XAFS on Vectorially-Oriented Single Monolayer Protein Samples

Polarized XAFS measurements were carried out on a single monolayer of cytochrome c tethered to a self assembled organic monolayer. The monolayer contains 1012 to 1013 protein molecules per cm2, which is 10 to 100 times more dilute than a single monolayer of small inorganic molecules. Thus, the spectra were collected under total reflection conditions to ensure a reasonable signal to noise ratio by increasing signal counts and, in the mean time, reducing background. The.spectrum with X-ray polarization parallel to the substrate surface is found to be similar to the spectrum'of the perpendicular orientation. However, the spectra are somewhat different from the spectrum of cytochrome c in a frozen solution. The differences may be attributed to thermal effects and/or the impurity signal from the silicon or quartz substrates. Optical linear dichroism measurements determined the tilt angle between the average heme-plane and the substrate surface plane to be about 40°, consistent with XAFS observations. XAFS experiments on a single protein monolayer provide new opportunities for probing metal centers in biology

The double porogen approach as a new technique for the fabrication of interconnected poly(L-lactic acid) and starch based biodegradable scaffolds

One of the most widely used fabrication methods of three dimensional porous scaffolds involves compression moulding of a polymer salt mixture, followed by salt leaching. However, the scaffolds prepared by this technique have typically limited interconnectivity. In this study, besides salt particles, an additional polymeric porogen, poly(ethylene oxide), PEO, was added to poly(L-lactic acid), PLLA, to enhance the interconnectivity of the scaffolds. Compression moulded specimens were quenched and put intowater, where PEO crystallized and phase separated. Following the leaching of PEO fraction, the permeability and interconnectivity among the macropores formed by salt leaching could be observed. The porosities obtained in the prepared scaffolds were between 76 to 86%. Moreover, the highest porosity of 86% was obtained with minimum fraction of total porogen. The water absorption of the porous scaffolds prepared with PEO could vary between 280 to 450% while water uptake of pure PLLA scaffolds was about 93%. The increase of interconnectivity induced by compounding PLLA with PEO could also be obtained in porous PLLA/starch blends and PLLA/hydroxyapatite composites demonstrating the versatility and wide applicability of this preparation protocol. The simplicity of this organic solvent free preparation procedure of three-dimensional porous scaffolds with high interconnectivity and high surface area to volume ratio holds a promise for several tissue engineering applications