MCD and EPR experiments gave insights into the coordination geometry and the spin state of the metal. Kinetic experiments showed that Fe(III)-MP3 possesses peroxidase-like activity comparable to R38A-hHRP, highlighting the possibility of mimicking the functional features of natural enzymes. The synergistic application of de novo design methods, synthetic procedures, and spectroscopic characterisation, described herein, demonstrates a method by which to implement and optimise catalytic activity for an enzyme mimetic.

Electronic Structure, Spectra, and Magnetic Circular Dichroism of Cyclohexa-, Cyclohepta-, and Cyclooctapyrrole

Three recently obtained expanded porphyrins represent nice examples of compounds for which the electronic and spectral properties can be predicted from symmetry considerations alone. Perimeter-model-based theoretical analysis of the electronic structure of doubly protonated cyclo[6], cyclo[7], and cyclo[8]pyrrole leads to the anticipation of qualitatively the same electronic absorption and magnetic circular dichroism patterns for all three compounds. These predictions are fully confirmed by experiments, as well as DFT and INDO/S calculations. Due to a characteristic pattern of frontier molecular orbitals, a degenerate HOMO and a strongly split LUMO pair, the three cyclopyrroles show comparable absorption intensity in the Q and Soret regions. Magnetic circular dichroism spectra reveal both A and B Faraday terms, of which the signs and magnitudes are in remarkably good agreement with theoretical expectations. The values of the magnetic moments of the two lowest degenerate excited states have also been obtained.

Histidine-tailed microperoxidase-10: a pH-dependent ligand switch

The electronic absorption and magnetic circular dichroism (MCD) spectra of ferric histidine-tailed microperoxidase-10 (His-MP10) change dramatically as the pH is raised from 1.8 to 11.8. Two distinct species are observed (pKa = 4.4). The spectra of acidic ferric His-MP10 nearly match those of ferric mesoporphyrin-reconstituted myoglobin and so the axial ligands are assigned to be histidine and water. The retention of histidine ligation below pH 4 contrasts to the behavior of myoglobin and horseradish peroxidase which convert to five-coordinate water ligated and then lose the heme prosthetic group at even lower pH. Neutral and alkaline ferric His-MP10 have spectra that are very similar to those of the imidazole complex of ferric mesoporphyrin-reconstituted myoglobin. Thus, we conclude that it is bis-histidine ligated with the C-terminal histidine bound as the sixth ligand. Thus, ferric His-MP10 exhibits a pH-dependent ligand switch with a change in axial ligation from water and histidine at low pH to bis-histidine at neutral and alkaline pH.

The metal binding site of the hepatitis C virus NS3 protease. A spectroscopic investigation

The NS3 region of the hepatitis C virus encodes for a serine protease activity, which is necessary for the processing of the nonstructural region of the viral polyprotein. The minimal domain with proteolytic activity resides in the N terminus, where a structural tetradentate zinc binding site is located. The ligands being been identified by x-ray crystallography as being three cysteines (Cys97, Cys99, and Cys145) and one histidine residue (His149), which is postulated to coordinate the metal through a water molecule. In this article, we present an analysis of the role of metal coordination with respect to enzyme activity and folding. Using NMR spectroscopy, the resonances of His149 were assigned based on their isotropic shift in a Co(II)-substituted protein. Data obtained with 15N-labeled NS3 protease were compatible with the involvement of the delta-N of His149 in metal coordination. pH titration experiments showed that the cooperative association of at least two protons is required in the protonation process of His149. Changes in the NMR signals of this residue between pH 7 and 5 are interpreted as evidence for a structural change at the metal binding site, which switches from a "closed" to an "open" conformation. Site-directed mutagenesis of His149 has shown the importance of this residue in the metal incorporation pathway and for achieving an active fold. The metal coordination of the protease was also investigated by circular dichroism and electronic absorption spectroscopies using a Co(II)-substituted enzyme. We show evidence for rearrangements of the metal coordination geometry induced by complex formation with an NS4A peptide cofactor. No such changes were observed upon binding to a substrate peptide. Also, CN- and N3- induced Co(II) ligand field perturbations, which went along with an 1.5-fold enhancement of protease activity.

A synthetic peptide encompassing the binding site of the second zinc atom (the 'structural' zinc) of alcohol dehydrogenase

A 23-residue peptide was synthesized that incorporates the loop which binds the structural zinc atom of mammalian alcohol dehydrogenases and contributes, in part, to subunit interactions in the native enzyme. Neither the amino acid composition nor the sequence of the peptide resemble those of zinc fingers. The reduced peptide stoichiometrically binds zinc or cobalt to form stable complexes with a dissociation constant of the peptide/CO2+ complex of 2.1 microM at pH 7.5. EDTA disrupts the complex. The absorption and magnetic circular dichroic spectra of the cobalt-peptide are indicative of a tetrahedral coordination geometry, and are similar to those of the cobalt-substituted structural site of horse and human (beta 1 beta 1) liver alcohol dehydrogenases. Consequently, the synthetic peptide can serve as a model for the metal-binding segment of alcohol dehydrogenase and for studies of fundamental problems concerning protein/metal interactions.

The active site structure of E. coli HPII catalase. Evidence favoring coordination of a tyrosinate proximal ligand to the chlorin iron

E. coli produces 2 catalases known as HPI and HPII. While the heme prosthetic group of the HPII catalase has been established to be a dihydroporphyrin or chlorin, the identity of the proximal ligand to the iron has not been addressed. The magnetic circular dichroism (MCD) spectrum of native ferric HPII catalase is very similar to those of a 5-coordinate phenolate-ligated ferric chlorin complex, a model for tyrosinate proximal ligation, as well as of chlorin-reconstituted ferric horseradish peroxidase, a model for 5-coordinate histidine ligation. However, further MCD comparisons of chlorin-reconstituted myoglobin with parallel ligand-bound adducts of the catalase clearly rule out histidine ligation in the latter, leaving tyrosinate as the best candidate for the proximal ligand.

Magnetic circular dichroism spectroscopy as a probe of axial heme ligand replacement in semisynthetic mutants of cytochrome c

Horse heart cytochrome c with either histidine or cysteine replacing the endogenous axial methionine ligand at position 80 has been characterized with magnetic circular dichroism (MCD) spectroscopy in the UV-visible region. Comparison of the MCD spectra of the mutant proteins in the ferric state to those of authentic bis-imidazole- and imidazole/thiolate-ligated ferric heme proteins clearly shows that the histidine-imidazole and cysteine-thiolate groups of the replacement amino acids at position 80 are coordinated to the heme iron in the mutant proteins. This study demonstrates the power of MCD spectroscopy in identifying axial ligands in mutant heme proteins. Accurate axial ligand assignment is essential for proper interpretation of the altered properties of such novel proteins.

Spectroscopic studies on the metal-ion-binding sites of Co2(+)-substituted D-xylose isomerase from Streptomyces rubiginosus

The coordination sphere of the two metal-binding sites/subunit of the homotetrameric D-xylose isomerase from Streptomyces rubiginosus has been probed by the investigation of the Co2(+)-substituted enzyme using electronic absorption, CD and magnetic circular dichroic spectroscopies in the visible region. The spectrum of the high-affinity site (B site) has an absorption coefficient, epsilon 545, of 18 M-1 cm-1, indicating a distorted octahedral complex geometry. The spectrum of the low-affinity site (A site) shows two absorption maxima at 505 nm and 586 nm with epsilon values of 170 M-1 cm-1 and 240 M-1 cm-1, respectively, which indicates a distorted tetrahedral or pentacoordinated complex structure as also observed for the enzyme from Streptomyces violaceoruber [Callens et al. (1988) Biochem. J. 250, 285-290] having the same feature but lower epsilon values. The first 4 mol Co2+ added/mol apoenzyme occupy both sites nearly equally. Subsequently the Co2+ located in the A site slowly moves into the B site. After equilibrium is reached, the next 4 mol Co2+/mol again occupy the A site with its typical spectrum, restoring full activity. Addition of 4 mol Cd2+ or Pb2+/mol Co4-loaded derivative displaces the Co2+ from the B site to form the Pb4/Co4 derivative containing Co2+ in the A site, reducing activity fourfold while the Pb4/Pb4 species is completely inactive. In contrast, Eu3+ displaces Co2+ preferentially from the A site. Thus, the high- and low-affinity sites may be different for different cations. After addition of the substrates D-xylose, D-glucose and D-fructose and the inhibitor xylitol the intense Co2+ A-site spectrum of both the active Co4/Co4 derivative and the less active Pb4/PCo4 derivative decreases, indicating that these compounds are bound to the A site, changing the distorted tetrahedral or pentacoordinated symmetry there to a distorted octahedral complex geometry.