Kinetic studies of primary processes of conformational ordering in gel
-forming biopolymers have suggested that a change in mechanism from in
termolecular to intramolecular multistrand formation occurs on lowerin
g the concentration of biopolymer. We report here ultrastructural obse
rvations consistent with intramolecular double stranding in a carbohyd
rate polymer, iota-carrageenan, by arresting this process of primary c
onformational ordering by an ultra-rapid freeze fixation technique. Hi
gh-resolution transmission electron microscopy (TEM) revealed isolated
iota-carrageenan chains showing a range of morphologies (linear, circ
ular, and hairpin) consistent with intramolecular stranding. Control e
xperiments in which iota-carrageenan was frozen in the disordered form
revealed longer and thinner strands.