Abstract
Pectins, forming a matrix for cellulose and hemicellulose, determine the mechanics of plant cell walls. They undergo salient structural changes during their development. In the presence of divalent cations, usually calcium, pectins can form gel-like structures. Because of their importance they have been the subject of many force spectroscopy experiments, which have examined the conformational changes and molecular tensions due to external forces. The most abundant unit present in the pectin backbone is polygalacturonic acid. Unfortunately, experimental force spectroscopy on polygalacturonic acid molecules is still not a trivial task. The mechanism of the single-molecule response to external forces can be inferred by theoretical methods. Therefore, in this work we simulated such force spectroscopy experiments using the Enforced Geometry Optimization (EGO) method. We examined the oligomeric (up to hexamer) structures of α-D-galacturonic acid exposed to external stretching forces. The EGO simulation of the force spectroscopy appropriately reproduced the experimental course of the enforced conformational transition: chair →inverted chair via the twisted boat conformation(s) in the pyranose ring of α-D-galacturonic acid. Additionally, our theoretical approach also allowed to determine the minimum oligomer size adequate for the description of nano-mechanical properties of (poly)-α-D-galacturonic acid.
Highlights
Atomic force microscopy (AFM) has proven itself as a very valuable tool for the investigation of the nano-structure and nano-mechanical properties of single biomolecules or assemblies, including cell walls
The atomic force microscopy (AFM) technique can be useful for analyzing the effect of different modifying agents on polysaccharide structures [1,2], cellulose in cell wall model materials [3] and cellulose in the cell wall of apples in relation to their texture [4]
Pectins are interesting molecular systems because they constitute a matrix for cellulose and hemicellulose microfibrils
Summary
Atomic force microscopy (AFM) has proven itself as a very valuable tool for the investigation of the nano-structure and nano-mechanical properties of single biomolecules or assemblies, including cell walls. It has been shown that the chemical composition of model cell walls has a significant influence on their surface topographic parameters (RMS roughness, height, ironed surface) [3,5]. Pectins are interesting molecular systems because they constitute a matrix for cellulose and hemicellulose microfibrils. These biomolecules undergo salient structural changes during their development. Enzymatic activeness results primarily in the depolymerization of polygalacturonic acid and its de-esterification [6]. These processes have a significant influence on rheological properties, swelling abilities, gelling under different pH and temperature conditions and networking properties, and on the mechanical properties of pectin compounds
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