Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan.
School of Life Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan.
J Phys Chem A. 2022 Sep 22;126(37):6387-6394. doi: 10.1021/acs.jpca.2c03789. Epub 2022 Sep 13.
Two diastereomers of the protonated diketopiperazine (DKP) dipeptide cyclo(Tyr-Tyr), namely, cyclo(LTyr-LTyr)H and cyclo(LTyr-DTyr)H, are studied in a cryogenic ion trap by means of IR photodissociation spectroscopy combined with quantum chemical calculations. The two diastereomers have similar structures in which one of the rings is folded over the DKP ring and the other one is extended in a trans geometry, allowing a strong OH···π interaction to take place. This contrasts to the observation of a stacked geometry for neutral cyclo(LTyr-LTyr) only under supersonic expansion conditions that do not exist for cyclo(LTyr-DTyr). In the protonated form, the strength of the OH···π interaction is different for the two diastereomers, resulting in a ∼110 cm difference in the ν(OH) frequency and a smaller but clearly identifiable difference in the protonated amide ν(NH) frequency. Stereochemical effects are therefore still evidenced despite the strong perturbation due to the excess charge.
两个质子化二酮哌嗪（DKP）二肽环（Tyr-Tyr）的非对映异构体，即环（LTyr-LTyr）H 和环（LTyr-DTyr）H，在低温离子阱中通过红外光解光谱结合量子化学计算进行了研究。这两种非对映异构体具有相似的结构，其中一个环折叠在 DKP 环上，另一个环以反式几何形状伸展，允许发生强烈的 OH···π 相互作用。这与中性环（LTyr-LTyr）仅在超音速膨胀条件下观察到堆叠几何形状形成鲜明对比，而在环（LTyr-DTyr）中则不存在这种条件。在质子化形式中，两种非对映异构体中 OH···π 相互作用的强度不同，导致 ν（OH）频率相差约 110 cm，质子化酰胺 ν（NH）频率也存在较小但可明显识别的差异。因此，尽管由于过剩电荷而受到强烈干扰，但仍存在立体化学效应。
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