Search

The aberrant fibrillization of huntingtin exon 1 (Httex1) characterized by an expanded polyglutamine (polyQ) tract is a defining feature of Huntington's disease, a neurodegenerative disorder. Recent investigations underscore the involvement of a small EDRK-rich factor 1a (SERF1a) in promoting Httex1 fibrillization through interactions with its N terminus. By establishing an integrated approach with size-exclusion-column-based small- and wide-angle X-ray scattering (SEC-SWAXS), NMR, and molecular simulations using Rosetta, the analysis here reveals a tight binding of two NT17 fragments of Httex1 (comprising the initial 17 amino acids at the N terminus) to the N-terminal region of SERF1a. In contrast, examination of the complex structure of SERF1a with a coiled NT17-polyQ peptide (33 amino acids in total) indicates sparse contacts of the NT17 and polyQ segments with the N-terminal side of SERF1a. Furthermore, the integrated SEC-SWAXS and molecular-simulation analysis suggests that the coiled NT17 segment can transform into a helical conformation when associated with a polyQ segment exhibiting high helical content. Intriguingly, NT17-polyQ peptides with enhanced secondary structures display diminished interactions with SERF1a. This insight into the conformation-dependent binding of NT17 provides clues to a catalytic association mechanism underlying SERF1a's facilitation of Httext1 fibrillization.

Abstract Peptidoglycan (PG) sacculi surround the cytoplasmic membrane, maintaining cell integrity by withstanding internal turgor pressure. During cell growth, PG endopeptidases cleave the crosslinks of the fully closed sacculi, allowing for the incorporation of new glycan strands and expansion of the peptidoglycan mesh. Outer-membrane-anchored NlpI associates with hydrolases and synthases near PG synthesis complexes, facilitating spatially close PG hydrolysis. Here, we present the structure of adaptor NlpI in complex with the endopeptidase MepS, revealing atomic details of how NlpI recruits multiple MepS molecules and subsequently influences PG expansion. NlpI binding elicits a disorder-to-order transition in the intrinsically disordered N-terminal of MepS, concomitantly promoting the dimerization of monomeric MepS. This results in the alignment of two asymmetric MepS dimers respectively located on the two opposite sides of the dimerization interface of NlpI, thus enhancing MepS activity in PG hydrolysis. Notably, the protein level of MepS is primarily modulated by the tail-specific protease Prc, which is known to interact with NlpI. The structure of the Prc-NlpI-MepS complex demonstrates that NlpI brings together MepS and Prc, leading to the efficient MepS degradation by Prc. Collectively, our results provide structural insights into the NlpI-enabled avidity effect of cellular endopeptidases and NlpI-directed MepS degradation by Prc.

Abstract The cyclic GMP-AMP synthase (cGAS)/stimulator of interferon gene (STING) signaling pathway plays a critical protective role against viral infections. Metazoan STING undergoes multilayers of regulation to ensure specific signal transduction. However, the mechanisms underlying the regulation of bacterial STING remain unclear. In this study, we determined the crystal structure of anti-parallel dimeric form of bacterial STING, which keeps itself in an inactive state by preventing cyclic dinucleotides access. Conformational transition between inactive and active states of bacterial STINGs provides an on-off switch for downstream signaling. Some bacterial STINGs living in extreme environment contain an insertion sequence, which we show codes for an additional long lid that covers the ligand-binding pocket. This lid helps regulate anti-phage activities. Furthermore, bacterial STING can bind cyclic di-AMP in a triangle-shaped conformation via a more compact ligand-binding pocket, forming spiral-shaped protofibrils and higher-order fibril filaments. Based on the differences between cyclic-dinucleotide recognition, oligomerization, and downstream activation of different bacterial STINGs, we proposed a model to explain structure-function evolution of bacterial STINGs.

Abstract Abnormal polyglutamine (polyQ) expansion and fibrillization occur in Huntington’s disease (HD). Amyloid modifier SERF enhances amyloid formation, but the underlying mechanism is not revealed. Here, the fibrillization and toxicity effect of SERF1a on Htt-exon1 are examined. SERF1a enhances the fibrillization of and interacts with mutant thioredoxin (Trx)-fused Httex1. NMR studies with Htt peptides show that TrxHttex1-39Q interacts with the helical regions in SERF1a and SERF1a preferentially interacts with the N-terminal 17 residues of Htt. Time-course analysis shows that SERF1a induces mutant TrxHttex1 to a single conformation enriched of β-sheet. Co-expression of SERF1a and Httex1-polyQ in neuroblastoma and lentiviral infection of SERF1a in HD-induced polypotent stem cell (iPSC)-derived neurons demonstrates the detrimental effect of SERF1a in HD. Higher level of SERF1a transcript or protein is detected in HD iPSC, transgenic mice, and HD plasma. Overall, this study provides molecular mechanism for SERF1a and mutant Httex1 to facilitate therapeutic development for HD.

The FtsB-FtsL-FtsQ complex is essential for regulating cell wall synthesis during bacterial cell division. Here, authors report the full-length trimeric structure showing a tilted V-shaped conformation and suggest an allosteric regulatory mechanism.

Structural and mutational analyses reveal interactions between NAD+-glycohydrolase and Streptolysin O critical for Group A Streptococcus virulence, validated in a murine infection model.

The electrostatic interaction between of poly(amidoamine) (PAMAM) G9 dendrimer and an achiral surfactant, dodecylbenzenesulfonic acid (DBSA), leads to the formation of nucleosome-like structure, where the globular dendrimer molecule directs the DBSA cylindrical micelle to wrap around it spirally with a regular pitch length of 3.2 ∼ 4.3 nm. Molecular dynamics simulation demonstrates that the spherical micelles of DBSA in the aqueous solution first condense on the dendrimer surface via the electrostatic attraction. The micelles then merge with their neighbors to form cylinders bending around the dendrimer, which further connect with one another to form a spring-like helix wrapping around the dendrimer. The discovery of the helical supramolecular structure attests the universality of soft cylindrical building block such as DNA and micelle to twist and wrap helically around the oppositely charged particle such as histone protein and dendrimer for attaining the optimal charge matching at the expense of the bending energy of the cylinder.

Epidemics caused by coronaviruses (CoVs), namely the severe acute respiratory syndrome (SARS) (2003), Middle East respiratory syndrome (MERS) (2012), and coronavirus disease 2019 (COVID-19) (2019), have triggered a global public health emergency. Drug development against CoVs is inherently arduous. The nucleocapsid (N) protein forms an oligomer and facilitates binding with the viral RNA genome, which is critical in the life cycle of the virus. In the current study, we found a potential allosteric site (Site 1) using PARS, an online allosteric site predictor, in the CoV N-N-terminal RNA-binding domain (NTD) to modulate the N protein conformation. We identified 5-hydroxyindole as the lead via molecular docking to target Site 1. We designed and synthesized four 5-hydroxyindole derivatives, named P4-1 to P4-4, based on the pose of 5-hydroxyindole in the docking model complex. Small-angle X-ray scattering (SAXS) data indicate that two 5-hydroxyindole compounds with higher hydrophobic R-groups mediate the binding between N-NTD and N-C-terminal dimerization domain (CTD) and elicit high-order oligomerization of the whole N protein. Furthermore, the crystal structures suggested that these two compounds act on this novel cavity and create a flat surface with higher hydrophobicity, which may mediate the interaction between N-NTD and N-CTD. Taken together, we discovered an allosteric binding pocket targeting small molecules that induces abnormal aggregation of the CoV N protein. These novel concepts will facilitate protein-protein interaction (PPI)-based drug design against various CoVs.

Chikungunya virus (CHIKV) is transmitted to humans through mosquitoes and causes Chikungunya fever. Nonstructural protein 2 (nsP2) contains an N-terminal RNA helicase with both nucleotide triphosphatase and RNA triphosphatase activities, and a C-terminal cysteine protease that is responsible for polyprotein processing. Both N-terminal RNA helicase and C-terminal cysteine protease are connected through a flexible linker. Although the structure of the C-terminal cysteine protease has been solved, the structure and the conformational arrangement of full-length nsP2 remains elusive. Here, we determined the crystal structure of the helicase part of the CHIKV nsP2 (nsP2h) bound to the conserved 3′-end of the genomic RNA and the nucleotide analogue ADP-AlF4. The structure of this ternary complex revealed the molecular basis for viral RNA recognition and ATP hydrolysis by the nsP2h. Unique hydrophobic protein–RNA interactions play essential roles in viral RNA replication. We also determined the solution structure of full-length nsP2 using small-angle X-ray scattering (SAXS). The solution architecture of the nsP2 was modeled using the available high-resolution structures and program CORAL (complexes with random loops). The CORAL model revealed that nsP2 is partially unfolded and the N-terminal protease domain is arranged near the N-terminal domain of the helicase domain. These findings expand our knowledge of CHIKV and related alphaviruses and might also have broad implications for antiviral and vaccine developments against pathogenic alphaviruses.

Melting of native tapioca starch granules in aqueous pastes upon heating is observed in situ using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) and solution viscometry. Correlated structure and viscosity changes suggest closely associated amylose and amylopectin chains in the semicrystalline layers, and the release of amylose chains for enhanced solution viscosity occurs largely after melting of the semicrystalline structure. Before melting, WAXS results reveal mixed crystals of A- and B-types (∼4:1 by weight), whereas SAXS results indicate that the semicrystalline layers are composed of lamellar blocklets of ca 43 nm domain size, with polydisperse crystalline (≃7.5 nm) and amorphous (≃1.1 nm) layers alternatively assembled into a lamellar spacing of ≃8.6 nm with 20% polydispersity. Upon melting, the semicrystalline lamellae disintegrate into disperse and molten amylopectin nanoclusters with dissolved and partially untangled amylose chains in the aqueous matrix which leads to increased solution viscosity. During subsequent cooling, gelation starts at around 347 K; successively increased solution viscosity coincides with the development of nanocluster aggregation to a fractal dimension ≃2.3 at 303 K, signifying increasing intercluster association through collapsed amylose chains owing to decreased solvency of the aqueous medium with decreasing temperature.

A self-assembly template with a photoreduction agent embedded along mesostructured silicate-surfactant channels is prepared at the air-water interface. The unique template allows uniform and continuous uptake of metallic precursors into the silicate-surfactant channels from the solution subphase. Upon UV irradiation, the metallic precursors result in near-monodisperse nanoparticles homogeneously arrayed in the mesostructured polyoxometalated-silicate-surfactant (PSS) channels until saturation. With three dimensionally densely packed Au NPs of ca. 1.7 nm in the PSS template, hence a very high number density of surface-enhanced Raman scattering (SERS) hot-spots and large absorptive volume, we show that impressive SERS can be measured with 4-nitrothiophenol adsorbed to Au-NPs@PSS.

Group A streptococcus (GAS, Streptococcus pyogenes) is a strict human pathogen that causes severe, invasive diseases. GAS does not produce catalase, but has an ability to resist killing by reactive oxygen species (ROS) through novel mechanisms. The peroxide response regulator (PerR), a member of ferric uptake regulator (Fur) family, plays a key role for GAS to cope with oxidative stress by regulating the expression of multiple genes. Our previous studies have found that expression of an iron-binding protein, Dpr, is under the direct control of PerR. To elucidate the molecular interactions of PerR with its cognate promoter, we have carried out structural studies on PerR and PerR-DNA complex. By combining crystallography and small-angle X-ray scattering (SAXS), we confirmed that the determined PerR crystal structure reflects its conformation in solution. Through mutagenesis and biochemical analysis, we have identified DNA-binding residues suggesting that PerR binds to the dpr promoter at the per box through a winged-helix motif. Furthermore, we have performed SAXS analysis and resolved the molecular architecture of PerR-DNA complex, in which two 30 bp DNA fragments wrap around two PerR homodimers by interacting with the adjacent positively-charged winged-helix motifs. Overall, we provide structural insights into molecular recognition of DNA by PerR and define the hollow structural arrangement of PerR-30bpDNA complex, which displays a unique topology distinct from currently proposed DNA-binding models for Fur family regulators.

Human Cep57 is a coiled-coil scaffold at the pericentriolar matrix (PCM), controlling centriole duplication and centrosome maturation for faithful cell division. Genetic truncation mutations of Cep57 are associated with the mosaic-variegated aneuploidy (MVA) syndrome. During interphase, Cep57 forms a complex with Cep63 and Cep152, serving as regulators for centrosome maturation. However, the molecular interplay of Cep57 with these essential scaffolding proteins remains unclear. Here, we demonstrate that Cep57 undergoes liquid-liquid phase separation (LLPS) driven by three critical domains (NTD, CTD, and polybasic LMN). In vitro Cep57 condensates catalyze microtubule nucleation via the LMN motif-mediated tubulin concentration. In cells, the LMN motif is required for centrosomal microtubule aster formation. Moreover, Cep63 restricts Cep57 assembly, expansion, and microtubule polymerization activity. Overexpression of competitive constructs for multivalent interactions, including an MVA mutation, leads to excessive centrosome duplication. In Cep57-depleted cells, self-assembly mutants failed to rescue centriole disengagement and PCM disorganization. Thus, Cep57's multivalent interactions are pivotal for maintaining the accurate structural and functional integrity of human centrosomes. [ABSTRACT FROM AUTHOR]

In this work, we present in depth structural investigations of nanocomposite ZnO: Ge thin films by utilizing a state of the art grazing incidence small angle x-ray spectroscopy (GISAXS) technique. The samples have been deposited by sequential r.f. and d.c. sputtering of ZnO and Ge thin film layers, respectively, on single crystal Si(100) substrates. Transformation of Ge layers into Ge nanoparticles (Ge-np) has been initiated by ex-situ rapid thermal annealing of asprepared thin film samples at 600 °C for 30, 60, and 90 s under forming gas atmosphere. A special attention has been paid on the effects of reactive and nonreactive growth of ZnO layers on the structural evolution of Ge-np. GISAXS analyses have been performed via cylindrical and spherical form factor calculations for different nanostructure types. Variations of the size, shape, and distributions of both ZnO and Ge nanostructures have been determined. It has been realized that GISAXS results are not only remarkably consistent with the electron microscopy observations but also provide additional information on the large scale size and shape distribution of the nanostructured components. [ABSTRACT FROM AUTHOR]

Water's behavior near hydrophobic surfaces has attracted great attention due to chemical and geological applications. Here, we report small angle X-ray scattering (SAXS) studies of water confined in the hydrophobic nanoporous carbon material, CMK-1-14, from ambient to deeply cooled temperatures. By monitoring the scattering intensity of the first Bragg peak, which is directly related to the scattering length density contrast between the carbon matrix and the confined water, the average density of the hydrophobically confined water was determined from 300 K to 150 K at ambient pressure. Furthermore, differential scanning calorimetry and X-ray diffraction measurements showed that the majority of such hydrophobically confined water did not crystallize in the investigated temperature range. By exploiting the fast speed of SAXS measurements and the continuous temperature ramping, the average density profile and the deduced thermal expansion coefficient (αp) were obtained. We found that the well-known density maximum of water at 277 K downshifted to 260 K, and the density minimum which has been observed in hydrophilic confinement disappeared. In addition, the previously measured large density decreasing of 18% at low temperature was recalibrated to a more reasonable 10% instead. Consequently, the recalculated αp peak was found to be quite similar to that of the water confined in hydrophilic MCM-41-S-15 suggesting an intrinsic property of water, which does not sensitively depend on the confinement surface. [ABSTRACT FROM AUTHOR]

Room temperature (RT) hydrogen uptake via bridged spillover is governed by the imperfect lattice structure and pore network in real metal-organic framework (MOF) crystals. The enhancement in RT hydrogen storage via bridged spillover has depended on the development of new MOF materials and also on the control of the lattice defects and pore network structure associated with the MOF crystal, which is optimized by synthesis conditions.

The article discusses a simple route for the synthesis of high quality polyaniline films, where in the polyaniline chains are deaggregated in the presence of phenol to form films. The conductivity of the films prepared by this method is shown to be higher than the films prepared by other methods.

The various local and global structural changes of cytochrome c induced by urea through an unfolding process are studied. The analysis reveals that the native, as well as the unfolded protein exhibit the same preedge features.

Hexanucleotide expansions, GGGGCC, in the non-coding regions of the C9orf72 gene were found in major frontotemporal lobar dementia and amyotrophic lateral sclerosis patients (C9FTD/ALS). In addition to possible RNA toxicity, several dipeptide repeats (DPRs) are translated through repeat-associated non-ATG-initiated translation. The DPRs, including poly(GA), poly(GR), poly(GP), poly(PR), and poly(PA), were found in the brains and spinal cords of C9FTD/ALS patients. Among the DPRs, poly(GA) is highly susceptible to form cytoplasmic inclusions, which is a characteristic of C9FTD/ALS. To elucidate DPR aggregation, we used synthetic (GA)15 DPR as a model system to examine the aggregation and structural properties in vitro. We found that (GA)15 with 15 repeats fibrillates rapidly and ultimately forms flat, ribbon-type fibrils evidenced by transmission electron microscopy and atomic force microscopy. The fibrils are capable of amyloid dye binding and contain a characteristic cross-β sheet structure, as revealed by x-ray scattering. Furthermore, using neuroblastoma cells, we demonstrated the neurotoxicity and cell-to-cell transmission property of (GA)15 DPR. Overall, our results show the structural and toxicity properties ofGADPR to facilitate future DPR-related therapeutic development. [ABSTRACT FROM AUTHOR]

We have investigated bismuth ferrite nanoparticles (~75 nm and ~155 nm) synthesized by a chemical method, using soft X-ray (1253.6 eV) and hard X-ray (3500, 5500, and 7500 eV) photo-electron spectroscopy. This provided an evidence for the variation of chemical state of bismuth in crystalline, phase pure nanoparticles. X-ray photoelectron spectroscopy analysis using Mg Kα (1253.6 eV) source showed that iron and bismuth were present in both Fe3+ and Bi3+ valence states as expected for bismuth ferrite. However, hard X-ray photoelectron spectroscopy analysis of the bismuth ferrite nanoparticles using variable photon energies unexpectedly showed the presence of Bi0 valence state below the surface region, indicating that bismuth ferrite nanoparticles are chemi-cally inhomogeneous in the radial direction. Consistently, small-angle X-ray scattering reveals a core-shell structure for these radial inhomogeneous nanoparticles [ABSTRACT FROM AUTHOR]

Biodegradable elastomers in the form of polyurethane nanoparticles (NPs) were successfully synthesized based on the combinations of two hydrolysis-prone polyester diols by a green water-based process. The anionic nature of the polymers successfully rendered polyurethane NPs (30-50 nm) consisting of approximately 200-300 polymer chains. The mechanical properties and degradation rate could be adjusted by the types and ratios of the component oligodiols in the soft segment. We demonstrated the feasibility using these biodegradable NPs as building blocks to generate self-assembled morphologies in nanometric, micrometric, or bulk scale, bearing excellent elasticity and biocompatibility. The elastic NPs and their various assembled forms represent a series of smart biodegradable elastomers with potential medical applications. [ABSTRACT FROM AUTHOR]

The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (.6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c]1,2,5 thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain. [ABSTRACT FROM AUTHOR]

Interleukin (IL)-33 is an important member of the IL-1 family that has pleiotropic activities in innate and adaptive immune responses in host defense and disease. It signals through its ligand-binding primary receptor ST2 and IL-1 receptor accessory protein (IL-1RAcP), both of which are members of the IL-1 receptor family. To clarify the interaction of IL-33 with its receptors, we determined the crystal structure of IL-33 in complex with the ectodomain of ST2 at a resolution of 3.27 Å. Coupled with structure-based mutagenesis and binding assay, the structural results define the molecular mechanism by which ST2 specifically recognizes IL-33. Structural comparison with other ligand–receptor complexes in the IL-1 family indicates that surface-charge complementarity is critical in determining ligand-binding specificity of IL-1 primary receptors. Combined crystallography and small-angle X-ray–scattering studies reveal that ST2 possesses hinge flexibility between the D3 domain and D1D2 module, whereas IL-1RAcP exhibits a rigid conformation in the unbound state in solution. The molecular flexibility of ST2 provides structural insights into domain-level conformational change of IL-1 primary receptors upon ligand binding, and the rigidity of IL-1RAcP explains its inability to bind ligands directly. The solution architecture of IL-33–ST2–IL-1RAcP complex from small-angle X-ray–scattering analysis resembles IL-1β–IL-1RII–IL-1RAcP and IL-1β–IL-1RI–IL-1RAcP crystal structures. The collective results confer IL-33 structure–function relationships, supporting and extending a general model for ligand–receptor assembly and activation in the IL-1 family. [ABSTRACT FROM AUTHOR]

The interaction between a polystyrene-b-polybutadiene-b-polystyrene (SBS) porogen and a low-k methylsilsesquioxane (MSQ) matrix under different curing profiles and their impact on porogen size were studied by grazing incidence small-angle X-ray scattering, viscosity measurement, and Fourier transform infrared analysis. For slow curing, significant porogen diffusion and aggregation occurred between 100 and 170°C, at which the porogen size increased from 12 to 32 nm. The interaction mechanism between porogen and the MSQ matrix and its correlation with porogen size during the thermal cure process were elucidated. The aggregation of the SBS porogen was greatly influenced by the microstructure of the MSQ matrix at three controlling temperatures; namely, (1) glass transition temperature, Tg (~100°C), (2) onset temperature (160°C) for transforming cage to network structure, and (3) immobilization temperature (170°C). In contrast, a rapid curing rate enabled the formation of small porogen size (~12 nm) in the hybrid films and yielded better mechanical strength. [ABSTRACT FROM AUTHOR]

At the National Synchrotron Radiation Research Center, a small/wide-angle X-ray scattering (SAXS/WAXS) instrument has been installed at the BL23A beamline with a superconducting wiggler insertion device. This beamline is equipped with double Si(111) crystal and double Mo/B4C multilayer monochromators, and an Si-based plane mirror that can selectively deflect the beam downwards for grazing-incidence SAXS (GISAXS) studies of air-liquid or liquid-liquid interfaces. The SAXS/WAXS instrument, situated in an experimental hutch, comprises collimation, sample and post-sample stages. Pinholes and slits have been incorporated into the beam collimation system spanning a distance of ∼5 m. The sample stage can accommodate various sample geometries for air-liquid interfaces, thin films, and solution and solid samples. The post-sample section consists of a 1 m WAXS section with two linear gas detectors, a vacuum bellows (1-4 m), a two-beamstop system and the SAXS detector system, all situated on a motorized optical bench for motion in six degrees of freedom. In particular, the vacuum bellows of a large inner diameter (260 mm) provides continuous changes of the sample-to-detector distance under vacuum. Synchronized SAXS and WAXS measurements are realized via a data-acquisition protocol that can integrate the two linear gas detectors for WAXS and the area detector for SAXS (gas type or Marl65 CCD); the protocol also incorporates sample changing and temperature control for programmable data collection. The performance of the instrument is illustrated via several different measurements, including (1) simultaneous SAXS/WAXS and differential scanning calorimetry for polymer crystallization, (2) structural evolution with a large ordering spacing of ∼250 nm in a supramolecular complex, (3) SAXS for polymer blends under in situ drawing, (4) SAXS and anomalous SAXS for unilamellar lipid vesicles and metalloprotein solutions, (5) anomalous GISAXS for oriented membranes of Br-labeled lipids embedded with peptides, and (6) GISAXS for silicate films formed in situ at the air-water interface. [ABSTRACT FROM AUTHOR]

In this study, we used simultaneous synchrotron grazing incidence X-ray scattering and diffraction to elucidate the overall morphologies of bulk heterojunction (BHJ) thin film (ca. 100 nm) solar cells containing phase-separated poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) domains. Specifically, the dimensions and orientation of the P3HT crystallites and the sizes of the PCBM aggregates in BHJ thin films were determined. The appropriate PCBM aggregate size and density required for an optimum performance of the film in the photovoltaics device resulted in deteriorated ordering in the out-of-plane direction, but improved the in-plane packing of the P3HT lamellae. When the P3HT crystallites and PCBM aggregates had comparable domain sizes and number densities, the interpercolated networks for electron- and hole-transport were optimized in the film. This new understanding of the underlying mechanism of carrier mobility in BHJ thin films might be crucial in improving the efficiency of future solar cells. [ABSTRACT FROM AUTHOR]