Evidence for double helical kappa-carrageenan in aqueous LiI-solution and model for iodide binding

Area of Science:

• Biophysics
• Polymer Science
• Carbohydrate Chemistry

Background:

• Carrageenans are sulfated polysaccharides with diverse biological activities.
• Kappa-carrageenan is known to form ordered helical structures in condensed states.
• Understanding solution structures is crucial for elucidating carrageenan functionality.

Purpose of the Study:

• To investigate the solution structure of kappa-carrageenan using NMR spectroscopy.
• To explore the interaction of kappa-carrageenan with iodide ions.
• To model the kappa-carrageenan double helix and its potential ion-binding sites.

Main Methods:

• Nuclear Magnetic Resonance (NMR) experiments utilizing a Redfield observation pulse.
• Analysis of hydrogen-bonded proton resonance at approximately 17.6 ppm.
• Molecular modeling using INSIGHTH-II software with published iota-carrageenan coordinates.

Main Results:

• A distinct hydrogen-bonded proton resonance indicates an ordered kappa-carrageenan structure in aqueous solution.
• The modeled kappa-carrageenan double helix possesses a wide interior, suitable for accommodating ions.
• In contrast, the iota-carrageenan double helix exhibits a narrow interior, unsuitable for large ions like iodide.

Conclusions:

• The observed NMR signal supports a double, parallel-stranded helical structure for kappa-carrageenan in solution, similar to its condensed state.
• The wide interior of the kappa-carrageenan double helix is proposed as the site for selective iodide binding.
• This structural insight explains previous observations of iodide binding to ordered carrageenan structures.