A new scanning electron microscopy approach to image aerogels at the nanoscale.

A new scanning electron microscopy (SEM) technique to image poor electrically conductive aerogels is presented. The process can be performed by non-expert SEM users. We showed that negative charging effects on aerogels can be minimized significantly by inserting dry nitrogen gas close to the region of interest. The process involves the local recombination of accumulated negative charges with positive ions generated from ionization processes. This new technique made possible the acquisition of images of aerogels with pores down to approximately 3 nm in diameter using a positively biased Everhart-Thornley (ET) detector.

Pancreatic exocrine hypofunction in the wasting syndrome of end-stage renal disease.

Pancreatic exocrine insufficiency in patients with end-stage renal disease on hemodialysis is not well documented despite the prevalence of a wasting syndrome, which may indicate pancreatic involvement. The present study intended to evaluate pancreatic exocrine function in this group. Eight patients were studied, none of whom had a history suggestive of pancreatic exocrine involvement. Routine 72-h stool collection was performed, and estimation for fecal fat showed steatorrhea in four patients. The secretin-pancreozymin test (direct stimulatory method) showed a statistically significant decrease in amylase and lipase levels in the duodenal aspirate, and increased basal and stimulated serum amylase (p less than 0.05). Values for volume, bicarbonate, and trypsin in the aspirate were normal. The role of pancreatic hypofunction in the etiology of the wasting syndrome of end-stage renal disease is therefore considered, as well as the advantages of pancreatic supplementation in improving nutritional status.

An analysis of molecular interactions involved in the assembly of tropomyosin tactoids.

Arrangements of parallel and antiparallel tropomyosin molecules are examined for residue interactions which might explain the paracrystal forms observed by electron microscopy. Molecular arrays which produce interaction maxima and corresponding computer graphics-simulated staining patterns are compared with observd electron micrographs. The best correlation of interactions with staining pattern occurs when cationic bridging of acid residues is maximized and supported by favourable ion pair interactions. In the antiparallel case, two main maxima occur and appear to correspond to divalent ion tactoids which have been previously reported. For the Cohen-Longley Mg2+ tactoid the best fit is obtained with a molecular overlap of 201 residues and an end overlap of 17-18 residues (based on a rational 287 peptide sequence). Secondary maxima correspond to other known tactoid forms. Binding of tropomyosin molecules to actin may involve hydrogen bonding to six serine residues which occur at approx. 40-residue intervals.

Conformational analysis of a chain reversal in alpha-chymotrypsin.

A complete conformational analysis of the fold (Asp-Lys-Thr-Gly) (residues 35-38), and additional adjacent residues of alpha-chymotrypsin has been performed. A comparison of these findings with those of Lewis et al. (1) is made, and a discussion of the implications of protein-fold models is discussed. This particular residue sequence prefers to bend over maintaining a helical conformation. However, the bend conformation of the tetramer is different from that of the native bend. The native bend conformation is nearly realized when an additional residue of the native primary structure is added to each side of the tetramer. Early and late folding-sequence studies suggests that while the native fold is of low energy, there are fold-points along the primary structure which are more stable. The structural implications of this finding are discussed.