Reductive methylation of proteins with sodium cyanoborohydride. Identification, suppression and possible uses of N-cyanomethyl by-products

Reductive methylation of protein amino groups with formaldehyde and sodium cyanoborohydride is shown to give up to 25% yield of N-cyanomethyl (-CH2CN) product; on work up of the reaction this is hydrolysed back to starting amine, lowering the methylation yield. Addition of metal ions such as Ni2+, which complex with free cyanide ion, improve reductive methylation yields by suppressing by-product formation. The N-cyanomethyl group itself, produced in good yield when cyanide ion replaces cyanoborohydride, may have some value as a reversible modifier of amino groups in proteins.

Acetaldehyde--enkephalins: structure proof and some conformational deductions from one- and two-dimensional proton nuclear magnetic resonance spectra

The structure of the adduct formed by reaction of acetaldehyde and Met5-enkephalin has been determined by analysis of 400-MHz proton spectra: two-dimensional J spectroscopy was used to resolve and measure virtually all the overlapping resonances, and decoupling difference spectroscopy was used to assign the resonances. Suitable manipulation of the two-dimensional data allowed analysis of alpha-CH resonances which were completely buried under a water signal and of amide NH resonances which overlapped in both dimensions. The adduct was shown to be a mixture of two diastereoisomers, each containing a 2-methylimidazolidin-4-one ring formed by condensation of an acetaldehyde molecule with the N-terminal amino group and Gly2 amide nitrogen. Analysis of the NMR data suggests that the folded conformation characteristic of native enkephalins in dimethyl-d6 sulfoxide is not important in these derivatives.