Structural change of polymer chains of gellan monitored by circular dichroism

Abstract

The random-coil-to-helix transition of gellan gum with various counter monocations, such as Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and tetramethyl ammonium (TMA⁺) in aqueous solution has been studied by circular dichroism (CD) and NMR methods in the absence of any additional salts. CD data show a sharp decrease in the molar ellipticity at 202 nm at the transition point, the temperature of which is defined as T _CD, i.e., the transition temperature determined by CD. T _CD depends on the concentration of gellan and on the countercation, for example, from 0.65 to 21.2 °C for 0.1–0.92% of Na-type gellan solution and from 6.7 to 26.1 °C for 0.13–1.0% Rb-type gellan solution. The molar ellipticity changes reversibly over the whole temperature range investigated for 1% Na, Rb, and TMA-type gellan solutions, while it shows a temperature delay in the recovery to random coil for 1% Li- and K-type gellan solutions. This phenomenon becomes more remarkable in higher concentration solutions. The water proton spin-spin relaxation time, T ₂, drastically decreases around the transition temperature. The temperature at which the decrease of T ₂ begins, however, is higher than T _CD in some cases. This observation indicates that a change in the state of water (mobility, hydrogen bonding, hydration structure and so on) may occur preceding the helix formation of the polymer chain. The T ₂ value shows the irreversibility of the transition in 1 % Li, K, and TMA gellan solutions. In comparison to the result obtained from CD, the aggregate formed at low temperature still remains as an aggregation of random coils at higher temperatures in the heating process.