Stoichiometry and Molecular Modeling

To determine the stoichiometry of the complex, we measured the CD spectra by changing the molar ratio Ms-spg/(Ms-spg + Mpoiy(o), where Ms-spg and Mpoiy(o are the molar concentrations of poly(C) and s-SPG, respectively. Fig. 4.29 plots [0]max against the molar ratio. The [0]max value increases linearly with increasing molar ratio, following an upward convex, and finally levels off at [0]max = 90 x 10 deg 

The critical chain length required to induce complexation. The subset (a) shows the Mw of s-SPG dependence of [0]max for the s-SPG + poly(C) system when the base number of poly(C) is fixed at 250. The subset (b) shows that the base number dependence of [0]max for the s-SPG -I- poly(dA) system, fixing Mw = 15000 for s-SPG. The inset of (b) compares the CD spectrum between poly(dA) and the s-SPG/poly(dA) complex. 

We can speculate a few possible structures based on the following facts or assumptions (1) since the second hydroxyl groups of the main chain, which are located inside the triple helix column, participate in the hydrogen-bonding interaction, the same hydroxyl groups should be involved in the complex formation (2) the side glucose chain only provides solubility and is not involved in the complex formation and (3) the CD and UV data suggest the formation of a helical structure in the complex. Fig. 4.30 illustrates the two possible structures for the poly(C)/s- 

SPG complex. In Model Dl, every glucose residue in the main chain interacts with the cytosine in a DNA-like double helix. Since the distance between the neighboring C-2 carbon s OH is about 4 A and the stacking distance between cytosine moieties is 3.0—3.6 A, this model is reasonable from the stereochemistry. However, the stoichiometric number n = 0.25) does not agree with the experi- 

Since the local polymer conformation in solution is sometimes related to the crystalline structure, the complex structure in solution should be represented by a triple helix similar to that of the original P-(1 3)-glucans. Based on the above discussion, we can construct a structural model for the complex with computer chemistry. First, we constructed the triple helix of schizophyllan (a right-handed 6i triple helix with a 17.4 A) according to the literatures, and took one chain 

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