A galactose-, 1, and a cellobiose derivative, 2, have been site selectively, post-translationally, incorporated into a folded helix-loop-helix dimer LA-42b in a one step reaction at room temperature. The structural effects on the folded peptide upon glycosylation have been studied by CD and NMR spectroscopy. The negative value of the mean residue ellipticity of the folded peptide, LA-42b, was raised from -19000 +/- 1000 to -21200 +/- 1000 deg cm(2) dmol(-1) upon introduction of the galactose derivative and to -19500 +/- 1000 deg cm(2) dmol(-1) upon introduction of the cellobiose derivative, showing that the helical content was increased. The dissociation constant of the dimer decreased from 120 to 30 mu M upon glycosylation. The introduction of 1 into GTD-C, a folded helix-loop-helix dimer with a well defined tertiary structure, had little structural impact. Glycosylation stabilises the folded structure of proteins with partially exposed hydrophobic cores but has little effect on well-packed proteins.