Chemical modification of lyophilized proteins in nonaqueous environments

Lyophilized proteins were reacted in vacuo with a volatile reagent or dispersed in octane and reacted with dissolved reagent. Three novel derivatives were formed with iodomethane: (a) quaternized trimethyl amino groups, (b) N1,N3-dimethylimidazolium cation, and (c) phenolic O-methyl ether. Acid anhydrides acylated amino groups and formed mixed anhydrides with side-chain carboxyl groups. Under nonaqueous conditions it was observed that: (i) The same derivatives are formed as under aqueous conditions. (ii) Hydrolytic breakdown of protein is prevented. (iii) Less reagent is required. (iv) Unreacted reagent can be recovered. (v) Water-labile derivatives can be isolated as stable intermediates. (vi) The yield of a derivatized functional group was directly related to its pK(a), its surface exposure, and the pH of the solution from which the protein was lyophilized. (vii) The physicochemical factors governing the reactivity of protein functional groups in nonaqueous environments appear to reflect the protein solution structure prior to lyophilization.