Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV-EHU), Apartado 644, 48080 Bilbao, Spain.
J Am Chem Soc. 2013 Feb 20;135(7):2845-52. doi: 10.1021/ja312393m. Epub 2013 Feb 7.
Fructose has been examined under isolation conditions using a combination of UV ultrafast laser vaporization and Fourier-transform microwave (FT-MW) spectroscopy. The rotational spectra for the parent, all (six) monosubstituted (13)C species, and two single D species reveal unambiguously that the free hexoketose is conformationally locked in a single dominant β-pyranose structure. This six-membered-chair skeleton adopts a (2)C(5) configuration (equivalent to (1)C(4) in aldoses). The free-molecule structure sharply contrasts with the furanose form observed in biochemically relevant polysaccharides, like sucrose. The structure of free fructose has been determined experimentally using substitution and effective structures. The enhanced stability of the observed conformation is primarily attributed to a cooperative network of five intramolecular O-H···O hydrogen bonds and stabilization of both endo and exo anomeric effects. Breaking a single intramolecular hydrogen bond destabilizes the free molecule by more than 10 kJ mol(-1). The structural results are compared to ribose, recently examined with rotational resolution, where six different conformations coexist with similar conformational energies. In addition, several DFT and ab initio methods and basis sets are benchmarked with the experimental data.
果糖在分离条件下使用组合的紫外超快激光蒸发和傅里叶变换微波（FT-MW）光谱进行了研究。母体、所有（六个）单取代（13）C 物种以及两个单 D 物种的旋转光谱明确表明，游离己酮糖在单一主导的β-吡喃糖结构中被构象锁定。这种六元椅骨架采用（2）C（5）构型（与醛糖中的（1）C（4）等效）。游离分子结构与在生物化学相关多糖（如蔗糖）中观察到的呋喃糖形式形成鲜明对比。使用取代和有效结构实验确定了游离果糖的结构。观察到的构象的增强稳定性主要归因于五个分子内 O-H···O 氢键的协同网络以及内消旋和外消旋效应的稳定化。打破单个分子内氢键会使游离分子不稳定超过 10 kJ mol(-1)。结构结果与核糖进行了比较，核糖最近也通过旋转分辨率进行了研究，其中六个不同的构象以相似的构象能共存。此外，还使用几种 DFT 和从头算方法和基组与实验数据进行了基准测试。
