To investigate protein folding dynamics in terms of compactness, we developed a continuous-flow mixing device to make smallangle x-ray scattering measurements with the time resolution of 160 s and characterized the radius of gyration (Rg) of two folding intermediates of cytochrome c (cyt c). The early intermediate possesses Ϸ20 Å of R g, which is smaller by Ϸ4 Å than that of the acid-unfolded state. The R g of the later intermediate is Ϸ18 Å, which is close to that of the molten globule state. Considering the ␣-helix content ( fH) of the intermediates, we clarified the folding pathway of cyt c on the conformational landscape defined by R g and fH. Cyt c folding proceeds with a collapse around a specific region of the protein followed by a cooperative acquisition of secondary structures and compactness. P roteins are unique heteropolymers that possess a remarkable property to fold quickly to compact and specific conformations. Interiors of proteins are densely packed with minimum void volumes (1), indicating the specific interresidue contacts that determine secondary and tertiary structures. The compactness is therefore an essential property of the folded conformations of proteins; however, the dynamics of compaction in the process of protein folding from the extended random-coil structures are still poorly understood. Two classical models of protein folding, the hydrophobic collapse (2) and the framework models (3), suppose that proteins acquire compactness in a distinct dynamical process separated from the secondary structure formations. In the hydrophobic collapse model (2), a nonspecific collapse of protein main chain is hypothesized to trigger the tertiary and secondary structure formations. In contrast, the framework model assumes that the initial formation of secondary structures urges the subsequent folding into compact conformations (3). Recent theoretical investigations, however, suggest that main-chain collapse and secondary structure formation are mostly concerted (4). Although the different equilibrium conformations of a certain protein indicate a linear correlation between the secondary structure content and compactness (5, 6), the relationship has not been confirmed directly for kinetic folding intermediates of proteins. Experimental investigations on the protein folding dynamics in terms of compactness are urgently needed to differentiate these models, that is, to understand how unfolded proteins kinetically explore for the native states on the conformational landscape defined by compactness and secondary structure content.Cytochrome c (cyt c) is a globular protein of 104 aa, whose folding dynamics has been the subject of extensive experimental investigations (7-21). A heme group is covalently connected to the main chain (22) and surrounded by the three major helices called N-terminal residues 6-14), C-terminal (residues 87-102), and 60's helices (residues 60-69). The time-resolved circular dichroism (CD) measurement on the folding process of cyt c clarified the stepwise formation of these helices...

No abstract

No abstract

No abstract

Self-consistent field theory predicts that the complex phase behavior of block copolymers does not originate solely from the interface seeking constant mean curvature as once thought, but instead reflects competing minimization of interfacial tension and packing frustration. To test this prediction, we directly measure interfacial curvature distributions from a 3D image reconstruction of the bicontinuous gyroid morphology. Results obtained here reveal that the gyroid interface is not constant mean curvature and confirm the importance of packing frustration in the stabilization of such complex nanostructures. PACS numbers: 68.35.Ct, 42.30.Wb, 47.20.Hw, 83.70.Hq Block copolymers exhibit periodic nanostructures due to immiscibility between the dissimilar (A and B) sequences [1]. Classical block copolymer nanostructures include spheres of A(B) on a body-centered cubic lattice in a B(A) matrix, cylinders of A(B) on a hexagonal lattice in a B(A) matrix, and coalternating lamellae. Of considerable recent interest are several complex (bicontinuous) nanostructures-the perforated lamellar (PL), gyroid (G), and double-diamond (D) morphologies [2][3][4][5][6][7]. These nanostructures may develop if the copolymer composition ( f) falls within a narrow range between the cylindrical and lamellar morphologies, and can be difficult to distinguish experimentally. Block copolymer nanostructures once believed [2] to be D, exemplified by a Schwarz D surface with Pn3m symmetry, have been reclassified [8] on the basis of their small-angle X-ray scattering (SAXS) signatures as G, which is represented by the Schoen G surface with Ia3d symmetry. Identification of complex nanostructures by transmission electron microscopy (TEM) is often inconclusive, since they appear identical along several projection axes.Complex nanostructures also develop in surfactant and lipid systems due to the formation of surfaces with constant mean curvature (CMC) that minimize contact between immiscible moieties [9]. Since block copolymer nanostructures share common topological features with those of other self-organized systems, the concept of CMC minimal surfaces has been used [3] to explain the stability of complex block copolymer nanostructures. On the basis of self-consistent field theory (SCFT), Matsen and Bates [10,11] have recently proposed that the area-averaged mean curvature ͑͗H͒͘ governs the gross morphology (lamellar, bicontinuous, cylindrical, or spherical), whereas the standard deviation of the mean curvature distribution (s H ) determines the delicate stability of the complex nanostructures (G, D, or PL). This additional consideration results from packing frustration [12] and implies that, while a surface strives toward CMC, the mean curvature cannot be constant everywhere along the interface since the microdomain-forming blocks must uniformly fill space in the most entropically favored manner. Thus far, neither ͗H͘ nor s H has been measured experimentally despite their apparent importance.Three-dimensional visualization of bicontinuous morpho...