Molecular-mechanical link in a shear-induced self-assembly of a functionalised biopolymeric fluid

Kappa-carrageenan is a linear sulphated anionic polysaccharide extracted from edible red seaweeds. It is widely used in food sciences, pharmaceutical industry, biotechnology, tissue engineering, medical applications and is also used in some unconventional areas like marbling [1].

Macromolecules of kappa-carrageenan solutions undergo conformational changes with temperature. The molecules are in random coil conformation at 313K and adapt isolated rod conformation upon cooling to room temperature. Further cooling leads to intermolecular synergy between rods resulting in gelation of a 0.5% kappa-carrageenan solution at 283K.

Mechanical behavior of the 0.5% carrageenan fluid changes from Newtonian (313K) to shear-thinning (283K). This mechanics is presumably correlated with conformational changes experienced by these biomacromolecules upon cooling. Therefore novel 23Na MQF rheo-NMR [2] methods were applied to establish this molecular-mechanical link.

The fluid was sheared in 1 mm gap at different flow rates in a Couette cell mounted inside 9.4Tmagnet. In addition, temperature in the Couette cell was varied from 285K to 313K with a range of flow rates sampled at each temperature. Na-23 nucleus frequency was used for the NMR signal detection in all experiments. We were able to demonstrate that Na-23 DQF MA and TQF signals were observed only in the presence of shear, however at 313K Na-23 MQF signals were absent with and without shear field. We have also shown a significant change in sodium dynamics under shear with the increase of flow rate and temperature. We also performed bulk rheological measurements of this fluid for all temperatures used in this study. We were able to correlate the detection of Na-23 MQF signals with the presence of flow-induced molecular alignment occurring only with macromolecules present in rod conformation.

This result is significant as to the best of our knowledge the correlation between shear-thinning, conformation of molecules in a fluid and mechanically induced behavior has never been shown experimentally. Sodium is a very important nucleus as it is naturally present in many biological systems including human bodies. This may result into shifting these studies towards in vivo to better our understanding about the mechanics of body fluids and their molecular behavior under shear. This could potentially impact anti-cancer drug delivery and management of other life threatening diseases.


References:

[1] Web of Science search for "kappa-carrageenan" results in 9,039 citations
[2] Pavlovskaya, GE; Meersmann, T, JPCL, 5(15), 2632-2636, (2014)