Stereochemistry of peptides containing 1-aminocyclopentanecarboxylic acid (Acc<SUP>5</SUP>): solution and solid-state conformations of Boc-Acc<SUP>5</SUP>-Acc<SUP>5</SUP>-NHMe

The conformational analysis of a protected homodipeptide of 1-aminocyclopentanecarboxylic acid (Acc<SUP>5</SUP>) has been carried out. <SUP>1</SUP>H-nmr studies establish a &#946;-turn conformation for Boc-Acc<SUP>5</SUP>-Acc<SUP>5</SUP>-NHMe in chloroform and dimethylsulfoxide solutions involving the methylamide NH in an intramolecular hydrogen bond. Supportive evidence for the formation of an intramolecular hydrogen bond is obtained from ir studies. X-ray diffraction studies reveal a type III &#946;-turn conformation in the solid state stabilized by a 4 &#8594;1 hydrogen bond between the Boc CO and methylamide NH groups. The &#934;, &#968; values for both Acc<SUP>5</SUP> residues are close to those expected for an ideal 3<SUB>10</SUB>-helical conformation ( &#934;&#x2243; &#177; 60&#176;, &#936;&#x223C; &#177; 30&#176;)