Site-specific synthesis of Amadori-modified peptides on solid phase.

Glycation of peptides and proteins is a slow chemical reaction of reducing sugars modifying the amino groups. The first intermediates of this nonenzymatic glycosylation are the Amadori products that can undergo further chemical reactions, finally leading to advanced glycation end products (AGEs). The formation of AGEs was not only linked to aging of tissues and organs in general but also to several diseases such as diabetes mellitus and Alzheimer's disease. Because of the importance of these modifications and their potential use as diagnostic markers, a global postsynthetic approach on solid phase was developed. The peptides were synthesized by Fmoc/(t)Bu-chemistry, with the lysine residue to be modified being protected with the very acid-labile methyltrityl group. Incubation of the peptides with D-glucose in DMF at elevated temperatures resulted in product yields of 35%. Neighboring residues with bulky protecting groups reduced the yields only slightly. The major by-products were the unmodified peptide and an oxidation product. Whereas the unmodified peptide eluted before the glycated peptide, all other by-products eluted later in RP-HPLC, allowing simple purification.