Conformation of Cyclo-(L-Threonine) sub(2) and Cyclo-(L-Allothreonine) sub(2) A Proton and Carbon N.M.R. Study

The diketopiperazines cyclo-L-Thr) sub(2) and cyclo-(L-allo Thr) sub(2) in water and in dimethyl sulfoxide were studied by proton and carbon-13 nuclear magnetic resonance, and the dominant conformations were deduced from proton-proton and proton-carbon coupling constants. In cyclo-(L-Thr) sub(2) the X super(1) = 60 degree , hydorxyl over the ring, side chain conformation is favored; this conformation is also favored for cyclo-(L-Ser) sub(2) and and cyclo-(L-Ser-D-Ser). However, the important side chain conformation for cyclo-(L-allo Thr) sub(2) is X super(1) = -60 degree , methyl group over the diketopiperazine ring. The determining factors are apparently steric. The diketopiperazine ring of cyclo-(L-Thr) sub(2) is puckered to hold the side chains more nearly axial than is that of cyclo-(L-allo Thr) sub(2), although the degree of ring folding is probably not large.