Glutaraldehyde-induced changes in the axially projected fine structure of collagen fibrils

The fine structure of the collagen fibril, as seen in axial projection, is changed by treatment with glutaraldehyde. The changes are detectable in electron-optical staining patterns and in the intensities of the low-angle meridional X-ray diffraction maxima. Current knowledge of the amino acid sequence of collagen and of the axial arrangement of molecules in fibrils permits interpretation in terms of specific alterations to the axial distribution of electron density along the fibril. Analysis of fibril staining patterns from glutaraldehyde-treated calf skin collagen shows that uptake of staining ions in positive staining patterns is inhibited at residues known to interact with glutaraldehyde (lysyl, hydroxylysyl and probably histidyl side-chains) and on other charged residues in the immediate neighbourhood of the glutaraldehyde-reactive residues. This can be seen as a "stain-exclusion effect" due to the presence of bulky polymeric complexes of glutaraldehyde molecules at cross-linking sites. Such stain exclusion accounts for the drastic changes in the negative staining pattern following treatment with glutaraldehyde. The intensity changes observed in the low-angle meridional X-ray reflections from rat tail tendon, similarly treated, also can be explained by the presence of these bulky complexes. Existing data have been used to predict a model of the altered electron density profile indicating the axial distribution of glutaraldehyde along a D-period of moist tendon collagen.