Iodinated media are derivatives of triiodobenzoic acid and capable of forming solutions that are uniquely capable of banding any biological particle, according to its buoyant density, often under isoosmotic conditions.
Both metrizoate (Fig. 1) and diatrizoate (Fig. 2) are ionic compounds which are able to interact with other charged groups on biological particles and also influence the distribution of ions across membranes, thus the development of nonionic iodinated compounds such as metrizamide and iohexol was an important step. Both of these non-ionic gradient media can form solutions of high density (>1 .30 g/mL at 60% w/v); in metrizamide the carboxyl group present in metrizoic acid is linked to glucosamine, while in iohexol the carboxyl group is linked to the amine group of 3-amino-1,2-propanediol. This difference gives iohexol some advantages over metrizamide, notably its lower toxicity towards cells and autoclavability. The systematic name of iohexol is 5-(N-2,3-dihydroxypropylacetamido)-2,4, 6-triiodo-N, N’-bis(2,3-dihydroxypropyl) isophthalamide whose molecular structure is given in Figure 3. Its molecular mass is 821.
The latest addition to the nonionic iodinated compounds is iodixanol. The systematic name of iodixanol is 5,5 -[(2-hydroxy-1,3-propanediyl)-bis acetylimino)]bis- [N,N bis (2,3dihydroxypropyl)-2,4,6-triiodo-benzenedicarboxamide] whose molecular structure is given in Figure 4. Its molecular mass is 1550 and it is essentially a dimer of Nycodenz.
The chemical properties and stability of Nycodenz and iodixanol are related to their structure (Figures 3 and 4). Their high density derives from the presence of the substituted triiodobenzene rings which are linked to a number of hydrophilic groups.