Effect of reductive alkylation on thermal stability of ribonuclease A and chymotrypsinogen A

In order to probe changes in the structural stability induced by the introduction of hydrophobic groups into proteins, the amino groups of ribonuclease A and chymotrypsinogen A were reductively alkylated by reaction with various aliphatic aldehydes, formaldehyde, acetaldehyde, n-butylaldehyde and n-hexylaldehyde, and their thermal stabilities were investigated by differential scanning calorimetry (DSC) at different acidic pH values. Ribonuclease A was thermally unstabilized by reductive alkylation, while chymotrypsinogen A was slightly stabilized, depending on both the size of the introduced alkyl groups and the extent of modification. These observations suggest that the effects induced by alkylation involve not only steric hindrance due to the entering bulky groups but also certain other factors such as the participation of the chemically introduced alkyl groups in hydrophobic interactions.