“Ordered” structure in solutions and gels of a globular protein as studied by small angle neutron scattering

Small-angle neutron scattering measurements were used for structural investigation of β-lactoglobulin solutions and heat-set gels in conditions of strong double layer repulsions. At pH 9 and low ionic strength, a correlation peak was observed on the scattering curves of the solutions whatever the protein concentration C used (in the range C = 0.02–0.10 g/mL). The wave vector value q_max corresponding to these maxima scaled as C^0.25. This exponent value is in agreement with those reported in the literature for other globular proteins in the same concentration range. Increasing the ionic strength decreased the peak which vanished without changing position at 0.1M NaCl. This polyelectrolyte-like behaviour suggests a local structure in the protein solution due to double layer repulsions. In the case of heat-set aggregates and gels (0.02–0.13 g/mL) formed at pH 9 and low ionic strength, a peak in the scattering curves was also observed indicating that even after gelation a correlation is still present; q_max varied as C^0.5. As in the case of the solutions, the correlation peak decreased with increasing ionic strength, and it vanished at 0.06M NaCl. The dilution of the aggregates in order to determine their intraparticle structure factor showed that the correlations were lost and that the aggregates displayed the same internal structure as the elementary subunit in the gels. At high ionic strength, fractal structures of the aggregates down to a length scale of about 40 Å were observed with d_f = 1.3–1.75 ± 0.05, increasing with protein concentration. Subsequent dilution didn't change the fractal dimension of these structures. © 1996 John Wiley & Sons, Inc.