Flexibility and length of human bronchial mucin studied using low‐shear viscometry, birefringence relaxation analysis, and electron microscopy

The glycoprotein mucin was isolated from the sputum of patients with chronic obstructive bronchitis. The fractionation procedure included treatment with 6 M urea at pH 12.5 followed by gel filtration in 6 M urea at neutral pH. (1) Using a low‐shear Cartesian diver viscometer, we found that the mucin intrinsic viscosity equals (0.32 ± 0.03) L/g in 1000 m M NaCl solution increasing to (12 ± 3) L/g in 0.1 m M NaCl (pH 7 and 20°C). (2) The relaxation of electrically induced birefringence in mucin solutions was measured and the relaxation spectrum calculated using a Fourier‐transform deconvoltion method. We found that the dominant relaxation time increased from 1 to 150 μs when the exitation pulse duration used was increased from 2 to 300 μs. (3) Mucin was vacuum‐dried from glycerol‐containing solutions followed by low‐angle rotary shadowing and electron microscopy. Mucin was found to be unbranched, with contour lengths ranging from 300 to 2500 nm and with an average of 900 nm. Our result indicate that mucin is an extended and flexible molecule with Kuhn length 0.3–0.5% of the contour length.