Variations in the turn‐forming characteristics of N‐Acyl proline units

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Abstract
We have examined intramolecular hydrogen bonding in four homologous compounds, N‐acetyl‐, N‐propionyl‐, N‐i‐butyryl‐, and N‐pivaloyl‐proline‐methylamide, in methylene chloride, by means of 1H‐nmr and ir measurements. At room temperature, the major trans conformer of MeCO‐Pro‐NHMe appears to be approximately 68% intramolecularly hydrogen bonded, the trans conformers of EtCO‐Pro‐NHMe and i‐PrCO‐Pro‐NHMe are approximately 75% intramolecularly hydrogen bonded, and t‐BuCO‐Pro‐NHMe is approximately 50% intramolecularly hydrogen bonded. Thus, the internally hydrogen‐bonded state (C7 or γ‐turn) is significantly less populated for the N‐pivaloyl compound than for the other three molecules in this series. Variable temperature measurements indicate that for each proline derivative there is very little enthalpic difference between the intramolecularly hydrogen‐bonded and nonhydrogen bonded states of the trans rotamer. Changing the N‐terminal acyl group also affects intramolecular hydrogen bonding (including β‐turn formation) in end‐blocked Pro‐Gly dipeptides.