Regenerated silk fibroin materials show properties dependent on the methods used to process them. The molecular structures of B. mori silk fibroin both in solution and in solid states were studied and compared using X-ray diffraction, FTIR, and (13)C NMR spectroscopy. Some portion of fibroin protein molecules dissolved in formic acid already have a beta-sheet structure, whereas those dissolved in TFA have some helical conformation. Moreover, fibroin molecules were spontaneously assembled into an ordered structure as the acidic solvents were removed from the fibroin-acidic solvent systems. This may be responsible for the improved physical properties of regenerated fibroin materials from acidic solvents. Regenerated fibroin materials have shown poor mechanical properties and brittleness compared to their original form. These problems were technically solved by improving the fiber forming process according to a method reported here. The regenerated fibroin fibers showed much better mechanical properties compared to the native silk fiber and their physical and chemical properties were characterized by X-ray diffraction, solid state (13)C NMR spectroscopy, SinTech tensile testing, and SEM.

Recently, genetic studies have revealed the entire amino acid sequence of Bombyx mori silk fibroin. It is known from X-ray diffraction studies that the beta-sheet crystalline structure (silk II) of fibroin is composed of hexaamino acid sequences of GAGAGS. However, in the heavy chain of B. mori silk fibroin, there are also present 11 irregular sequences, with about 31 amino acid residues (irregular GT approximately GT sequences). The structure and role of these irregular sequences have remained unknown. One of the most frequently appearing irregular sequences was synthesized and its 3-D solution structure was studied by high-resolution 2-D NMR techniques. The 3-D structure determined for this peptide shows that it makes a loop structure (distorted omega shape), which implies that the preceding backbone direction is changed by 180 degrees, i.e., reversed, by this sequence. This may facilitate the beta-sheet formation between the crystal-forming building blocks, GAGAGS/GY approximately GY sequences, in the fibroin heavy chain.

Low-molecular-weight liquid poly(ethylene glycol) (PEG) spontaneously forms an inclusion compound (IC) when combined with R-cyclodextrin (R-CD) powder at room temperature. This process can be followed with wide-angle X-ray diffraction (WAXD). The WAXD data shows that the R-CD crystals undergo a solid-state crystal-crystal transformation from the cage to the channel crystal structure upon IC formation over a period of about 8 h. The time dependence of the 2θ ) 20°R-CD channel structure X-ray peak can be described by a simple first-order kinetic model. The effects of changing the temperature, PEG:R-CD molar ratio, PEG molecular weight, and vacuum-drying the CD have been studied. The barrier opposing the PEG inclusion-induced solid-state transformation of R-CD from the cage to the channel crystal structure appears to be dominated by changes in the packing/interactions of R-CDs, rather than the loss in the conformational entropy experienced by the PEG chains during the inclusion process.

Length scales of mixing in amorphous blends of solid PCL (polycaprolactone) and PLLA (poly-l-lactic acid) were investigated as a function of preparation method. A recently described two-dimensional heteronuclear correlation (Hetcor) spin-diffusion technique (Jia et al. Macromolecules 2003, 36, 712) revealed that PCL/PLLA blends with shorter length scales of mixing, relative to solution blending, could be prepared using inclusion-compound coalescence methods (Rusa et al. Macromolecules 2000, 33, 5321). These biocompatible and biodegradable polymer blends provide a clear example of the utility of the 2D Hetcor spin-diffusion method for quantitative miscibility and phase analysis in amorphous macromolecules and their blends. The rates for intrapolymer polarization transfer vs interchain/interdomain polarization equilibration were easily differentiated using the 2D technique for either blend. As a result, spin-diffusion coefficients and miscibility length scales could be calculated by direct measurement on the blend constituents, while more traditional methods involving NMR relaxation proved inconclusive.

Cyclomaltohexaose (alpha-cyclodextrin, alpha-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal structure in which alpha-CD molecules stack to form a molecular tube. Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer/alpha-CD ICs to determine their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, polyethylene glycol (PEG, MW = 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/alpha-CD IC are similar to those obtained for pure alpha-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/alpha-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure alpha-CD, due to dethreading of PEG. This behavior does not occur for HTC/alpha-CD IC. Sorption isotherms collected at 20, 30, 40, and 50 degrees C allowed the calculation of the isosteric heats of adsorption and the integral entropies of adsorbed water which are characterized by minima that indicate the monolayer concentration of water in the ICs.