Bovine serum albumin and its behaviour in acid solution

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Abstract
New measurements of the depolarization of the fluorescence of conjugates of bovine serum albumin with fluorescent dye-stuffs were carried out over a range of pH and ionic strengths. Parallel investigations of the light absorption of the conjugating molecule, of the total fluorescence intensity and of the fluorescence spectra were made in order to detect changes in the life time of fluorescence. When such changes are allowed for, a decrease in the rotational relaxation time with decrease in pH below 4 was confirmed. Light-scattering oleasurements have shown that, down to pH 1.9, over a range of ionic strengths, no change in molecular weight occurs. Sedimentation velocity measurements, with diffusion data already published, confirmed this conclusion, but also indicated a swelling and increasing asymmetry of the molecule as the pH was lowered from 4 to 2. Increase in the ionic strength at constant pH only partially reversed such processes. Changes in viscosity, optical rotation and heat content of the solution occurred over a similar pH range. The decrease in rotational relaxation time was not directly connected with the increased molecular asymmetry at acid pH values, but must be attributed to increasing freedom of rotational motion within the molecule. Possible models in explanation of the observations are suggested. New sedimentation data for the protein under neutral conditions are reported, and a molecular weight of 67, 500, in good agreement with estimates from other methods, is suggested.