The conformations of oligopeptides derived from palanine and co-oligomers of i.-alanine with y-methyl-L-glutamate were studied in several solvents via optical rotation and far-ultraviolet spectroscopy. Calculated values for optical rotation based on model compounds were compared with experimental values for the oligomers. In trifluoroacetic and dichloroacetic acids, the oligomers and co-oligomers exhibit rotations in close agreement with predicted values based on model compounds. Thus, in these solvents only nonhelical conformations exist. I n trifluoroethanol, the experimental points of molar rotation for the pentamer and larger oligomers no longer follow the predicted values. In addition, the benzyloxycarbonyl and acetyl cononamers show bo values of about -150, which demonstrates the presence of stable helical forms for these peptides. We also examined the molar extinction coefficients of oligopeptides in the 190 mp region and determined the values for nonhelical peptide groups, The molar extinction coefficients per amide bond for the benzyloxycarbonyl and acetyl cononamers show extensive hypochromism, once again indicating the presence of stable helices for these compounds in trifluoroethanol.
* For previous papers in this series see Goodman and
Langsam.' A. Monoalanine (Simple) Model Compounds Z-( Ma)-( Gly )-( Gly )-OEt -0.385 -0.382 Z-( Gly)-(A1a)-(G1y)-OEt -1.151 -1.29 Z-(Gly )-( G1y)-(Ala)-OEt -0.930 -0.651 B. Dialanine (Complex) Model Compounds Z-(A1a)-(A1a)-( Gly)--OEt -2.35 -2.079 Z-( Gly )-( Ala)*-( Gly )-OE t -1.86 -1.49 Z-( Gly )-( Ala)-( Ah)-OE t -1.91 -1.67 C. Alanine-Glutamate Model Compounds OMe I Z-(Gly)-(Ala)-(Glu~Gly>-OEt -I .64 -1.19 Z-(Gly)-(Gl?)-(Ale,t(Gly)-OEt -I .68 -1.24 OMe D. Glutamate Model Compounds OMe -0.661 -0.678 1 Z-( Gly)-( Glu)-( Gly)-ORlle OMe -0.538 -0.468 I Z-(Glu)-( Gly)-OM@ OMe Z-( Gly )-( Glu)*-( Gly)-OMea -1.03 -1.06 z-( G1?)-( &-( Giy)-oMe -1.14 -1.16 I OMe OMe -0.