Analysis of polypeptide inter-chain entanglements using swelling dynamics of a spin coated protein layer

Highlights: • Swelling model of electrolyte fits well with the dynamics of a protein film. • Denatured protein demonstrates slow swelling rate but high swellability. • Internal morphology of the films is very sensitive to spinning speed. • Inter-chain entanglement was indirectly analyzed from swelling dynamics. -- Abstract: A study of swelling dynamics was applied to analyze the internal structure with regard to inter-chain entanglements of a bio-molecular film. Lysozyme – an enzyme protein was spun coated on silicon substrates to study swelling dynamics when exposed to water vapor. Two different samples were prepared with two different spinning speeds with 500 and 1000 rpm. The changes in the thickness of the films were progressively monitored using x-ray reflectivity technique as a function of time till the saturation of thickness. The swelling dynamics for protein layer were fitted with an existing polyelectrolyte chain swelling model. The fittings were used to obtain diffusion coefficient and charge fraction. It was found that the spinning speed strongly affects the swelling dynamics, charge fraction and saturation thickness. The slower dynamics in 1000 rpm film were attributed to inter-chain entanglements arising due to higher degree of compression during spinning. The enhanced degree of inter-chain entanglements leads to complete loss of swelling functionality on a mild thermal treatment. This study has stronger implication for the analysis of internal structure of homogeneous biomolecular films for inter-chain entanglements which are otherwise difficult to infer from direct space measurement techniques.