Self assembly of coiled-coil peptide-porphyrin complexes.

We are interested in the controlled assembly of photoelectronic materials using peptides as scaffolds and porphyrins as the conducting material. We describe the integration of a peptide-based polymer strategy with the ability of designed basic peptides to bind anionic porphyrins in order to create regulated photoelectronically active biomaterials. We have described our peptide system in earlier work, which demonstrates the ability of a peptide to form filamentous materials made up of self-assembling coiled-coil structures. We have modified this peptide system to include lysine residues appropriately positioned to specifically bind meso-tetrakis(4-sulfonatophenyl)porphine (TPPS(4)), a porphyrin that contains four negatively charged sulfonate groups at neutral pH. We measure the binding of TPPS(4) to our peptide using UV--visible and fluorescence spectroscopies to follow the porphyrin signature. We determine the concomitant acquisition of helical secondary structure in the peptide upon TPPS(4) binding using circular dichroism spectropolarimetry. This binding fosters polymerization of the peptide, as shown by absorbance extinction effects in the peptide CD spectra. The morphologies of the peptide/porphyrin complexes, as imaged by atomic force microscopy, are consistent with the coiled-coil polymers that we had characterized earlier, except that the heights are slightly higher, consistent with porphyrin binding. Evidence for exciton coupling in the copolymers is shown by red-shifting in the UV--visible data, however, the coupling is weak based on a lack of fluorescence quenching in fluorescence experiments.

Self-assembly of peptide porphyrin complexes: toward the development of smart biomaterials.

The anionic porphyrin, meso-tetrakis(4-sulfonatophenyl)porphine, is found to tightly bind to an engineered 14-residue peptide, resulting in induced alpha-helix formation when mixed in aqueous solutions. The small porphyrin-peptide dissociation constant (2 muM) observed is related to the energetics of peptide helix formation coupled with electrostatic interactions between the anionic porphyrin and cationic residues in the coiled peptide. Analytical ultracentrifugation measurements indicate the porphyrin-peptide complexes dimerize, probably into a coiled coil, and weakly associate to form even higher order structures.

Self-assembling porphyrin-modified peptides.

[structure: see text] We report the synthesis and characterization of a novel supramolecular assembly that features long-range electronic coupling between porphyrins covalently attached to a designed peptide scaffold. The resulting construct self-assembles to form extended organized aggregates in which the porphyrins engage in exciton coupling.

Raman spectra and normal coordinate analyses of low-frequency vibrations of oxo-bridged manganese complexes.

The active sites of certain metalloenzymes involved in oxygen metabolism, such as manganese catalase and the oxygen-evolving complex of photosystem II, contain micro -oxo-bridged Mn clusters with ligands that include H(2)O and micro (1,3)-carboxylato bridges provided by protein side chains. In order to understand better the vibrational spectra of such clusters, the low-frequency resonance Raman spectra of a series of structurally characterized Mn-oxo model complexes were examined. The series includes complexes of the type [Mn(2)(O)(OAc)(2)(bpy)(2)(L)(2)] and [Mn(2)(O)(2)(OAc)(bpy)(2)(L)(2)], where bpy=2,2'-bipyridine, OAc=acetate and L=H(2)O or Cl(-). Complexes containing the isotopomers OAc- d(3) and D(2)O, as well as those containing both isotopomers, were also examined. Normal coordinate analyses (NCA) were performed on the various complexes using theGF matrix method. Selected vibrational modes in the 200-600 cm(-1) region were assigned based on the spectra and NCA, which identify vibrational modes arising from the metal-ligand bonds. These results will be useful in interpreting the vibrational spectra obtained from metalloproteins containing Mn-oxo complexes in their active sites.

Synthesis, Transient Absorption, and Transient Resonance Raman Spectroscopy of Novel Electron Donor-Acceptor Complexes: [5,15-Bis[(4'-nitrophenyl)ethynyl]-10,20- diphenylporphinato]copper(II) and [5-[[4'-(Dimethylamino)phenyl]ethynyl]-15-[(4''-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II).

We report the synthesis, transient absorption, FT Raman, resonance Raman, time-resolved resonance Raman, and transient resonance Raman spectra of pseudo-D 2h symmetric [5,15-bis[(4'-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (I) and electronically asymmetric [5-[4'-(dimethylamino)phenyl]ethynyl]-15-[(4''-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (II), which bears both electron-releasing and electron-withdrawing groups conjugated directly to the porphyrin periphery. The spectroscopic results suggest extensive electronic communication between the 5- and 15-arylethynyl groups and the porphyrin core. Relative to the parent compound, (tetraphenylporphinato)copper(II) (CuTPP), the arylethynyl substituents increase the lifetime of the excited trip-multiplet states. CuTPP, as well as compounds I and II, however, shows similar solvent-dependent dynamics: the trip-multiplet lifetimes are longer in a noncoordinating solvent such as benzene than in a coordinating solvent such as THF. This behavior is consistent with the existence of a quenching state whose effect is more pronounced upon coordination of solvent. The time-resolved resonance Raman spectrum of compound II shows features commonly associated with the relatively long-lived triplet excited states of copper(II) porphyrins. The transient resonance Raman spectrum of a short-lived excited state present in both compounds I and II is characterized by marked shifts in the nitro and porphyrin stretching frequencies relative to that observed for the ground states of both (4-nitrophenyl)ethyne and (tetraphenylporphinato)copper(II). We interpret these results for the compounds I and II as arising from (i) a short-lived excited state present at early time that possesses enhanced porphyrin-to-nitro charge-transfer character with respect to the ground state and (ii) a longer-lived excited state deriving from this initially probed charge-transfer state that is largely porphyrin localized.