Gas phase infrared dissociation spectra of the radical cation, deprotonated and protonated forms of the hormone melatonin, and its complexes with alkali (Li⁺, Na⁺, and K⁺) and alkaline earth metal ions (Mg²⁺, Ca²⁺, and Sr²⁺) are measured in the spectral range 800–1800 cm⁻¹. Minimum energy geometries calculated at the B3LYP/LACVP++** level are used to assign structural motifs to absorption bands in the experimental spectra. The melatonin anion is deprotonated at the indole-N. The indole-C linking the amide chain is the most favored protonation site. Comparisons between the experimental and calculated spectra for alkali and alkaline earth metal ion complexes reveal that the metal ions interact similarly with the amide and methoxy oxygen atoms. The amide I band undergoes a red shift with increasing charge density of the metal ion and the amide II band shows a concomitant blue shift. Another binding motif in which the metal ions interact with the amide-O and the π-electron cloud of the aromatic group is identified but is higher in energy by at least 18 kJ/mol. Melatonin is deprotonated at the amide-N with Mg²⁺ and the metal ion coordinates to the amide-N and an indole-C or the methoxy-O. These results provide information about the intrinsic binding of metal ions to melatonin and combined with future studies on solvated melatonin-metal ion complexes may help elucidate the solvent effects on metal ion binding in solution and the biochemistry of melatonin. These results also serve as benchmarks for future theoretical studies on melatonin-metal ion interactions.

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褪黑激素的自​​由基阳离子，去质子和质子化形式及其与碱金属（Li ⁺，Na ⁺和K ⁺）和碱土金属离子（Mg 2 ⁺，Ca ²⁺和Sr ²⁺）在800–1800 cm ^-1的光谱范围内测量。在B3LYP / LACVP ++ **级别计算的最小能量几何用于将结构图案分配给实验光谱中的吸收带。褪黑激素阴离子在吲哚-N处被去质子化。连接酰胺链的吲哚-C是最优选的质子化位点。碱金属和碱土金属离子配合物的实验光谱和计算光谱之间的比较表明，金属离子与酰胺和甲氧基氧原子的相互作用相似。酰胺I谱带随金属离子电荷密度的增加而发生红移，而酰胺II谱带则伴有蓝移。鉴定出另一个结合基序，其中金属离子与酰胺基-O和芳族基团的π电子云相互作用，但是其能量至少高出18kJ / mol。褪黑激素在酰胺-N上被镁去质子化²⁺，并且金属离子与酰胺-N和吲哚-C或甲氧基-O配位。这些结果提供了有关金属离子与褪黑激素固有结合的信息，并结合对溶剂化的褪黑激素-金属离子复合物的未来研究可能有助于阐明溶剂对溶液中金属离子结合和褪黑素生物化学的影响。这些结果也可作为将来有关褪黑素与金属离子相互作用的理论研究的基准。

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