Temperature‐Induced conformational transition in xanthans with partially hydrolyzed side chains

Abstract

The conformational properties of xanthans with partially hydrolyzed side chains were investigated by optical rotation, CD, and differential scanning calorimetry (DSC). All variants displayed the well-known temperature-driven, cooperative order-disorder transition, and both optical rotation and DSC showed that the transition temperature was essentially independent of the content of terminal beta-mannose. It was found that up to 80% of the changes in the specific optical rotation accompanying the transition reflects conformational changes linked to the terminal beta-mannose in the side chains. Modification of the side chains also affected the CD when xanthan was in the ordered state, but in this case the data suggest that the glucuronic acid is the major component determining the magnitude of the CD signal. DSC measurements showed that the transition enthalpy (delta Hcal) increased linearly with the fraction of beta-mannose, again indicating that a significant part (up to 80%) of delta Hcal reflects conformational changes in the side chains. The conformational transition of the xanthan variants generally showed a higher degree of cooperativity (sharper transition) than unmodified, pyruvated xanthan. Calculation of the cooperativity parameter sigma by means of the Zimm-Bragg theory (OR data) or from the ratio between delta Hcal and the van't Hoff enthalpy (delta HvH) using DSC data showed a correlation between sigma and the content of beta-mannose, but the two methods gave different results when the content of beta-mannose approached 100%. The ionic strength dependence of the transition temperature, expressed as d (log I)/d(T-1m), was nearly identical for intact xanthan and a sample containing only 6% of the terminal beta-mannose. Application of the Manning polyelectrolyte theory does not readily account for the observed delta Hcal values, neither does it provide new information on the nature of the ordered and disordered conformations in xanthan.

Effects of carboxylation of the side chains on the order-disorder transition in aqueous solution of schizophyllan, a triple helical polysaccharide

“…This suggests that the intact side chain (glucose residue) and the modified side chain (Figure 1) contribute differently to the total optical rotation, reflecting differences in the number of asymmetric carbons in the two cases (Jackson & Hudson, 1937). This finding parallels the observations for xanthan with partially hydrolysed (removed) side chains (Christensen, Knudsen, Smidsrød, Kitamura, Takeo, 1993), where the total optical rotation (and transition enthalpy) changed linearly with changes in the number of side chains. Secondly, it is noted that the transition temperatures obtained from OR agree well with those obtained by DSC, i.e.…”

“…Chemical modifications of polysaccharide side chains generally influence such transition markers. For example, partial acid hydrolysis was used in a double helical polysaccharide, xanthan, to investigate the influence of the removal of side chains on the conformational transition of xanthan (Christensen, Knudsen, Smidsrød, Kitamura, Takeo, 1993). Since the order-disorder transition of schizophyllan is characteristic to the triple helical conformation (Kitamura & Kuge, 1989), it is necessary to investigate both of the molecular conformation and the transition behavior after the chemical modification.…”

The influence of charge on the multiple thermal transitions observed in xanthan

“…This is apparently contradicted by the observation that in the absence of the terminal mannose and consequently pyruvate there is still an increase in transition temperature with increasing salt content (Christensen, Knudsen, Smidsrød, Kitamura, & Takeo, 1993). Table 2 shows the values in the present work for the slope of the logarithm of the change in concentration divided by the change in reciprocal absolute transition temperature, compared with literature values.…”

“…As the salt might be expected to act similarly on both the glucuronic acid and the pyruvate groups, both having similar pKa values, the effect of the pyruvate can be viewed as superimposed upon but acting independently of the effect of glucuronic acid and the glucuronic acid effect becoming dominant when the pyruvate is removed. (Christensen, et al 1993) compared with values from the present work. Also shown are the results for the removal of the terminal mannose and slight reduction in glucuronic acid content.…”

Thermally induced coil‐to‐helix transition of sodium gellan gum with different molar masses in aqueous salt solutions

“…In order to estimate the DH vH , it is necessary to decide which slope should be selected. Christensen and his coworkers 48 reported that the van't Hoff plots of xanthan solutions showed two slopes below T ch , as found in the present study. They estimated DH vH from the data just below T ch (steep slope).…”