Selective replacement of the amorphous peptide domain of a spider silk with poly(ethylene glycol) gave N. clavipes silk-inspired polymers having similar solid-state structures and very good mechanical properties. The tendency of poly(alanine) having appropriate chain length to form beta-sheets and the facility with which the beta-sheets self-assemble have been retained in the polymers. Solid-state (13)C NMR, solid-state FTIR, X-ray diffraction, and AFM studies showed that the polymers formed predominantly antiparallel beta-sheets that self-assembled into discrete nanostructures. The longer the peptide segment was, the greater was the tendency to self-assemble into antiparallel beta-sheet aggregates. AFM revealed that the morphology of the polymers was a microphase-separated architecture that contained irregularly shaped 100-200 nm poly(alanine) nanodomains interspersed within the PEG phase. The results suggest that the poly(alanine) domain influences the solid-state properties of spider silk through beta-sheet self-assembly into temporary cross-links. The results further demonstrate that by selectively replacing certain segments of a naturally occurring biopolymer with a judiciously selected nonnative segment while, at the same time, retaining other segments known to be critical for the essential properties of the native biopolymer, a synthetic polymer with similar properties and function can be obtained.

Biomolecular Bombyx mori silk-based multiblock copolymers of poly(ethylene oxide)(PEO) and GlyAlaGlyAla prepared by step-growth polymerization based on a modular chemical method have been found to self-assemble into nanostructures. Solid-state 13C NMR, solid-state FTIR, and powder X-ray diffraction provided evidence for the β-sheet self-assembly. The overall β-sheet content was as high as 90%. The building blocks had significant impact on the solid-state structure of the resulting polymer. Longer PEO spacers lowered the tendency to self-assemble into antiparallel β-sheets and implicated the existence of PEO-sequestered isolated β-sheets. Differential scanning calorimetry, transmission electron microscopy, and atomic force microscopy confirmed nanostructure formation and microphase separation. The microphase-separated morphology of the polymers contained 20−50 nm peptide domains dispersed in a continuous PEO phase on which was superimposed a 100−150 nm superstructure due to the polymers' polydispersity and multiblock character. Instron measurements suggested that mechanical properties could be modulated by manipulating the building blocks.

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Microwave spectra have been observed with Fourier transform and cold jet expansion techniques in the 6−17 GHz range for trans-3,4-difluorocyclobutene, its two 13C1 modifications, two d 1 modifications, and the perdeutero species. Watson-type Hamiltonians have been fit with a full set of quartic centrifugal distortion constants for all species except the d 4 species. Complete structures fit with an r s/r 0 treatment and an r 0 treatment are in satisfactory agreement. Preferred r s/r 0 results with Costain uncertainties in parentheses are 1.349(4) Å for the CC bond, 1.503(9) Å for the contiguous C−C bonds, 1.534(4) Å for the distant C−C bond, 1.398(6) Å for the C−F bond, 1.081(3) Å for the C−H bond, and 1.102(5) Å for the −C−H bond. Consistent with its C 2 symmetry, the ring is slightly puckered, and the C−F bonds are roughly equatorial. Compared with the structure of cyclobutene, the structure of trans-3,4-difluorocyclobutene shows a modest lengthening of the CC bond and more significant decreases in the C−C bond lengths. These effects are consistent with observations for other fluorine-substituted rings in which contiguous fluorine atom substitution causes CC bond shortening and distant fluorine atom substitution causes CC bond lengthening. The CC bond adjustments in trans-3,4-difluorocyclobutene are, however, surprisingly similar to the adjustments observed for 3,3,4,4-tetrafluorocyclobutene.