SEC-MALS Analysis of Linear and Branched Synthetic Amyloses

Amylose is an occasionally branched biopolymer and, together with amylopectin, the hyper-branched component, a constituent of starch. Determination of branching in amylopectin, an important property affecting viscosity, stiffness, and thermo-mechanical performance, requires a fully linear amylose analogue. This application note describes how synthetic amyloses from enzymatic (phosphorolytic) reaction were checked for their linearity using size-exclusion chromatography-multi-angle light scattering (SEC-MALS).

Materials and Methods

Separations were performed on Shodex OHpak HQ-806, -805, -804, and -803 SEC columns. A DAWN® MALS detector and Optilab® differential refractometer were placed downstream of the columns to analyze molar mass and size of each eluting fraction. Data were acquired and analyzed by ASTRA® chromatography software to calculate differential weight distributions, conformation plots of mean-square radius versus molar mass, and branching per molecule.

The molar mass M and root-mean-square radius R_g were calculated at each slice using dn/dc = 0.146 mL/g. At least three data sets per sample were averaged together.

Results and Discussion

Branching calculations (branching ratio g_M, branches per molecule B, and long-term branching λ) were performed using the Zimm‑Stockmayer equations (trifunctional random branching). The molar mass distributions of low molar mass (LMM) synthetic amyloses are narrow in the range of 1–4 × 10⁵ g/mol, and the branching calculations show that these polymers are strictly linear. The differential weight distribution plots of high molar mass (HMM) amyloses show a broader distribution extending into the 10⁶ g/mol range. A distinct halt to the rise of R_g with increasing M is noticeable beginning around 1 million g/mol (Figure 1). Rigorous analysis shows that branching starts (B = e⁰) at approximately 8 × 10⁵ g/mol (Figure 2).

Conclusions

A fully-linear amylose was synthesized for use in branching analysis of naturally hyperbranched amylopectin. The synthetic amylose samples analyzed here were linear in the LMM range up to ~ 800,000 g/mol, but exhibited branching at higher molar masses. Even though the linear samples ended at 800,000 g/mol, the conformation plot of linear amylose can be safely extrapolated up to 10⁸ g/mol, enabling analysis of quite large amylopectins. Hence the SEC-MALS method using a DAWN and Optilab has proved to be useful to check branching in HMM biopolymers on the basis of linear

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