Na+ ions-induced coil-single helix transition of κ-carrageenan: Direct evidence from single-molecule studies

According to the principle of polymer science, the gelation process of κ-carrageenan should start with polymer chains in a disordered coil conformation. However, the coil-helix model has been challenged by recent studies, which pointed out that in deionized water, κ-carrageenan exists in single-helix conformation rather than random coil. Here, we employ single-molecule force spectroscopy (SMFS) to investigate its conformation under different conditions at the single-chain level. Direct evidence for the existence of random coil state is obtained. In addition, it is found that coil-single helix transition can be induced by external Na + ions, which can screen the charges of the sulfated groups in κ-carrageenan. Thus, the mechanism of the first step of κ-carrageenan gelation is revealed, which lays a solid foundation for better understanding of the entire gelation process.