We investigate the UV absorption spectra of a series of cationic GxG peptides (where x denotes a guest residue) in aqueous solution and find that only a subset of these spectra show a strong dependence with temperature. To explore whether or not this observation reflects conformational dependencies, we carry out time-dependent density functional calculations for the polyproline II (pPII) and β-strand conformations in implicit and explicit water. We find that the calculated CD spectra for pPII can qualitatively account for the experimental spectra irrespective of the water model. The β-strand UV-CD spectra, however, require the explicit consideration of water. Contrary to conventional wisdom, we find that both the NV₁ and NV₂ band are the envelopes of contributions from multiple transitions that involve more than just the HOMOs and LUMOs of the peptide groups. A natural transition orbital analysis reveals that some of the transitions have a charge-transfer character. The overall manifold of transitions depends on the peptide’s backbone conformation, peptide hydration, and side chain of the guest residue. Our results reveal that peptide groups, side chains, and hydration shells must be considered as an entity for a physically valid characterization of UV absorbance and circular dichroism.

中文翻译：

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寡肽的水介导电子结构，通过其紫外圆二色性，吸收光谱和随时间变化的DFT计算进行探测

我们研究了水溶液中一系列阳离子G x G肽（其中x表示客体残基）的紫外线吸收光谱，发现这些光谱中只有一部分显示出对温度的强烈依赖性。为了探索该观察结果是否反映构象依赖性，我们对隐性和显性水中的脯氨酸II（pPII）和β链构象进行了时间依赖性密度函数计算。我们发现，无论水模型如何，计算出的pPII的CD光谱都可以定性地说明实验光谱。然而，β链UV-CD光谱需要明确考虑水。与传统观点相反，我们发现NV ₁和NV ₂带是来自多个转变的贡献的包络，这些转变不仅涉及肽组的HOMO和LUMO。自然过渡轨道分析表明，某些过渡具有电荷转移特性。过渡的总流形取决于肽的主链构象，肽水合和来宾残基的侧链。我们的结果表明，肽基团，侧链和水合壳必须被视为对紫外线吸收和圆二色性进行物理有效表征的实体。