alpha 1 acid glycoprotein: a small-angle neutron scattering study of a human plasma glycoprotein

Small-angle neutron scattering experiments on alpha 1 acid glycoprotein showed that it has a molecular weight of 37000 and a matchpoint of 44.7% 2H2O. The molecular weight, the matchpoint and a v of 0.704 ml/g are in agreement with the primary sequence and standard residue volumes for amino acids and carbohydrates. The radius of gyration RG of alpha 1 acid glycoprotein was found to be independent of concentration in the range 2-11 mg/ml, but increases on going from a buffer containing 0.2 M NaCl to one containing 1 M NaCl. A contrast variation study showed that the RG at infinite contrast is 2.47 nm for the expanded form and 2.19 nm for the contracted form, and that the two Stuhrmann alpha values are similar at 27 X 10(-5). The latter is greater than that expected for globular proteins and are explained by the surface disposition of the five glycan chains on a core of protein in alpha 1 acid glycoprotein. Modelling calculations account for the two RG values in which for the expanded form the glycan chains extend out into the solvent and for the contracted form they either fold back or are splayed out such that they are able to interact with the surface of the protein core.