X -ray scattering studies of maquette peptide monolayers

We have applied x-ray scattering techniques to characterize the structure of Langmuir and Langmuir-Blodgett monolayers containing maquette peptides. These small, soluble, synthetic, de novo designed peptides were developed as test systems for studying electron transfer in proteins, but may also be useful as a new class of thin film materials for endowing surfaces with functionalities. In the family of maquettes under consideraton, the 31-mer unit synthesized forms an amphipathic α-helix with a flexible [special characters omitted] tether at the N-terminus. In solution, pairs of such units associate and dimerize covalently via a disulfide bond between the cysteine residues, and then further associate into 4-helix bundles. Each dihelix forms a pair of bis-His sites for binding prosthetic groups. We studied the prototype peptide of this family, H10H24, and a derivative, BBC16, palmitoylated at the N-terminus. Both peptides form monolayers at the air/water interface with a plateau in their isotherms. The more amphiphilic BBC16 forms more stable Langmuir monolayers that could be studied via synchrotron-based x-ray reflectivity at surface pressures above and below the plateau, while H10H24 could only be studied below the plateau. The data, analyzed by slab-model fitting and a model-independent refinement procedure, demonstrate that BBC16 undergoes an orientational transition as pressure increases. At low pressures, both peptides exist as dihelices parallel to the plane of the air/water interface, but at high pressure BBC16\u27s helices are approximately normal to the interface. The addition of free DLPE to BBC16 monolayers increased their stability sufficiently to allow collection of off-specular inplane scattering for long periods. The difference signal between a 2:1 DLPE:BBC16 and a pure DLPE monolayer indicates that the upright helices form a 2-dimensional glass. X-ray interferometry demonstrates that BBC16 monolayers LB deposited onto alkylated solid supports can maintain the perpendicular orientation that the peptide has in the precursor Langmuir film. H10H24 monolayers LB deposited at low pressure onto a thiol surface also have the peptide oriented perpendicular to the plane of the support, indicating that rearrangement of the molecules must occur during or after deposition. The suitability of these systems for future electrochemical and neutron scattering studies is discussed

X -ray scattering studies of maquette peptide monolayers

We have applied x-ray scattering techniques to characterize the structure of Langmuir and Langmuir-Blodgett monolayers containing maquette peptides. These small, soluble, synthetic, de novo designed peptides were developed as test systems for studying electron transfer in proteins, but may also be useful as a new class of thin film materials for endowing surfaces with functionalities. In the family of maquettes under consideraton, the 31-mer unit synthesized forms an amphipathic α-helix with a flexible [special characters omitted] tether at the N-terminus. In solution, pairs of such units associate and dimerize covalently via a disulfide bond between the cysteine residues, and then further associate into 4-helix bundles. Each dihelix forms a pair of bis-His sites for binding prosthetic groups. We studied the prototype peptide of this family, H10H24, and a derivative, BBC16, palmitoylated at the N-terminus. Both peptides form monolayers at the air/water interface with a plateau in their isotherms. The more amphiphilic BBC16 forms more stable Langmuir monolayers that could be studied via synchrotron-based x-ray reflectivity at surface pressures above and below the plateau, while H10H24 could only be studied below the plateau. The data, analyzed by slab-model fitting and a model-independent refinement procedure, demonstrate that BBC16 undergoes an orientational transition as pressure increases. At low pressures, both peptides exist as dihelices parallel to the plane of the air/water interface, but at high pressure BBC16\u27s helices are approximately normal to the interface. The addition of free DLPE to BBC16 monolayers increased their stability sufficiently to allow collection of off-specular inplane scattering for long periods. The difference signal between a 2:1 DLPE:BBC16 and a pure DLPE monolayer indicates that the upright helices form a 2-dimensional glass. X-ray interferometry demonstrates that BBC16 monolayers LB deposited onto alkylated solid supports can maintain the perpendicular orientation that the peptide has in the precursor Langmuir film. H10H24 monolayers LB deposited at low pressure onto a thiol surface also have the peptide oriented perpendicular to the plane of the support, indicating that rearrangement of the molecules must occur during or after deposition. The suitability of these systems for future electrochemical and neutron scattering studies is discussed

Reconstruction of evolving nanostructures in ultrathin films with X-ray waveguide fluorescence holography

The authors introduce X-ray waveguide fluorescence holography based on the waveguiding properties of thin films. Combined with model-independent reconstruction algorithms, they show that the method can be used for real-time nanostructure kinetic studies

A Tale of Two Transitions: Unraveling Two Distinct Polymorph Transition Mechanisms in One n-Type Single Crystal for Dynamic Electronics

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the simultaneous, concerted displacement of molecules in a crystalline material, in stark contrast to the more typical molecule-by-molecule nucleation and growth mechanism often breaking the single crystallinity. Cooperative transitions have acquired much attention in the research community for their low transition barriers, ultrafast kinetics, and structural reversibility. On the other hand, cooperative transitions are rarely observed in molecular crystals and the molecular origin is not well understood. Single crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two thermally-activated, reversible phase transitions with one exhibiting a cooperative mechanism and the second exhibiting a nucleation and growth mechanism. In situ microscopy, single crystal and grazing incidence X-ray diffraction (GIXD), along with Raman spectroscopy suggest a reorientation of the alkyl side chains results in a cooperative transition behavior. On the other hand, the nucleation and growth transition is coincident with both side chain melting and the emergence of new spin-spin interactions between conjugated cores, confirmed through in situ electron paramagnetic resonance spectroscopy (EPR). This is the first observation of biradical interactions directly initiating a structural transition. Through studying these fundamental mechanisms, we establish alkyl chain conformation and disorder as integral to rationally controlling these polymorphic behaviors for novel electronic applications

Direct evidence of conformational changes associated with voltage gating in a voltage sensor protein by time-resolved X-ray/neutron interferometry.

The voltage sensor domain (VSD) of voltage-gated cation (e.g., Na(+), K(+)) channels central to neurological signal transmission can function as a distinct module. When linked to an otherwise voltage-insensitive, ion-selective membrane pore, the VSD imparts voltage sensitivity to the channel. Proteins homologous with the VSD have recently been found to function themselves as voltage-gated proton channels or to impart voltage sensitivity to enzymes. Determining the conformational changes associated with voltage gating in the VSD itself in the absence of a pore domain thereby gains importance. We report the direct measurement of changes in the scattering-length density (SLD) profile of the VSD protein, vectorially oriented within a reconstituted phospholipid bilayer membrane, as a function of the transmembrane electric potential by time-resolved X-ray and neutron interferometry. The changes in the experimental SLD profiles for both polarizing and depolarizing potentials with respect to zero potential were found to extend over the entire length of the isolated VSD's profile structure. The characteristics of the changes observed were in qualitative agreement with molecular dynamics simulations of a related membrane system, suggesting an initial interpretation of these changes in terms of the VSD's atomic-level 3-D structure

Direct evidence of conformational changes associated with voltage gating in a voltage sensor protein by time-resolved X-ray/neutron interferometry.

The voltage sensor domain (VSD) of voltage-gated cation (e.g., Na(+), K(+)) channels central to neurological signal transmission can function as a distinct module. When linked to an otherwise voltage-insensitive, ion-selective membrane pore, the VSD imparts voltage sensitivity to the channel. Proteins homologous with the VSD have recently been found to function themselves as voltage-gated proton channels or to impart voltage sensitivity to enzymes. Determining the conformational changes associated with voltage gating in the VSD itself in the absence of a pore domain thereby gains importance. We report the direct measurement of changes in the scattering-length density (SLD) profile of the VSD protein, vectorially oriented within a reconstituted phospholipid bilayer membrane, as a function of the transmembrane electric potential by time-resolved X-ray and neutron interferometry. The changes in the experimental SLD profiles for both polarizing and depolarizing potentials with respect to zero potential were found to extend over the entire length of the isolated VSD's profile structure. The characteristics of the changes observed were in qualitative agreement with molecular dynamics simulations of a related membrane system, suggesting an initial interpretation of these changes in terms of the VSD's atomic-level 3-D structure

Molecular Origin of Photovoltaic Performance in Donor-<i>block</i>-Acceptor All-Conjugated Block Copolymers

All-conjugated block copolymers may be an effective route to self-assembled photovoltaic devices, but we lack basic information on the relationship between molecular characteristics and photovoltaic performance. Here, we synthesize a library of poly­(3-hexyl­thiophene) (P3HT) <i>block</i> poly­((9,9-dialkyl­fluorene)-2,7-diyl-<i>alt</i>-[4,7-bis­(alkyl­thiophen-5-yl)-2,1,3-benzo­thiadiazole]-2′,2″-diyl) (PFTBT) donor-<i>block</i>-acceptor all-conjugated block copolymers and carry out a comprehensive study of processing conditions, crystallinity, domain sizes, and side-chain structure on photovoltaic device performance. We find that all block copolymers studied exhibit an out-of-plane crystal orientation after deposition, and on thermal annealing at high temperatures the crystal orientation flips to an in-plane orientation. By varying processing conditions on polymer photovoltaic devices, we show that the crystal orientation has only a modest effect (15–20%) on photovoltaic performance. The addition of side chains to the PFTBT block is found to decrease photovoltaic power conversion efficiencies by at least an order of magnitude. Through grazing-incidence X-ray measurements we find that the addition of side chains to the PFTBT acceptor block results in weak segregation and small (<10 nm) block copolymer self-assembled donor and acceptor domains. This work is the most comprehensive to date on all-conjugated block copolymer systems and suggests that photovoltaic performance of block copolymers depends strongly on the miscibility of donor and acceptor blocks, which impacts donor and acceptor domain sizes and purity. Strategies for improving the device performance of block copolymer photovoltaics should seek to increase segregation between donor and acceptor polymer domains

Understanding Interfacial Alignment in Solution Coated Conjugated Polymer Thin Films

Domain alignment in conjugated polymer thin films can significantly enhance charge carrier mobility. However, the alignment mechanism during meniscus-guided solution coating remains unclear. Furthermore, interfacial alignment has been rarely studied despite its direct relevance and critical importance to charge transport. In this study, we uncover a significantly higher degree of alignment at the top interface of solution coated thin films, using a donor–acceptor conjugated polymer, poly­(diketopyrrolopyrrole-<i>co</i>-thiophene-<i>co</i>-thieno­[3,2-<i>b</i>]­thiophene-<i>co</i>-thiophene) (DPP2T-TT), as the model system. At the molecular level, we observe in-plane π–π stacking anisotropy of up to 4.8 near the top interface with the polymer backbone aligned parallel to the coating direction. The bulk of the film is only weakly aligned with the backbone oriented transverse to coating. At the mesoscale, we observe a well-defined fibril-like morphology at the top interface with the fibril long axis pointing toward the coating direction. Significantly smaller fibrils with poor orientational order are found on the bottom interface, weakly aligned orthogonal to the fibrils on the top interface. The high degree of alignment at the top interface leads to a charge transport anisotropy of up to 5.4 compared to an anisotropy close to 1 on the bottom interface. We attribute the formation of distinct interfacial morphology to the skin-layer formation associated with high Peclet number, which promotes crystallization on the top interface while suppressing it in the bulk. We further infer that the interfacial fibril alignment is driven by the extensional flow on the top interface arisen from increasing solvent evaporation rate closer to the meniscus front