NMR diffusometry and dynamic light scattering studies of amylopectin: effect of shearing and heating on the size distribution and diffusion behaviour

The influence of different factors on the diffusion behaviour of cationized potato amylopectin (CAP) in water was investigated by NMR Diffusometry and Dynamic Light Scattering (DLS). The sample preparation procedure was found to have a fundamental influence on the size and size distribution evolution of the different samples, whereas the addition of NaN₃ or the replacement of CAP by non-cationic potato amylopectin (PAP) did not influence the diffusion behaviour. NMR samples were prepared by redissolving a freeze-dried CAP sample in D₂O at ambient temperature or by heating. Some samples were sheared, reducing their average aggregate size by a factor of 40. The unheated samples had a large average molecular size, indicating incomplete aggregate dissolution. The single-fit average diffusion coefficient D_av,init showed a strong time evolution for the unheated samples, increasing with a factor of 1000 over 20 days for both sheared and unsheared samples. This effect was mainly attributed to incomplete sample dissolution, leading to a time evolution of the molecular size and size distribution. For heated samples, D_av,init oscillated around a much higher value indicating the occurrence of sub-molecular sized fragments and possibly also a time-coherent rearrangement of the sample. The size distribution was very large for both heated and unheated samples, with σ_InD values of up to 2.5 and diffusion coefficients ranging from 10^-14 to 10^-11 m²/s in the same sample. DLS measurements corroborated the conclusions drawn from the results found by NMR diffusometry, but the complementary use of DLS and NMR diffusometry also highlighted the difference between the two methods for size determination. With NMR diffusometry, the proportion of small molecules may be slightly exaggerated, whereas the scattering methods mostly used in previous studies may underestimate their importance, resulting in a larger mean aggregate size than may actually be the case. This study thus draws the attention to the importance of the analysis method for the size and size distribution of amylopectin samples.