We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet–infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S₁ state and the vibrationally excited triplet state(s) formed by intersystem crossing, T_n(v). Photofragment mass spectra are recorded out of the S₁ origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH₃ loss mediated by electron transfer. In YGGFL, IR excitation in the S₁ state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH₃⁺ group leading to loss of neutral NH₃. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH₃⁺. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S₁ and T_n(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S₁ or T_n(v) states fragments the peptide backbone exclusively at amide(4), producing the b₄ cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.

中文翻译：

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沿着质子化肽支架的位点特异性光化学

我们对两种质子化五肽亮氨酸脑啡肽（酪氨酸-甘氨酸-甘氨酸-苯丙氨酸-亮氨酸，YGGFL）及其发色团交换类似物 FGGYL 的已知构象的时间依赖性光物理和光化学进行了详细研究，该研究是在低温下进行的-气相冷却条件。利用紫外-红外 (UV-IR) 双共振，我们记录激发态红外光谱作为紫外和红外脉冲之间时间延迟的函数。我们确定了由于 S _{1态和系间窜越 T n} (v)形成的振动激发三重态而产生的独特 Tyr OH 拉伸跃迁。从 S ₁原点并遵循 UV-IR 双共振记录光碎片质谱。发现了几种竞争性位点特异性断裂途径，涉及肽主链裂解、Tyr 侧链丢失和电子转移介导的N 末端 NH ₃丢失。在YGGFL中，S ₁态的IR激发促进电子转移(ET)从芳环到N端R-NH ₃ ⁺基团，导致中性NH ₃的损失。由于 ET 从 Y(4) 到 NH ₃ ⁺的距离较大，因此 FGGYL 中缺少该产物通道。 Tyr 侧链的选择性损失发生在 UV 激发后的激发态过程中，并通过YGGFL 和 FGGYL 的S ₁​​ 和 T _n (v) 状态下的 IR 激发进一步增强。最后，S ₁或 T _n (v) 状态下的红外激发仅在酰胺 (4) 处断裂肽主链，产生 b ₄阳离子。我们假设这种选择性碎片是由系间穿越产生的，产生具有足够能量的振动激发三重态，以沿着已知起始结构中存在的质子管道发射质子。