Image analysis in the electron microscopy of cellulose protofibrils II. Digital correlation methods

Abstract

Electron micrographs of parallel arrays of negatively stained ramie cellulose protofibrils were analyzed using the two-dimensional digital autocorrelation function (ACF). The method is based upon the statistical analysis of images in real space. The ACF shows strong parallel streaks of high correlation, and the lateral distance between adjacent streaks allows the mean interfibrillar distance to be estimated as 3.7 nm. The intensity profile along the streaks shows a weak modulation with peaks occurring at integral multiples of 3 or 6 nm. These results provide direct evidence that there is a regular axial texture in the protofibrils, and corroborate the conclusions previously drawn from optical diffraction analysis. Using the difference vectors found in the ACF it has been possible to reduce the picture noise level by linear integration, thereby obtaining an enhanced image. A preliminary result obtained in this way suggests that the projected protofibril morphology associated with the observed axial periodicity is a ribbon-like zigzag structure. Possible applications of the method for future work are discussed.

One-Dimensional Image Simulation in the Electron Microscopy of Negatively Stained Cellulose Protofibrils

“…The optical diffraction pattern of small localized areas in appropriately defocused images showed a pseudo-lattice of discrete maxima from which it was deduced that the protofibrils have an axial periodicity of about 6 nm, and that their lateral periodicity is about 5 nm . These conclu sions were essentially corroborated by further analysis using the two-dimensional digital autocorrelation function 2) It was observed that the image contrast changed dramatically with focal setting (defocus) of the objective lens, especially in the lateral dimension . Since the structure of the specimen is unknown , this poses the problem of recognizing the image that reflects the correct structure of the proto fibrils.…”