Variations in the turn-forming characteristics of N-acyl proline units.
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 gamma-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 beta-turn formation) in end-blocked Pro-Gly dipeptides.
Liang
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GB
S. M. McElvain Laboratory of Organic Chemistry, Department of Chemistry, University of Wisconsin, Madison 53706.
Rito
C J
CJ
Gellman
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SH
D004151:Dipeptides / Q000737:chemistry; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D011392:Proline / Q000031:analogs & derivatives* / Q000737:chemistry; D011487:Protein Conformation; D013055:Spectrophotometry, Infrared; D013696:Temperature
D004151:Dipeptides; C057749:prolylglycine; D011392:Proline