Double-Globular Structure of Porcine Stomach Mucin: A Small-Angle X-ray Scattering Study

We present evidence from small-angle X-ray scattering synchrotron experiments that porcine stomach mucin (MUC6) contains a double-globular comb structure. Analysis of the amino acid sequence of the peptide comb backbone indicates that the globular structure is determined by both the charge and hydrophobicity of the amino acids and the placement of the short hydrophilic carbohydrate side chains (similar to 2.5 nm). The double-globular structure is, thus, due to a block copolymer type hydrophobic polyampholyte charge instability in contrast to the random copolymer instabilities observed previously with synthetic polyclectrolytes (particularly polystyrene sulfonates). Careful filtering was required to exclude multimonomer aggregates from the X-ray measurements. A double Guinier analysis (R-g similar to 26 nm) and a double power law fit are consistent with two globules per chain in low salt conditions. The average radius of the globules is similar to 10 nm in salt-free condition (double Guinier fit) and the average distance of intrachain separation of the globules is 48 nm. The addition of salt causes a significant decrease in the radius of gyration (14 nm 100 mM NaCl) of the chains and is attributed to the contraction of the glycosylated peptide spacer between the two globules (the globular size continues to be similar to 10 nm and the globule separation is then 18 nm). Without salt, the scaling of the semidilute mesh size () as a function of the mucin concentration (C) is xi similar to c(-0.45) compared with xi similar to c(-0.28) in high salt conditions, highlighting the globular nature of the chains. In contrast, hydrophilic flexible polyclectrolytes have a stronger concentration dependence of when excess salt is added.