Lysyl residues of rapeseed napin (2S) and cruciferin (12S) were acylated and sulfamidated by means of anhydrides and sulfonyl chlorides, respectively. The secondary and tertiary structures as well as the surface hydrophobicity of the modified proteins were studied using circular dichroism, intrinsic fluorescence, and binding of anilinonaphthalenesulfonic acid. The results showed clearly that grafting of hydrophobic chains induced different structural modifications and surface hydrophobicities on the monomeric (2S) and on the hexameric (12S) proteins. Thus, the original structure of the 2S modified protein seemed to be preserved. Therefore, the surface hydrophobicity increased proportionally with the number of groups grafted. Conversely, after modification, 12S was shown to be expanded. As a result, hydrophobic regions were exposed, leading to a much greater hydrophobization of the protein surface. Acylation and sulfamidation appeared, therefore, to be good methods to hydrophobize efficiently the surface of the two proteins and thus might probably induce new functional properties.

Bovine serum albumin was chosen as a model protein to study the effect of the functionalization of the epsilon-NH2 of lysine residues with different carbon chains on the physical properties of proteins. Thus, BSA has been acylated and sulfonylated by means of anhydrides and sulfonyl chlorides, respectively. The secondary structures of modified BSA, studied by far-UV CD, showed very slight changes except after sulfamidation. However, near-UV CD and intrinsic fluorescence spectra revealed important conformational perturbations for proteins bearing long carbon chains. Furthermore, the binding of an apolar probe (ANS) to BSA revealed an improvement of surface hydrophobicity after modification. Meanwhile, Scatchard plot results indicate that only 20% of the hexanoyl carbon chains lie at the surface of the proteins. Solvent conditions should influence the exposure of these chains and consequently the surface hydrophobicity of proteins.

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