Structure of the alanine dipeptide in condensed phases determined by 13C NMR

We report solution-state and solid-state C-13 NMR data for the alanine dipeptide (N-acetyl-L-alanine-N'-methylamide, AcAlaNHMe) in polycrystalline, lyophilized, and solvated states. Changes in the C-13 chemical shifts of individual carbonyl carbons between the solvated and lyophilized states for the alanine dipeptide dissolved in H2O and CHCl3 suggest preferential solvation at different carbonyl carbons. This effect depends on the solvent, and it suggests an alteration in secondary structure on removal of water. We employ a novel external referencing scheme (neat tetramethylsilane under magic-angle spinning), which allows for a direct comparison of solution- and solid-state chemical shifts. The structure of the alanine dipeptide in the crystalline state and after lyophilization out of H2O and CHCl3 solutions was determined using double-quantum dipolar recoupling solid-state NMR at high magnetic field strength (600 MHz). The compound adopts the same polyproline-II-like secondary structure when lyophilized from hydrogen-bonding (H2O) and non-hydrogen-bonding (CHCl3) solvents.