A pH-Induced Reversible Conformational Switch Able to Control the Photocurrent Efficiency in a Peptide Supramolecular System.

External stimuli are potent tools that Nature uses to control protein function and activity. For instance, during viral entry and exit, pH variations are known to trigger large protein conformational changes. In Nature, also the electron transfer (ET) properties of ET proteins are influenced by pH-induced conformational changes. In this work, a pH-controlled, reversible 310 -helix to α-helix conversion (from acidic to highly basic pH values and vice versa) of a peptide supramolecular system built on a gold surface is described. The effect of pH on the ability of the peptide SAM to generate a photocurrent was investigated, with particular focus on the effect of the pH-induced conformational change on photocurrent efficiency. The films were characterized by electrochemical and spectroscopic techniques, and were found to be very stable over time, also in contact with a solution. They were also able to generate current under illumination, with an efficiency that is the highest recorded so far with biomolecular systems.

The Self-Aggregation of Porphyrins with Multiple Chiral Centers in Organic/Aqueous Media: The Case of Sugar- and Steroid-Porphyrin Conjugates.

An overview of the solvent-driven aggregation of a series of chiral porphyrin derivatives studied by optical methods (UV/Vis, fluorescence, CD and RLS spectroscopies) is herein reported. The investigated porphyrins are characterized by the presence in the meso-positions of glycol-, steroidal- and glucosteroidal moieties, conferring amphiphilicity and solubility in aqueous media to the primarily hydrophobic porphyrin platform. Aggregation of the macrocycles is driven by a change in bulk solvent composition, forming architectures with supramolecular chirality, steered by the stereogenic centers on the porphyrin peripheral positions. The aggregation behavior and chiroptical properties of the final aggregated species strongly depend on the number and stereogenicity of the ancillary groups that dictate the mutual spatial arrangement of the porphyrin chromophores and their further organization in larger structures, usually detectable by different microscopies, such as AFM and SEM. Kinetic studies are fundamental to understand the aggregation mechanism, which is frequently found to be dependent on the substrate concentration. Additionally, Molecular Mechanics calculations can give insights into the intimate nature of the driving forces governing the self-assembly process. The critical use of these combined methods can shed light on the overall self-assembly process of chirally-functionalized macrocycles, with important implications on the development of chiral porphyrin-based materials.

DPPTE Thiolipid Self-Assembled Monolayer: A Critical Assay.

Supported lipid membranes represent an elegant way to design a fluid interface able to mimic the physicochemical properties of biological membranes, with potential biotechnological applications. In this work, a diacyl phospholipid, the 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE), functionalized with a thiol group, was immobilized on a gold surface. In this molecule, the thiol group, responsible for the Au-S bond (45 kJ/mol) is located on the phospholipid polar head, letting the hydrophobic chain protrude from the film. This system is widely used in the literature but is no less challenging, since its characterization is not complete, as several discordant data have been obtained. In this work, the film was characterized by cyclic voltammetry blocking experiments, to verify the SAM formation, and by reductive desorption measurements, to estimate the molecular density of DPPTE on the gold surface. This value has been compared to that obtained by quartz crystal microbalance measurements. Ellipsometry and impedance spectroscopy measurements have been performed to obtain information about the monolayer thickness and capacitance. The film morphology was investigated by atomic force microscopy. Finally, Monte Carlo simulations were carried out, in order to gain molecular information about the morphologies of the DPPTE SAM and compare them to the experimental results. We demonstrate that DPPTE molecules, incubated 18 h below the phase transition temperature (T = 41.1 ± 0.4 °C) in ethanol solution, are able to form a self-assembled monolayer on the gold surface, with domain structures of different order, which have never been reported before. Our results make possible rationalization of the scattered results so far obtained on this system, giving a new insight into the formation of phospholipids SAMs on a gold surface.

Tuning the chiroptical and morphological properties of steroidal-porphyrin aggregates: a mechanistic, structural, and MM investigation.

The aggregation of a steroid-functionalised porphyrin derivative occurs with the formation of J-type chiral species. Spectroscopic and SEM studies indicate that the initial concentration of the macrocycle strongly influences the morphology of the final mesoscopic structures, as a consequence of a change in the mechanistic course of the self-assembly process. Fibrillar structures are obtained at low porphyrin concentration, whereas aggregates of globular shapes are formed on increasing the substrate concentration. Molecular mechanics investigations gave insights into the intimate nature of the driving forces that govern the self-assembly process, pointing out the importance of ring distortion, of intramolecular steroidal OH-π hydrogen bonds, as well as dispersion forces among the tetrapyrrolic platforms.

Influence of natural additives on the properties of a milk-based compostable bioplastic.

The ongoing revolution in the plastic sector is the use of renewable and compostable materials obtained from biomass. However, their mechanical strength and thermal stability are generally not sufficient for practical applications. This study investigates the influence of natural additives on the physical-mechanical properties of a new biobased compostable bioplastic, SP-Milk®, produced from milk scraps. To provide this matrix the appropriate mechanical and thermal properties for daily use while leaving its compostability unchanged, the effect of incorporating vegetal fibres and organic particulates into the bulk bioplastic was investigated. Mechanical tests showed that fibres with a length of 2 mm are capable of increasing ductility by up to 97% compared with the original matrix, whereas fibres with a length of 10 mm led to a more effective reinforcement due to the residual resistance effect, increasing the final compressive strain from 20% (original matrix) to 70.9%. The addition of particulate yielded a harder and more resistant material, and the elastic modulus increased by 21%, although with loss of ductility, compared to SP-Milk® alone. The combination of fibres and particles resulted in the preservation of the positive effects of both components, showing a higher elastic modulus (240 ± 20 MPa, compared to 199 ± 12 MPa for the matrix), higher ductility (+50%) and higher strain at failure (+30%), compared with the matrix. Excellent compatibility between the polymeric matrix and both the fibres and the granules was confirmed using scanning electron microscopy. The thermal analysis demonstrated improved thermal stability particularly because of the effect of the combination of granules and fibres. The results validate that natural reinforcement agents are effective and ecologically advantageous.

Looking for the peptide 2.0(5) -helix: a solvent- and main-chain length-dependent conformational switch probed by electron transfer across c(α,α) -diethylglycine homo-oligomers.

The elusive, multiple fully extended (2.0(5) -helix) peptide conformation was searched with a series of C(α,α) -diethylglycine homo-oligomers (n = 1 to 5) functionalized by an electron transfer (ET) donor···acceptor (D···A) pair in acetonitrile and chloroform solutions. In the former solvent, all peptides investigated were shown to populate the 3(10) -helix conformation, whereas in chloroform the two shortest members of the series (n = 1 and n = 2) adopt predominantly the 2.0(5) -helix. Interestingly, for the longest components (n = 3 to n = 5) in this latter solvent, an equilibrium between the 2.0(5) - and 3(10) -helices takes place, the latter conformation becoming progressively more populated as the peptide main-chain length increases. Time-resolved fluorescence (TRF) experiments and molecular mechanics (MM) calculations were used in a combined approach to analyze the ET efficiencies and to associate a specific conformer (from MM) to an experimentally determined ET rate constant (from TRF). Therefore, because of the high sensitivity of the ET process to the D···A distance, ET can be used as a kinetic spectroscopic ruler, allowing for the characterization of the transition from a pure 3(10) -helix conformation to a 2.0(5) -/3(10) -helix equilibrium for the longest Deg homo-peptides of this series upon changing the solvent from acetonitrile to chloroform. To our knowledge, this is the first time that the electronic coupling factor ß for ET across a peptide chain in the 2.0(5) -helix conformation is provided.

Non-Conventional Peptide Self-Assembly into a Conductive Supramolecular Rope.

Structures composed of alternating α and ß amino acids can give rise to peculiar secondary structural motifs, which could self-assemble into complex structures of controlled geometries. This work describes the self-assembly properties of an α,ß-peptide, containing three units of syn H2-(2-F-Phe)-h-PheGly-OH, able to self-organize on surfaces into a fascinating supramolecular rope. This material was characterized by AFM, electronic conduction and fluorescence measurements. Molecular dynamics simulations showed that this hexapeptide can self-assemble into an antiparallel ß-sheet layer, stabilized by intermolecular H-bonds, which, in turn, can self-assemble into many side-by-side layers, due to π-π interactions. As a matter of fact, we demonstrated that in this system, the presence of aromatic residues at the intramolecular interface promoted by the alternation of α,ß-amino-acids in the primary sequence, endorses the formation of a super-secondary structure where the aromatic groups are close to each other, conferring to the system good electron conduction properties. This work demonstrates the capability and future potential of designing and fabricating distinctive nanostructures and efficient bioelectronic interfaces based on an α,ß-peptide, by controlling structure and interaction processes beyond those obtained with α- or ß-peptides alone.

Design and studies of novel polyoxysterol-based porphyrin conjugates.

New types of steroid-porphyrin conjugates derived from 20-hydroxyecdysone (20E) and 24-epibrassinolide (EBl) were synthesized. An exceptional regioselectivity in the reaction of both steroids with porphyrin boronic acids was found to give side-chain-conjugated boronic esters as sole products. UV-Vis-, fluorescence and NMR spectroscopy yielded similar data for all the studied compounds confirming the solvent driven supramolecular assembly with formation of J-aggregates. CD measurements of water diluted solutions showed a clear difference between 20E and EBl conjugates. The latter showed a strong supramolecular chirality, whereas 20E J-aggregates did not.