Tyrosine-containing cyclic dipeptides based on a diketopiperazine (DKP) ring are studied under jet-cooled conditions using resonance-enhanced multi-photon ionisation (REMPI), conformer-selective IR-UV double resonance vibrational spectroscopy and quantum chemical calculations. The conformational landscape of the dipeptide containing natural L tyrosine (Tyr), namely c-LTyr–LTyr strongly differs from that of its diastereomer c-LTyr–DTyr. A similar family of conformers exists in both systems, with one aromatic ring folded on the dipeptide DKP ring and the other one extended. Weak NH⋯π and CH⋯π interactions are observed, which are slightly different in c-LTyr–LTyr and c-LTyr–DTyr. These structures are identical to those of LL and LD cyclo diphenylalanine, which only differ from c-Tyr–Tyr by the absence of hydroxyl on the benzene rings. While this is the only conformation observed for c-LTyr–DTyr, c-LTyr–LTyr exhibits an additional form stabilised by the interaction of the two hydroxyls, in which the two aromatic rings are in a stacked geometry. Stereochemical effects are still visible in the radical cation, for which one structure is observed for c-LTyr–DTyr, while the spectrum of the c-LTyr–LTyr radical cation is explained in terms of two co-existing structures.

中文翻译：

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依赖立体化学的氢键可稳定喷射冷却的环状二肽中的堆叠构象：（LD）vs.（LL）环酪氨酸-酪氨酸

基于二酮哌嗪（DKP）环的含酪氨酸的环状二肽在射流冷却条件下使用共振增强多光子电离（REMPI），构象选择性IR-UV双共振振动光谱和量子化学计算进行了研究。含有天然L酪氨酸（Tyr）的二肽，即c-LTyr-LTyr的构象结构与其非对映体c-LTyr-DTyr的构象结构存在很大差异。两个系统中都存在类似的构象异构体家族，一个芳香环折叠在二肽DKP环上，另一个延伸。观察到弱的NH⋯π和CH⋯π相互作用，在c-LTyr–LTyr和c-LTyr–DTyr中略有不同。这些结构与LL和LD环二苯丙氨酸的结构相同，仅与c-Tyr–Tyr不同，因为苯环上没有羟基。尽管这是观察到的c-LTyr–DTyr的唯一构象，但c-LTyr–LTyr表现出另一种形式，该形式通过两个羟基的相互作用而稳定，其中两个芳香环呈堆叠的几何形状。立体化学效应仍然在自由基阳离子中可见，对于c-LTyr–DTyr观察到一种结构，而c-LTyr–LTyr自由基阳离子的光谱则根据两种共存结构进行了解释。