Experimental and Quantum Chemical Modeling Studies of the Interactions of l‑Phenylalanine with Divalent Transition Metal Cations

Encoded by the UUU and UUC codons of the genetic code, l-phenylalanine (LPA) serves as an important precursor for tyrosine and various other compounds that are necessary to support life on earth. Here, we report the synthesis (both in solid and solvent phases) and characterization of the Ni²⁺, Cu²⁺, and Zn²⁺ complexes of LPA by several analytical, spectral, thermal, and electrochemical techniques. The results reveal that the products formed by following the two synthetic approaches are the same, and the metal ions bind to the LPA molecules in a 1:2 molar ratio (M⁺²/LPA). Complementary geometries of the metal complexes are modeled involving the most predominant LPA conformers predicted at the MP2/6-311++G­(d,p) level. The gaseous and aqueous phase interaction enthalpies and free energies; theoretical IR and UV–vis spectra; HOMO–LUMO energy gaps; dipole moments; Wiberg bond indices as well as the partial atomic charges in LPA and its metallic complexes are calculated and evaluated using B3LYP/6-311++G­(d,p) as the main computational method. This study also incorporates analyses on the efficacy of the DFT-D2 level in describing dispersion contributions, performance of the BHandHLYP functional for the open-shell Cu²⁺-LPA system, and relative metal binding affinities of the singlet versus triplet states of the Ni²⁺-LPA complex. Metal−π interactions established via the aromatic side chain of LPA add to the thermodynamic stability of the complexes, whereas metal coordination induces considerable intrinsic structural rearrangements in the molecular geometry of LPA. The LPA binding affinity order of the three Lewis acids investigated emerges as Cu²⁺ > Ni²⁺ > Zn²⁺, paralleling the Irving–Williams series. The illustrative evidence offered by the present work suggests that the B3LYP/6-311++G­(d,p) level in combination with an empirical dispersion-correction term performs well in describing the vibrational frequencies and cation−π interactions, which are undoubtedly of immense significance for natural sciences

Amberlyst A21 Catalyzed Chromatography-Free Method for Multicomponent Synthesis of Dihydropyrano[2,3‑<i>c</i>]pyrazoles in Ethanol

Amberlyst A21 was found to be an extremely efficient catalyst for synthesis of a series of 6-amino-4-alkyl/aryl-3-methyl-2,4-dihydropyrano­[2,3-<i>c</i>]­pyrazole-carbonitriles by a four-component reaction of a mixture of ethyl acetoacetate, hydrazine hydrate, aldehyde, and malononitrile in ethanol at room temperature. The catalytic efficiency of Amberlyst A21 was compared with some other resin-bound free and anionic bases in order to ascertain the best catalyst for the said conversion. The catalyst was found to work extremely well also for acyclic/cyclic ketones to give corresponding dihydropyrano­[2,3-<i>c</i>]­pyrazoles or their spirocyclic variants. Easy recovery of the catalyst and its reusability, room temperature reaction conditions, short reaction time, excellent yields, no chromatographic purification, and evasion of environmentally hazardous solvents in the entire reaction process may make this protocol very useful for academia and industry

Metalation of Glycylglycine: An Experimental Study Performed in Tandem with Theoretical Calculations

Interactions of the Ni²⁺, Cu²⁺, and Zn²⁺ ions with the simplest dipeptide glycylglycine (GlyGly) are explored using various experimental and computational techniques. Solid and aqueous phase syntheses of the metalated GlyGly complexes (by solid-state grinding and by coprecipitation respectively) lead to the same products, as confirmed by physicochemical and spectral properties which indicate metal-coordination through the −NH₂ and −CO₂^– groups of the dipeptide. Phase-diagram and kinetic studies of the solid-phase reaction between GlyGly and copper acetate suggest that complexation occurs in 1:2 (metal/ligand) stoichiometry via a facile kinetic pathway (a barrier of only 22.22 kJ/mol). The right-handed α-helical conformer of GlyGly is considered in DFT modeling studies in gas and aqueous phases elucidating the effects of metalation and solvation upon structural, electronic, and vibrational properties of the complexes. The complexes are found to follow the stability order Cu²⁺ > Ni²⁺ > Zn²⁺ corroborating the Irving-Williams series. The Ni­(GlyGly)₂ complex is predicted to exist in its low-spin state. Hydration effects on structural aspects of the complexes are also investigated computationally. The BHandHLYP/6-311++G­(d,p) level describes the Cu­(GlyGly)₂ complex more efficiently than the B3LYP/6-311++G­(d,p) level (which, however, better predicts the vibrational spectra of the systems). Absorption titration experiments with calf thymus DNA together with in silico docking and molecular mechanical studies reveal that these metal–dipeptide complexes are DNA minor-groove binders primarily through H-bonding interactions, yielding a DNA-binding affinity order of Ni²⁺ > Zn²⁺ > Cu²⁺