Positive Staining of (Pectin) Carboxyl Groups for Electron Microscopy

Abstract

The demonstration of pectic acid elementary fibrils by negative staining has prompted an attempt to demonstrate such structures by positive staining. Positive staining of specific reactive groups could give insight into elementary fibril organization as well as cell wall morphology.Because pectin staining based on esterified groups is insensitive to pectic acid, and pectin esterification is rarely total, attempts were made to stain free carboxyl groups. The formation of heavy metal salts or unsaturated esters with subsequent osmification are two methods for imparting electron density to carboxyl groups.The labile nature of pectin in water is probably responsible for amorphous configurations observed after formation of heavy metal pectates in aqueous systems. Morphology can be maintained by rapid dehydration of pectin solutions in organic solvents followed by salt formation using silver acetate dissolved in the organic solvent. Attempts to form uranyl pectates under these conditions resulted in large amorphous regions presumably because of molecular cross-linking by the divalent uranyl ion.