  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
(Mmol = 401.3 g/mol); two molecular weights, Mw = 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, clusters are observed by
SANS in the dense phase of the diphasic samples. In between, close 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 one or a few HA chains. For the short chains (Mw ~ 6000),
the rod length of the complexes is close to the chain contour length (~ 15 nm).
