The intrinsic structures of biomolecules in the gas phase may not reflect their native solution geometries. Microsolvation of the molecules bridges the two environments, enabling a tracking of molecular structural changes upon hydration at the atomistic level. We employ density functional calculations to compute a large pool of structures and vibrational spectra for a gas-phase complex, in which a doubly protonated decapeptide, gramicidin S, is solvated by two water molecules. Though most vibrations of this large complex are treated in a harmonic approximation, the water molecules and the vibrations of the host ion coupled to them are locally described by a quantum mechanical vibrational self-consistent field theory with second-order perturbation correction (VSCF-PT2). Guided and validated by the available cold ion spectroscopy data, the computational analysis identifies structures of the three experimentally observed conformers of the complex. They, mainly, differ by the hydration sites, of which the one at the Orn side chain is the most important for reshaping the peptide toward its native structure. The study demonstrates the ability of a quantum chemistry approach that intelligently combines the semiempirical and ab initio computations to disentangle a complex interplay of intra- and intermolecular hydrogen bonds in large molecular systems.

中文翻译：

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十水合的十肽：通过理论确定并通过冷离子光谱验证的构象异构体

气相中生物分子的固有结构可能无法反映其天然溶液的几何形状。分子的微溶剂化在两个环境之间架起了桥梁，使人们能够在原子级水合时跟踪分子结构的变化。我们使用密度泛函计算来计算气相复合物的大量结构和振动光谱，其中双质子化的十肽，短杆菌肽S被两个水分子溶剂化。尽管此大分子复合物的大多数振动都以谐波近似处理，但水分子和与其耦合的主体离子的振动是通过具有二阶微扰校正（VSCF-PT2）的量子机械振动自洽场理论来局部描述的）。由可用的冷离子光谱数据指导和验证，计算分析确定了该复合物三个实验观察到的构象异构体的结构。它们的主要区别在于水合位点，其中Orn侧链的水合位点对于将肽重塑成其天然结构最重要。这项研究证明了量子化学方法的能力，该方法可以智能地结合半经验和从头算起，即可解开大分子系统中分子内和分子间氢键的复杂相互作用。