Bovine serum albumin observed by infrared spectrometry. I. Methodology, structural investigation, and water uptake

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Abstract
The results of preliminary infrared (IR) spectrometry experiments on bovine serum albumin (BSA) films are presented. An analysis of spectral variations due to raising the temperature and deuteration of N--H groups leads to the assignment of most IR bands of BSA. From this analysis we furthermore deduce that at 115 degrees C only hydrogen bonds established by N&bond;H groups on the still present H(2)O molecules, which are so strongly bound to the protein that they do not evaporate, are weakened, some of which are broken. These N--H...OH(2) groups represent some 5% of all N--H groups in the dried protein. Spectral changes due to hydration by water vapor are also analyzed and a precise method to measure the water-vapor pressure of the atmosphere surrounding the BSA film, or equivalently the relative humidity, is described. Various procedures to measure the number of H(2)O molecules embedded in BSA are then presented and evaluated. One of them is selected as the best one for proteins, because it matches previous measurements based on gravimetric methods. This procedure is subsequently used in a study that is devoted to the determination of the various hydrogen-bond configurations, or interaction configurations, which are adopted by H(2)O molecules during the various steps of hydration of BSA. This first analysis of hydration spectra allows the completion of the assignment of IR bands. The various spectral components of the amide I band, which are interchanged during the hydration process, cannot be assigned to various secondary structures, as is usually proposed. It suggests that this usual assignment should be used with care, especially by taking into account the state of hydration, when one wishes to obtain structural information from it.