Simple Construction lo Determine Protein Molecular Weights by the Osmotic Pressure Method I n large laboratory classes determination of the molecular weight of a macmmaleeule by physical methods presents certain problems. Mea\ surement of osmotic pressure by s. capillary tube i? the simplest met,hod, but the apparatus is usually fragile and difficult to handle. We have successfully used the device pictured. I t consists of a disposable plastic syringe with a 1-mm bore capillary tube cemented (we used ICI "Tensal" cement) to the tip. The open end of the syringe is romded off with a knife so that a piece of ViskingB tubing can be stretched over it and held in place with a rubber band. I n order t o introduce the protein solution t,o a paint in the lower part of the capillary, a hole is cut in the barrel and covered with a section of 3-mm thick rubber tubing. The lower end of the syringe is immersed in a beaker of the solvent. Since the cost of each assembly is negligible, it may be discarded after use. I f a capillary correction is required, an identical piece of capillary may he sbtxehed with a pin. The molecular weight is calculated from the capillary rise using the relationship r = c a c (g/lW ml) X RTlmolemlar 300 (standard weight. h olas of 22 st,udents rising s. 3% hemoglobin solution obtained a mesn value of 70,200 deviation). This value is sufficient t.o determine t,rue molecular weight if minimal molecular weight is found by another method such ss iron determination
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Volume 45, Number 2, February 1968
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93
