Kopple K D, Narutis V
Int J Pept Protein Res. 1981 Jul;18(1):33-40.
The diketopiperazines cyclo-(L-Thr)2 and cyclo-(L-allo Thr)2 in water and in dimethyl sulfoxide were studied by proton and carbon-13 nuclear magnetic resonance, and the dominant conformation were deduced from proton-proton and proton-carbon coupling constants. In cyclo-(L-Thr)2 the chi 1 = 60 degrees, hydroxyl over the ring, side chain conformation is favored; this conformation is also favored for cyclo-(L-Ser)2 and cyclo-(L-Ser-D-Ser). However, the important side chain conformation for cyclo-(L-allo Thr)2 is chi 1 = -60 degrees, methyl group over the diketopiperazine ring. The determining factors are apparently steric. The diketopiperazine ring of cyclo-(L-Thr)2 is puckered to hold the side chains more nearly axial than is that of cyclo-(L-allo Thr)2. although the degree of ring folding is probably not large.
通过质子和碳-13核磁共振研究了二酮哌嗪环（L-苏氨酸）₂和环（L-别苏氨酸）₂在水和二甲基亚砜中的情况，并根据质子-质子和质子-碳耦合常数推导出主要构象。在环（L-苏氨酸）₂中，χ₁ = 60°，羟基在环上方，侧链构象更受青睐；这种构象对于环（L-丝氨酸）₂和环（L-丝氨酸-D-丝氨酸）也更受青睐。然而，环（L-别苏氨酸）₂的重要侧链构象是χ₁ = -60°，甲基在二酮哌嗪环上方。决定性因素显然是空间位阻。环（L-苏氨酸）₂的二酮哌嗪环比环（L-别苏氨酸）₂的二酮哌嗪环更皱，以使侧链更接近轴向，尽管环折叠程度可能不大。
