Emanuela Gatto

Large photocurrent generation in multiwall carbon nanotubes

The authors demonstrate the ability of multiwall carbon nanotubes to generate photocurrents in the near ultraviolet and visible spectral ranges using electrochemical photocurrent measurements. The photogenerated current depends on the excitation wavelength similar to that for single wall carbon nanotubes. Its intensity and modulation can be related to the carbon nanotubes morphology. The maximum photon-to-current conversion efficiency is approximately 7%, about 50 times higher than that reported for single wall carbon nanotubes. This result is of particular relevance for photovoltaic nanodevices and solar energy conversion applications.

Tetraferrocenylporphyrins as active components of self-assembled monolayers on gold surface

Novel tetraferrocenylporphyrins-containing self-assembled monolayers were prepared employing two different approaches. Self-assembled monolayers were characterized using UV-Vis spectroscopy and cyclic voltammetry (CV) whereas their photoelectrochemical properties were investigated by photocurrent generation (PG) experiments.

Polychromatic fingerprinting of excitation emission matrices

The ability of a ubiquitous polychromatic excitation to support the fingerprinting of the Excitation Emission Matrices (EEM) of fluorescent indicators was reported. Solutions of fluorescent molecules, with concentration of 2µM for the absorption and 10µM for the emission measurements were prepared. Spectroscopic grade THF and distilled water passed through Milli-Q purification system were used as solvents. EEM measurements were carried out on a spectrofluorimeter, operating in the 390-700 nm detection range, with excitations at 20 nm intervals within the same range. During the measurement, the screen displayed a sequence of 50 colors and for every color, the transmission and emission spectrum of both the reference and the sample were recorded. Illuminating colors in the red region are not able to excite fluorescence and this can be seen in the spectra for red light.

Supramolecular chirality in solvent-promoted aggregation of amphiphilic porphyrin derivatives: kinetic studies and comparison between solution behavior and solid-state morphology by AFM topography.

The solvent-promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L-proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (nitrogen-quaternized L-proline, 3H(2)) or anionic (carboxylate residue, 6H(2)) group. Formation of anionic 6H(2) aggregates shows a diffusion-limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H(2) aggregates are formed by a cooperative, fractal-type decay, and appear as long-fibrous, looser structures. In the templated aggregation of 3H(2) over preformed 6H(2) aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter interchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid-state aggregate structures, corroborating the sergeant-soldier effect observed in the templated aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereoselective sensors and devices.

Dynamics of formation of a helix-turn-helix structure in a membrane-active peptide: a time-resolved spectroscopic study.

Trichogin GA IV (nOct-Aib-Gly-Leu-Aib-Gly-Gly-Leu-Aib-Gly-IleLol, in which nOct is n-octanoyl and Lol is leucinol) is an antimicrobial peptide member of the lipopeptaibol family, a unique group of membrane-active compounds of fungal origin, characterized by a high content of the nonproteinogenic C-disubstituted glycine Aib (a-aminoisobutyric acid). Owing to the gem-dimethyl substitution on the C atom, Aib exhibits a strong propensity to induce 310/a-helical conformations in peptides, and b-turns as well. X-ray diffraction studies on trichogin GA IV show that the first few residues at the N terminus adopt a right-handed 310-helix, while the rest of the peptide chain forms a distorted, right-handed a-helix. A NMR investigation indicates that this mixed 310/a-helical structure is basically maintained in solution, although the C-terminal region appears to be in equilibrium with disordered states. The hypothesis that the flexible -Gly5-Gly6motif gives rise to a hinge point between the two helical segments has also been advanced. Therefore, trichogin GA IV could represent an interesting model for examining an elementary step in protein folding, that is, the coalescence of a pair of helical segments to form a helix–turn–helix motif. Knowledge of the kinetics of formation of such locally structured domains would be useful for discriminating between different folding mechanisms. To this goal, we applied time-resolved optical spectroscopy techniques, from the nanosecond to the microsecond timescale, on a new analogue of trichogin GA IV with the sequence Fmoc-Aib-Gly-Leu-Aib-Gly-Gly-Leu-TOAC-Gly-Ile-Leu-OMe, denoted F0T8. F0T8 differs from trichogin GA IV by the substitution of i) the n-octanoyl lipid chain at the N terminus by Fmoc (fluoren-9-ylmethyloxycarbonyl), a well-known fluorescent group, ii) -Aib8by TOAC (4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-4-carboxylic acid), and iii) the 1,2-aminoalcohol Lol moiety group at the C terminus by Leu-OMe. Permeabilization experiments on phospholipid bilayers (Supporting Information) proved that these substitutions enhance the membrane activity of the peptide investigated. Fmoc populates singlet and triplet states with high yields, while TOAC, a stable nitroxidebased radical species, exhibits a strong quenching activity on the excited states of aromatic molecules. We have recently shown that for Fmoc···TOAC interchromophoric distances larger than 9 A, long-range dipolar interactions determine the photophysical relaxation of the interacting pair (Fçrster energy transfer), while for distances shorter than 4–5 A an electron exchange (Dexter energy transfer) mechanism largely predominates. Photophysical-quenching efficiencies were determined with respect to the reference compound Fmoc-Aib-Gly-Leu-Aib-GlyGly-Leu-Aib-Gly-Ile-Leu-OMe (denoted as F0), in which the TOAC quencher is missing. To characterize the conformational heterogeneity and the dynamical behaviour of F0T8, we carried out a time-resolved spectroscopic investigation in three solvents: i) methanol, a polar medium exhibiting a relatively low viscosity and a low competition for hydrogen bond formation; ii) glycerol (Glc) /methanol (8:2, v/v), a mixed solvent showing strong temperature-dependent viscosity that allowed us to sample the peptide dynamics over a broad range of conditions; iii) sodium dodecyl sulfate (SDS) micelles, a well-known membrane-mimicking environment that reproduces the encapsulation effect of an organized medium on the conformational heterogeneity of F0T8. The circular dichroism (CD) spectra of F0T8 in the three solvents are reported in Figure 1.

Photocurrent generation through peptide‐based self‐assembled monolayers on a gold surface: antenna and junction effects

The photocurrent generation properties of mono‐ and bi‐component peptide‐based self‐assembled monolayers (SAMs) immobilized on a gold surface were studied by electrochemical and spectroscopic techniques. The peptides investigated comprised almost exclusively Cα‐tetrasubstituted α‐amino acids. These non‐coded residues, because of their unique conformational properties, forced the peptide backbone to attain a helical conformation, as confirmed by X‐ray crystal structure and CD determinations in solution. The peptide helical structure promoted the formation of a stable SAM on the gold surface, characterized by an electric macrodipole directed from the C(δ−) to the N(δ+) terminus, that remarkably affected the electron transfer (ET) process through the peptide chain. The peptides investigated were derivatized with chromophores strongly absorbing in the UV region to enhance the efficiency of the photocurrent generation (antenna effect). The influence of the nature of the peptide–gold interface on the ET process (junction effect) was analyzed by comparing the photocurrent generation process in peptide SAMs immobilized on a gold surface through AuS linkages with that in a bi‐component SAM embedding a photoactive peptide into the linked palisade formed by disulfide‐functionalized peptides. Copyright

Optoelectronic properties in quantum-confined germanium dots

Photocurrent generation of nanometric Ge dots has been investigated by using electrochemical measurements. Photocurrent features have been ascribed, for large Ge dots, to Ge bulk direct electronic transitions at L and X points as evidenced by their close correspondence with the optical absorption coefficient. A blueshift of the photocurrent features has been detected by reducing the Ge dot size. These changes have been interpreted as due to quantum confinement effect. This result suggests that Ge dots could be applied in photovoltaic nanodevices and quantum dot based lasers.

Conformational effects on the electron-transfer efficiency in peptide foldamers based on alpha,alpha-disubstituted glycyl residues.

Peptide foldamers based on α,α‐disubstituted glycyl residues were synthesized and chemically characterized to investigate the effects of the electric field generated by a 310‐helix on the rate of intramolecular photoinduced electron‐transfer reactions. To this end, two new octapeptides having identical sequences were suitably side‐chain functionalized with the same electron‐transfer donor–acceptor pair, but inverting the position of the pair along the main chain. The electron‐transfer rate constants, measured by time‐resolved spectroscopy techniques (nanosecond transient absorption and time‐resolved fluorescence), indicated that, in the case of the 310‐helix, the electrostatic effect is significant, but smaller than that obtained for α‐helical peptides. This finding can be likely ascribed to the distortion of the H‐bond network with respect to the helical axis taking place in the former secondary structure. Overall, these results could have implications on electron‐transfer phenomena in model and biomembranes facilitated by peptaibiotics.

Mimicking Nature: A Novel Peptide-based Bio-inspired Approach for Solar Energy Conversion

A bioinspired approach is applied to photoelectric conversion devices. A 3(10)-helical hexapeptide bearing a pyrene unit is immobilized on a gold-covered TiO2 surface. The device is integrated for the first time in a dye-sensitized solar cell, exhibiting stability after several measurements. The approach could have promising applications in the field of optoelectronics.

Playing with Peptides: How to Build a Supramolecular Peptide Nanostructure by Exploiting Helix···Helix Macrodipole Interactions

A novel method to build bicomponent peptide self-assembled monolayers (SAMs) has been developed, by exploiting helix···helix macrodipole interactions. In this work, a peptide-based self-assembled monolayer composed of two helical peptides was immobilized on a gold surface. Specifically, a pyrene-containing octapeptide, devoid of any sulfur atom (A8Pyr), and a hexapeptide, functionalized at the N-terminus with (S,R) lipoic acid, for binding to gold substrates (SSA4WA) via a Au-S linkage, have been employed. Both peptides investigated attain a helical structure, because they are almost exclusively formed by strongly folding inducer C(α)-tetrasubstituted α-amino acids. We demonstrate that the two peptides generate a stable supramolecular nanostructure (a densely packed bicomponent peptide monolayer), where A8Pyr is incorporated into the SSA4WA palisade by exploiting helix···helix macrodipole interactions. The presence of both peptides on the gold surface was investigated by spectroscopic and electrochemical techniques, while the morphology of the monolayer was analyzed by ultra high-vacuum scanning tunnelling microscopy. The composition of the bicomponent SAM on the surface was studied by a combination of electrochemical and spectroscopic techniques. In particular, the amount of Au-S linkages from the sulfur-containing peptides was quantified from reductive desorption of the peptide-based SAM, while the amount of A8Pyr was estimated by fluorescence spectroscopy. The antiparallel orientation of the A8Pyr and SSA4WA peptide chains minimizes the interaction energy between the helix dipoles, suggesting that this kind of electrostatic phenomenon is the driving force that stabilizes the bicomponent SAM.