Rodlike Complexes of a Polyelectrolyte (Hyaluronan) and a Protein (Lysozyme) Observed by SANS

We study by small-angle neutron scattering (SANS) the structure of hyaluronan -lysozyme complexes. Hyaluronan: (HA). is a polysaccharide of 9 nm intrinsic persistence length that bears one negative charge per disaccharide monomer (M-mol = 401.3 g/mol); two molecular weights, M-w = 6000 and 500 000 Da were used. The pH was adjusted at 4.7 and 7.4 so that lysozyme has a global charge of +10 and +8, respectively. The lysozyme concentration was varied from 3 to 40 g/L at constant HA, concentration (10 g/L). At low protein concentration, samples are monophasic, and SANS experiments reveal only fluctuations of concentration, although, at high protein concentration, dusters ire observed by SANS in the dense phase of the diphasic samples. In between, dose to the onset of the phase separation, a distinct original scattering is observed. It is characteristic of a rod-like shape; which could Characterize "single" complexes involving one or a few polymer chains. For the large molecular weight (500 000), the rodlike rigid domains extend to much larger length scale than the persistence length of the HA chain alone in solution and the range of the SANS investigation. They can be described as a necklace of proteins attached along a backbone of diameter of One Or a few HA :chains. For the short chains (M-w approximate to 6000), the rod length of the complexes is dose to the chain contour length. (similar to 15 nm).