β-Lactoglobulin as an ideal random polymer coil

Unfolding of globular proteins by breaking the bonds and interactions that shape their structure, results in more flexible macromolecular chains with a relatively high percentage of apolar side group. Such properties may be beneficial for application in coatings and adhesives. It is then essential to define the flexibility of the chains at very high protein concentrations. As a measure for the stiffness of an unfolded protein chain the average persistence length P̄, which varies with the experimental conditions, is introduced. We demonstrate that P̄ at very high protein concentrations can be predicted by using measuring data of dilute solutions, as is required by the available analytical techniques. In this study the globular protein β-lactoglobulin is used as a model. Its secondary structure can be disrupted by applying 98% formic acid as a solvent and by additional chemical modification of the thiol groups to break the two remaining covalent SS bonds. The conformation of both unfolded structures, with and without chemical modification, can be described according to the Kuhn formalism, leading to a P̄ value of ±2 nm This value is determined by viscosimetry and by SANS and can be held representative for very concentrated protein layers where θ-conditions are assumed.