We use Small Angle Neutron Scattering (SANS), with an original analysis
method, to obtain both the characteristic sizes and the inner composition of
lysozyme-pectin complexes depending on the charge density. Lysozyme is a
globular protein and pectin a natural anionic semiflexible polysaccharide with
a degree of methylation (DM) 0, 43 and 74. For our experimental conditions
(buffer ionic strength I = 2.5 10-2 mol/L and pH between 3 and 7), the
electrostatic charge of lysozyme is always positive (from 8 to 17 depending on
pH). The pectin charge per elementary chain segment is negative and can be
varied from almost zero to one through the change of DM and pH. The weight
molar ratio of lysozyme on pectin monomers is kept constant. The ratio of
negative charge content per volume to positive charge content per volume, -/+,
is varied between 10 and 0.007. On a local scale, for all charged pectins, a
correlation peak appears at 0.2 {\AA}-1 due to proteins clustering inside the
complexes. On a large scale, the complexes appear as formed of spherical
globules with a well defined radius of 10 to 50 nm, containing a few thousands
proteins. The volume fraction Phi of organic matter within the globules derived
from SANS absolute cross-sections is around 0.1. The protein stacking, which
occurs inside the globules, is enhanced when pectin is more charged, due to pH
or DM. The linear charge density of the pectin determines the size of the
globules for pectin chains of comparable molecular weights whether it is
controlled by the pH or the DM. The radius of the globules varies between 10 nm
and 50 nm. In conclusion the structure is driven by electrostatic interactions
and not by hydrophobic interactions. The molecular weight also has a large
influence on the structure of the complexes since long chains tend to form
larger globules. This maybe one reason why DM and pH are not completely
equivalent in our system since DM 0 has a short mass, but this may not be the
only one. For very low pectin charge (-/+ = 0.07), globules do not appear and
the scattering signals a gel-like structure. We did not observe any
beads-on-a-string structure
