Wang Xiaocong, Woods Robert J
Complex Carbohydrate Research Center, University of Georgia, Athens, GA, 30602, USA.
J Biomol NMR. 2016 Apr;64(4):291-305. doi: 10.1007/s10858-016-0028-y. Epub 2016 Mar 12.
A two-state model is commonly used for interpreting ring conformations of furanoses based on NMR scalar (3) J-coupling constants, with the ring populating relatively narrow distributions in the North and the South of the pseudorotation itinerary. The validity of this simple approach has been questioned, and is examined here in detail employing molecular dynamics (MD) simulations with a new GLYCAM force field parameter set for furanoses. Theoretical (3) J-coupling constants derived from unrestrained MD simulations with the new furanose-specific parameters agreed with the experimental coupling constants to within 1 Hz on average. The results confirm that a two state model is a reasonable description for the ring conformation in the majority of methyl furanosides. However, in the case of methyl α-D-arabinofuranoside the ring populates a continuum of states from North to South via the eastern side of the pseudorotational itinerary. Two key properties are responsible for these differences. Firstly, East and West regions in β- and α-anomers, respectively, are destabilized by the absence of the anomeric effect. And, secondly, East or West conformations can be further destabilized by repulsive interactions among vicinal hydroxyl groups and ring oxygen atoms when the vicinal hydroxyl groups are in syn-configurations (such as in ribose and lyxose) more so than when in anti (arabinose, xylose).
基于核磁共振标量（3）J耦合常数，双态模型通常用于解释呋喃糖的环构象，在假旋转行程的北部和南部，环具有相对较窄的分布。这种简单方法的有效性受到了质疑，本文使用针对呋喃糖的新GLYCAM力场参数集进行分子动力学（MD）模拟对此进行了详细研究。使用新的呋喃糖特异性参数从无约束MD模拟中得出的理论（3）J耦合常数与实验耦合常数平均相差在1赫兹以内。结果证实，双态模型对于大多数甲基呋喃糖苷中的环构象是一种合理的描述。然而，对于甲基α-D-阿拉伯呋喃糖苷，环通过假旋转行程的东侧从北部到南部占据一系列连续的状态。有两个关键特性导致了这些差异。首先，β-和α-端基异构体中的东部和西部分别由于缺乏端基异构效应而不稳定。其次，当邻位羟基处于顺式构型（如核糖和来苏糖）时，邻位羟基与环氧原子之间的排斥相互作用比处于反式（阿拉伯糖、木糖）时更能使东部或西部构象进一步不稳定。
