In situ monitoring of proteins during lyophilization using micro-Raman spectroscopy: a description of structural changes induced by dehydration.

Raman investigations were carried out in situ in real time during the lyophilization of three proteins (β-lactoglobulin, bovine serum albumin, and chymotrypsinogen) characterized by different structural properties. Structural changes in the proteins were only and systematically detected after the primary drying step of the lyophilization, through a frequency shift and a general broadening of amide I and III bands. These spectral changes have been interpreted in terms of local disordering related to the distortion of the structural elements induced by ice desorption. Structural changes of the secondary structure were found almost reversible upon rehydration, whereas changes in the solvent accessibility to protein residues are detected and related to the alteration of the tertiary and/or quaternary structures. The influence of the lyophilization parameters, corresponding to different stress conditions, on the degree of protein denaturation has been analyzed.