Hydration of proteins: SAXS study of native and methoxy polyethyleneglycol (mPEG)-modified L-asparaginase and bovine serum albumin in mPEG solutions

Two mPEG-modified globular proteins [mPEG: methoxy poly(ethylene glycol)], and their native unmodified forms, were examined by small-angle x-ray scattering to evaluate the extent of their surface hydration. The effects of free and protein-bound mPEG on the hydration shell were modeled with discrete electron density profiles. We show that an mPEG-depleted layer can account for the decrease in the measured radius of gyration Rg from 34.1 to 31.1 (angstrom) in native L-asparaginase, and from 32.4 to 31.0 (angstrom) in native bovine serum albumin (BSA) in mPEG-containing solvents. For mPEG-modified proteins in mPEG-free solvents, we attribute the observed increase in the Rg over that of the native proteins (~3% in L-asparaginase, and 10% in BSA) to the presence of mPEG on the protein surface. The Rg of the mPEG-modified proteins in mPEG solutions generally decrease with mPEG concentration. Relative to the corresponding unmodified protein, this decrease in Rg is much larger in BSA (from 35.6 to 31.2 (angstrom)) but much smaller (from 34.9 to 34.3 (angstrom)) in L-asparaginase. From these studies, the thickness of the hydration layer around L-asparaginase and BSA is estimated to be ~15 (angstrom). Exclusion of polyols from the protein domain could be related to the presence of the hydration shell around the protein.