Solvent dependent changes in proteoglycan subunit conformation in aqueous guanidine hydrochloride solutions

Prior studies of the viscosity of the subunit proteoglycan obtained by dissociative extraction of bovine nasal septum cartilage suggested that proteoglycan subunit molecule behaves as a rigid rod in adequeous solution over the range of 0.5 to 4.0 M guanidine hydrochloride. A more direct view of dimension and shape can be obtained from light scattering and viscosity of bovine nasal cartilage proteoglycan subunit over the range from 0.05 to 4.0 M guanidine hydrochloride. The molecular weight, radius of gyration, second virial coefficient, and intrinsic viscosity were determined. The weight average molecular weight of the proteoglycan subunit was constant (2.3 X 10⁵) at all guanidine hydrochloride concentrations. The radius of gyration decreased from a value of 1590 A in 0.05 M guanidine hydrochloride to 570 A in 4.0 M guanidine hydrochloride, and there were changes in the second virial coefficient indicating a major change in solventproteoglycan subunit interaction in the 0.15 to 4.0 M guanidine hydrochloride range. The second virial coefficient goes from a minimum value of -7.0 X 10^-5 ml g^-2 mole in 0.15 M guanidine hydrocloride to a value of 5.7 X 10^-5 ml g^-2 mole in 4.0 M guanidine hydrochloride. At concentrations of guanidine hydrochloride greater than 0.5 M, the viscosity was nearly constant. These data suggest that at low ionic strength in guanidine hydrochloride, the proteoglycan subunit is a rather rigid structure, but at approximately 0.3 M guanidine hydrochloride, a conformational change occurs, providing a more flexible structure which persists up to concentrations as high as 4.0 M guanidine hydrochloride. Over the same range, the separate chondroitin 4 sulfate chains show typical polyelectrolyte effects and a positive second virial coefficient ranging from +4.8 X 10^-3 in 0.1 M guanidine hydrochloride to 0.6 X 10^-3 in 3.0 M guanidine hydrochloride. This comparison indicates that the behavior of the intact proteoglycan subunit cannot be understood in terms of the properties of the isolated chondroitin 4 sulfate chains. It is likely that both the protein core itself and the packing of the polysaccharide chains along the core are involved in this difference.