Mechanochemical Preparation, Solid-State Characterization, and Antimicrobial Performance of Copper and Silver Nitrate Coordination Polymers with L- and DL-Arginine and Histidine

Abstract

The antimicrobial activity of the novel coordination polymers obtained by co-crystallizing the amino acids arginine or histidine, as both enantiopure L and racemic DL forms, with the salts Cu(NO₃)₂ and AgNO₃ has been investigated to explore the effect of chirality in the cases of enantiopure and racemic forms. The compounds [Cu·AA·(NO₃)₂]_CPs and [Ag·AA·NO₃]_CPs (AA = L-Arg, DL-Arg, L-His, DL-His) were prepared by mechanochemical, slurry, and solution methods and characterized by X-ray single-crystal and powder diffraction in the cases of the copper coordination polymers, and by powder diffraction and by solid-state NMR spectroscopy in the cases of the silver compounds. The two pairs of coordination polymers, [Cu·L-Arg·(NO₃)₂·H₂O]_CP and [Cu·DL-Arg·(NO₃)₂·H₂O]_CP, and [Cu·L-Hys·(NO₃)₂·H₂O]_CP and [Cu·DL-His·(NO₃)₂·H₂O]_CP, have been shown to be isostructural in spite of the different chirality of the amino acid ligands. A similar structural analogy could be established for the silver complexes on the basis of SSNMR. The activity against the bacterial pathogens Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus was assessed by carrying out disk diffusion assays on lysogeny agar media showing that, while there is no significant effect arising from the use of enantiopure or chiral amino acids, the coordination polymers exert an appreciable antimicrobial activity comparable, when not superior, to that of the metal salts alone.

Keywords: amino acids; antimicrobials; co-crystallization; coordination polymers; copper; crystal engineering; mechanochemistry; silver.

Figure 1

Polymeric chains in L-Arg·Cu (left) and DL-Arg·Cu (right). Note the remarkable structural similarity, with all arginine ligands having the same chirality on the left and alternate chirality on the right. Hydrogens omitted for clarity.

Figure 2

Polymeric chains in L-His·Cu form II (left) and DL-His·Cu (right). As in the case of arginine (Figure 1), the coordination polymers are structurally very similar despite the different chirality. Hydrogens omitted for clarity.

Figure 3

(a) From bottom to top: calculated pattern from single-crystal data of L-Arg·Cu (in red); powder pattern from ball milling experiment (in black); powder pattern from slurry experiment resulting in the more stable 2:1 product LAHNOX (in gray) and calculated pattern of LAHNOX [59] from database (in orange).; (b) From bottom to top: calculated pattern from single-crystal data of DL-Arg·Cu (in red); powder pattern from ball milling experiment (in blue) and from slurry (in gray).

Figure 4

XRPD of ball-milling product of L-His·Cu (in black); calculated pattern from SCXRD of DL-His·Cu (in orange); XRPD of ball-milling product of DL-His·Cu synthesis (in gray).

Figure 5

Overlay of the ¹³C CPMAS spectra of L-Arg·Ag (in black) and pure L-Arg (in red).

Figure 6

Overlay of a detail of the ¹⁵N CPMAS spectra of L-Arg·Ag (in black) and pure L-Arg (in red).

Figure 7

Overlay of the ¹³C CPMAS spectra of DL-Arg·Ag (in blue) and pure DL-Arg (in red).

Figure 8

Overlay of the ¹³C CPMAS spectra of L-Arg·Ag (in black) and DL-Arg·Ag (in blue).

Figure 9

From bottom to top: calculated pattern from single-crystal data of IWOFUX (L-Arg·Ag) in red), experimental powder pattern of L-Arg·Ag (in black), and experimental powder pattern.

Figure 10

Overlay of the ¹³C CPMAS spectra of L-His₂·Ag (in purple) and pure L-His (in red).

Figure 11

Overlay of a detail of the ¹⁵N CPMAS spectra of L-His₂·Ag (in purple) and pure L-His (in red).

Figure 12

Overlay of the ¹³C CPMAS spectra of L-His·Ag (in black) and pure L-His (in red).

Figure 13

Overlay of the ¹³C CPMAS spectra of DL-His₂·Ag (in green), DL-His·Ag (in blue), and pure DL-His (in red).

Figure 14

Overlay of a detail of the ¹⁵N CPMAS spectra of DL-His₂·Ag (in green), DL-His·Ag (in blue), and pure DL-His (in red).

Figure 15

Overlay of the ¹³C CPMAS spectra of L-His₂·Ag (in purple) and DL-His₂·Ag (in green).

Figure 16

(a) L-His₂·Ag (TIGHEY) [56] calculated pattern from database (in red), experimental XRPD pattern from slurry synthetic procedure of L-His₂·Ag (in black), and DL-His₂·Ag (in blue); (b) experimental XRPD pattern from slurry of L-His·Ag (in black) and DL-His·Ag (in blue).

Figure 17

Antimicrobial activity. (A) Normalized antimicrobial activity from disk diffusion assay on lysogeny agar media for the coordination polymers of the metal salts with the amino acids; (B) mixture of the amino acid with the metal salt at 1:1 ratio. Values normalized to silver nitrate value of 1.0. Values above 1.05 are more antimicrobial than silver nitrate alone.