Dracocephalum kotschyi Boiss.: Phytochemical profiling, green synthesis of silver nanoparticles, molecular docking, and antibacterial evaluation

Technical Summary: Phytochemical Profiling, Green Synthesis of Silver Nanoparticles, Molecular Docking, and Antibacterial Evaluation of Dracocephalum kotschyi Boiss.

Problem Statement and Research Gaps
Dracocephalum kotschyi Boiss., a medicinal plant native to Iran, possesses documented pharmacological properties, yet comprehensive data regarding its specific phytochemical profile and nanotechnological applications remains limited. This study addresses four specific gaps in the existing literature:

1. The absence of a comprehensive LC-MS-based phytochemical profiling of the hydromethanolic extract of this species.
2. The lack of reported green synthesis of silver nanoparticles (Ag NPs) using the aqueous extract of D. kotschyi.
3. The absence of a systematic comparative evaluation of the antibacterial activity of the essential oil, extract, and biosynthesized Ag NPs against standard bacterial strains.
4. The lack of molecular docking studies to predict the binding interactions of identified phytochemicals with specific bacterial protein targets.

Methodology
The research employed a multi-faceted approach combining phytochemical analysis, nanotechnology synthesis, biological assays, and computational modeling:

• Plant Material and Extraction: D. kotschyi was collected from Sisakht, Iran. Hydromethanolic extracts were prepared via ultrasonic-assisted extraction (UAE), while essential oils were isolated using the simultaneous distillation extraction (SDE) method.
• Phytochemical Profiling:
  • LC-MS: Used to identify phenolic compounds in the hydromethanolic extract.
  • GC-MS: Used to analyze the chemical composition of the essential oil.
• Green Synthesis and Characterization of Ag NPs: Silver nanoparticles were biosynthesized by reacting silver nitrate (AgNO₃) with the aqueous D. kotschyi extract. The resulting nanoparticles were characterized using:
  • UV-Vis spectroscopy (for Surface Plasmon Resonance).
  • FT-IR (to identify capping biomolecules).
  • XRD (to determine crystalline structure).
  • SEM/EDS (to analyze morphology, size, and elemental composition).
• Antibacterial Evaluation: Activity was tested against two Gram-positive (Bacillus subtilis, Staphylococcus aureus) and two Gram-negative (Escherichia coli, Pseudomonas aeruginosa) strains using:
  • Disk diffusion (well-diffusion) assays.
  • Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) assays.
• Molecular Docking: Computational docking was performed using AutoDock Vina to predict binding affinities of identified phenolic compounds against B. subtilis PBP4a (PDB ID: 2J9P) and P. aeruginosa FimX EAL (PDB ID: 3HV8).

Key Results

• Phytochemical Composition:
  • Extract (LC-MS): Ten phenolic compounds were identified, including gallic acid, benzoic acid, syringic acid, 3,4-dimethoxycinnamic acid, p-coumaric acid, caffeic acid, chlorogenic acid, myricetin, apigenin, and luteolin. Notably, gallic acid, benzoic acid, syringic acid, 3,4-dimethoxycinnamic acid, p-coumaric acid, and myricetin are reported for the first time in D. kotschyi.
  • Essential Oil (GC-MS): Twenty-nine compounds were identified. The major constituents were 2-Propanone,1-bicyclo[2.2.1]hept-2-yl (25.67%) and 1(7),4,8-O-Menthatriene (15.49%). The oil was dominated by oxygenated monoterpenes (58.98%) and monoterpene hydrocarbons (28.01%).
• Nanoparticle Synthesis:
  • The aqueous extract successfully reduced Ag⁺ to Ag⁰, indicated by a color change to dark brown and a UV-Vis absorption peak at ~410 nm.
  • Characterization confirmed the Ag NPs were predominantly spherical, well-dispersed, and ranged in size from 40 to 80 nm.
  • XRD confirmed a face-centered cubic (FCC) crystalline structure. FT-IR indicated that phenolic and flavonoid compounds acted as reducing and capping agents.
• Antibacterial Activity:
  • Extract: Showed the highest inhibition zone (15 mm) against B. subtilis at 1200 μg/well. It exhibited bactericidal activity (MIC = MBC = 250 μg/mL) against S. aureus and P. aeruginosa.
  • Essential Oil: Demonstrated the highest sensitivity against E. coli (MIC = MBC = 250 μg/mL) and the lowest MIC against P. aeruginosa (125 μg/mL), though its action against P. aeruginosa was bacteriostatic (MBC = 1000 μg/mL).
  • General Trend: Gram-positive bacteria were generally more susceptible to the extract, while the essential oil showed broad activity against both Gram-positive and Gram-negative strains, attributed to the hydrophobic nature of its terpenoid constituents.
• Molecular Docking:
  • Apigenin and Luteolin showed favorable binding energies (-8.2 and -8.1 kcal/mol, respectively) against B. subtilis PBP4a.
  • Chlorogenic acid exhibited the highest binding affinity (-8.2 kcal/mol) among the plant compounds against P. aeruginosa FimX EAL.
  • Hydrogen bonding was identified as the primary stabilizing interaction for these complexes.

Significance and Claims
The authors conclude that Dracocephalum kotschyi is a rich source of bioactive phenolics and terpenoids. The study establishes the plant's dual potential:

1. As a source of novel bioactive compounds, expanding the known phytochemical profile of the species.
2. As an effective agent for the green synthesis of antibacterial silver nanoparticles, offering an eco-friendly alternative to chemical synthesis methods.

The research supports the potential of D. kotschyi for pharmaceutical applications, particularly in developing natural antibacterial agents and nanomedicine. The authors suggest that future work should focus on isolating the newly identified compounds for structure-activity relationship analysis and evaluating the in vivo efficacy of the biosynthesized Ag NPs. The study maintains a modest scope, focusing on in vitro and in silico validation without proposing immediate clinical applications.