The History of Alginate Chemistry – Structure 6

Structure – Tertiary structure

The tertiary structure of alginate has been comprehensively reviewed (Morris, 1986). Alginate adopts an extended, ribbon like conformation, in the solid state, being a two fold structure for the free acid form (Atkins, 1973) and a three fold structure in the salt forms so far studied (Mackie, 1973). Alginate is biosynthesised as a polymer of β-D-mannuronic acid, a portion of which is subsequently epimerised at the C-5 position enzymatically to give α-L-guluronic acid (Lin 1966). The D-mannuronic acid residues have been shown to be in the ⁴C₁ conformation and the L-guluronic acid residues in the ¹C₄ conformation (Rendleman, 1978). Recently a fully unified mechanism for the action of lyases and epimerases in the biosynthesis of alginate has been proposed (Gacesa, 1987).

Molecular modelling has played its part in the structural analysis of alginate. A semi empirical technique was used which assumes that the total conformational energy of a polymer can be broken down into various contributions from bond length and angles superimposed onto he contributions from favourable non bonding interactions. Each of these contributions can be computer modelled and a contour map calculated representing the energy requirements for the various possible angles of the two C-O bonds joining a dimer together. Hence the probability of a dimer being in a given conformation can be calculated, this can then be extrapolated to give an average structure for a polymer chain. The average structure of the polymer was then related to the viscosity of the polymer solution, alginate was noted as having a very stiff chain in comparison to carrageenan (Brant & Buliga, 1984). The comment on chain stiffness is interesting when compared with the direct electron microscopy studies on the tetra-methyl-ammonium salt of alginate that described alginate as a “linear, unbranched and flexible structure” (Stokke et al, 1987). However no absolute comparisons where possible.

The use of electron microscopy could well prove useful in the future study of polymer tertiary structure. However at present the effect the treatment of the sample has on the structure of the polymer prior to examination is unknown. These problems are partly overcome with the newer technique of scanning tunnelling microscopy.

Additional information on alginate structure, production and bacterial alginate can be accessed using the arrows in the Further Reading box below.