Browsing by Author "Qian, Shuo"

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    Amphiphilic Bottlebrush Block Copolymers: Analysis of Aqueous Self-Assembly by Small-Angle Neutron Scattering and Surface Tension Measurements
    (American Chemical Society, 2019) Alaboalirat, Mohammed; Qi, Luqing; Arrington, Kyle J.; Qian, Shuo; Keum, Jong K.; Mei, Hao; Littrell, Kenneth C.; Sumpter, Bobby G.; Carrillo, Jan-Michael Y.; Verduzco, Rafael; Matson, John B.
    A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs) containing polystyrene and poly(N-acryloylmorpholine) (PACMO) side chains were prepared by a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization, and ring-opening metathesis polymerization (ROMP). The grafting-through method used to prepare the polymers enabled a high degree of control over backbone and side-chain molar masses for each block. Surface tension measurements on the self-assembled amphiphilic bottlebrush BCPs in water revealed an ultralow critical micelle concentration (cmc), 1–2 orders of magnitude lower than linear BCP analogues on a molar basis, even for micelles with >90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting of small-angle neutron scattering traces (SANS) allowed us to evaluate solution conformations for individual bottlebrush BCPs and micellar nanostructures for self-assembled macromolecules. Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-d8, which is a good solvent for both blocks, reflecting an extended conformation for the PACMO block. SANS traces of bottlebrush BCPs assembled into micelles in D2O, a selective solvent for PACMO, were fitted to a core–shell–shell model, suggesting the presence of a partially hydrated inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters ranging from 16 to 18 nm. A general trend of increased stability of micelles (i.e., resistance to precipitation) was observed with increases in PACMO content. These results demonstrate the stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with ultralow (<70 nmol/L) cmc’s across a broad range of compositions.
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    Structural determination of Enzyme-Graphene Nanocomposite Sensor Material
    (Springer Nature, 2019) Rai, Durgesh K.; Gurusaran, Manickam; Urban, Volker; Aran, Kiana; Ma, Lulu; Qian, Shuo; Narayanan, Tharangattu N.; Ajayan, Pulickel M.; Liepmann, Dorian; Sekar, Kanagaraj; Álvarez-Cao, María-Efigenia; Escuder-Rodríguez, Juan-José; Cerdán, María-Esperanza; González-Siso, María-Isabel; Viswanathan, Sowmya; Paulmurugan, Ramasamy; Renugopalakrishnan, Venkatesan; Li, Pingzuo
    State-of-the-art ultra-sensitive blood glucose-monitoring biosensors, based on glucose oxidase (GOx) covalently linked to a single layer graphene (SLG), will be a valuable next generation diagnostic tool for personal glycemic level management. We report here our observations of sensor matrix structure obtained using a multi-physics approach towards analysis of small-angle neutron scattering (SANS) on graphene-based biosensor functionalized with GOx under different pH conditions for various hierarchical GOx assemblies within SLG. We developed a methodology to separately extract the average shape of GOx molecules within the hierarchical assemblies. The modeling is able to resolve differences in the average GOx dimer structure and shows that treatment under different pH conditions lead to differences within the GOx at the dimer contact region with SLG. The coupling of different analysis methods and modeling approaches we developed in this study provides a universal approach to obtain detailed structural quantifications, for establishing robust structure-property relationships. This is an essential step to obtain an insight into the structure and function of the GOx-SLG interface for optimizing sensor performance.
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    Structure and mechanism of peptide-induced membrane pores
    (2009) Qian, Shuo; Huang, Huey W.
    This thesis reports the studies of the structure and mechanism of peptide-induced membrane pores by antimicrobial peptide alamethicin and by a peptide named Baxalpha5, which is derived from Bax protein. Alamethicin is one of best known antimicrobial peptides, which are ubiquitous throughout the biological world. Bax-alpha5 peptide is the pore-forming domain of apoptosis regulator protein Bax, which activates pore formation on outer mitochondrial membrane to release cytochrome c to initiate programmed cell death. Both peptides as well as many other pore-forming peptides, induce pores in membrane, however the structure and mechanism of the pore formation were unknown. By utilizing grazing angle x-ray diffraction, I was able to reconstruct the electron density profile of the membrane pores induced by both peptides. The fully hydrated multiple bilayers of peptide-lipid mixture on solid substrate were prepared in the condition that pores were present, as established previously by neutron in-plane scattering and oriented circular dichroism. At dehydrated conditions, the inter bilayer distance of the sample shortened and the interactions between bilayers caused the membrane pores to become long-ranged correlated and formed a periodically ordered lattice of rhombohedral symmetry, so that x-ray diffraction can be applied. To help solving the phase problem of diffraction, a brominated lipid was used and multi-wavelength anomalous diffraction was performed below the bromine K-edge. The reconstructed electron density profiles unambiguously revealed that the alamethicin-induced membrane pore is of barrel-stave type, while the Bax-alpha5 induced pore is of lipidic toroidal (wormhole) type. The underlying mechanism of pore formation was resolved by observing the time-dependent process of pore formation in vesicles exposed to Bax-alpha5 solutions, as well as the membrane thinning experiment. This demonstrated that Bax-alpha5 exhibited the same sigmoidal concentration dependence as alamethicin: below a threshold concentration, peptide only binds to membrane surface, and thins the membrane; when the concentration exceeds a critical value, pores are formed. The structure and mechanism of peptide-induced membrane pores provide insight onto how alpha-pore-forming proteins and peptides interact with membrane. The results also suggest that formation of lipidic pores is the major mechanism for alpha-pore-forming proteins.