Alkali modification of carrageenans. Part V. The iota–nu hybrid carrageenan from Eucheuma denticulatum and its cyclization to iota-carrageenan

A homogeneous iota/nu-hybrid carrageenan (71% iota- and 21% nu-) isolated from Eucheuma denticulatum was used as a model compound to study the cyclization reaction of α-d-galactose 2,6-disulfate units to 3,6-anhydro-α-d-galactose 2-sulfate. The rate of cyclization, at 70 °C, of this carrageenan is about 50 times faster than that of a porphyran (non-sulfated β-d-galactose linked to α-l-galactose 6-sulfate) and 210 times faster when compared with a lambda-carrageenan (2-sulfated β-d-units linked to α-d-galactose 2,6-disulfate). The use of this model compound confirms the previous hypothesis of the accelerating effect of the β-d-4-sulfate group as well as suggests the influence of the 2-sulfate of the α-d-galactose units on the 4C1→1C4 chair forms interchange. The easy of cyclization indicates that to produce commercial iota-carrageenans milder alkaline treatments could be used, avoiding degradation and increasing the gel strength.