p-Sulfonato-Calix[4]arene Micelles Stabilize a Povidone Iodine Solution: Supramolecular Interactions, Iodine Retention, and Bactericidal Activity

Abstract

Povidone iodine (PVPI) is an antiseptic widely used against a broad spectrum of pathogens. However, undesired side-effects are still associated with PVPI treatment due to the irritant effect of iodine. Reducing the concentration of a PVPI formulation could provide safer and more friendly formulations, for routine use and applications in very delicate organs such as the eye. However, managing the storage of a low-concentration solution of PVPI is challenging due to the high iodine volatility. In this study, we demonstrated that an amphiphilic p-sulfonato-calix[4]arene derivative forming micelles (SC4OC6) improves the stability of a 0.1% PVPI aqueous buffered solution. UV-vis and NMR spectra as well as dynamic and electrophoretic light scattering measurements showed that SC4OC6 establishes non-covalent supramolecular interactions with PVPI, resulting in the formation of nanoaggregates with a negatively charged surface. Isothermal titration calorimetry provided the aggregation parameters and evidenced that the formation of the supramolecular assembly is an enthalpically favored process. The interaction of SC4OC6 with PVPI enhances the iodine retention and stability of the solution without affecting the rapid and effective bactericidal activity of PVPI, as demonstrated by a time-killing assay with Staphylococcus epidermidis.

Keywords: antibacterial activity; micellar aggregate; non-covalent interactions; povidone-iodine; sulfonato-calix[4]arene.

Figure 1

Schematic representation of the SC4OC6 structure and its self-assembling in a micellar aggregate.

Figure 2

Schematic representation of the structure of the PVPI complex.

Figure 3

Pictures of 0.1% PVPI solutions alone and with different amounts of SC4OC6 (0.25, 0.5, and 1 mg/mL) in PCB.

Figure 4

UV-vis spectra of 0.1% PVPI solutions alone and with SC4OC6 (0.25, 0.5, and 1 mg/mL) in PCB.

Figure 5

¹H NMR spectra (400 MHz, D₂O, 297 K) of SC4OC6, PVPI, and PVPI/SC4OC6.

Figure 6

ITC titration of a 2.5 mg/mL PVPI solution into PCB at 25 °C (top); enthalpy of reaction as a function of the total concentration of PVPI in the calorimetric cell (C_PVPI is calculated as the monomer unit; squares: enthalpy values, red line: curve fitting, black line: first derivative) (bottom).

Figure 7

ITC titration of a 2.5 mg/mL PVPI solution into SC4OC6 1 mM in PCB at 25 °C (top); enthalpy of reaction as a function of the total concentration of PVPI in the calorimetric cell (C_PVPI is calculated as the monomer unit; squares: enthalpy values, red lines: curve fitting and first derivative) (bottom).

Figure 8

Samples of 0.1% PVPI solution alone (far left) and with different amounts of SC4OC6, after 3 months in plastic dropper containers at 4 °C.

Figure 9

UV-vis spectra of iodine in cyclohexane (10 s shaking) from PVPI alone and with different amounts of SC4OC6; inset: pictures of the iodine extraction from a solution of 0.1% PVPI (left) and PVPI/SC4OC6 (right).

Figure 10

Diffusion of molecular iodine from 0.1% PVPI buffered solution (1 mL) to cyclohexane (1 mL) in the absence and in the presence of SC4OC6 (0.25 mg/mL) at different time intervals (5, 10, 20, 30, and 60 min); inset: picture of the samples after 24 h.

Figure 11

(a) Representative time-killing experiment performed against Staphylococcus Epidermidis for the vehicle PBS (CTR), PVPI 0.1%, SC4OC6 (0.25 mg/mL), and PVPI/SC4OC6; (b) inhibition (%) of the bacterial cell viability (3 time-killing experiments) by PVPI/SC4OC6. Statistical analysis (Two-Way ANOVA) of time-killing assay for PVPI/SC4OC6 vs. SC4OC6: 10, 20, and 40 s, 1 and 2 min, p < 0.01; 4 and 8 min p < 0.001; 60 min p < 0.05.