Fig. 3.

Dry-down reactions of glc and Lys produce primarily linear oligomers by amidation at the α-amine rather than the side-chain ε-amine. The ¹H NMR spectra of (A) a 5:1 mixture of glc and Lys monomers, before the dry-down reaction, with blue (ε-resonance) and red (α-resonance) highlighting; (B) a synthetic standard of Lys acylated with glc at the ε-amine demonstrating the downfield shift of the ε-resonance; (C) a synthetic standard of Lys acylated with glc at the α-amine; and (D) dry-down of a 5:1 mixture of glc and Lys at 85 °C for 7 d. Upon α-amidation of Lys with glc, the Lys α-proton resonance shifted downfield to 4.43 ppm, and the ε-proton resonance shifted slightly upfield. An additional Lys resonance observed at 4.62 ppm corresponds to Lys species that are both α-amidated and esterified, as confirmed by comparison with a glc-Lys(-α)-glc standard. The colored envelopes in D indicate species that are either amidated (am) or not (free) at α or ε positions. An asterisk indicates all states, including monomer and all possible oligomeric species.