Femtosecond Transient Absorption Spectroscopy Study of the Early Events of Norfloxacin in Aqueous Solutions with Varying pH Values

Abstract

The photophysics and photochemistry of norfloxacin (NF) have been investigated in aqueous solutions of different pH using femtosecond transient absorption spectroscopy (fs-TA). Resonance Raman spectroscopic experiments on NF have also been conducted in aqueous solutions of different pH to characterize the vibrational and structural information on the initial forms of NF. The experimental results in combination with density functional theory calculations of the key intermediates help us to elucidate the early events for NF after photoexcitation in aqueous solutions with varying pH values. The fs-TA results indicate that NF mainly underwent photophysical processes on the early delay time scale (before 3 ns), and no photochemical reactions occurred on this time scale. Specifically, after the irradiation of NF, the molecule reaches a higher excited singlet Sn and then decays to the lowest-lying excited singlet state S1 followed by intersystem crossing to transform into the lowest-lying triplet state T1 with a high efficiency, with an exception that there is a lower efficiency observed in basic aqueous solution due to the generation of an intramolecular electron transfer as an additional pathway to waste energy.

Impact of Hydrogen Bonding on the Fluorescence of N‐Amidinated Fluoroquinolones

“…This bonding motif can be disturbed/weakened by competing intermolecular hydrogen‐bonding, e.g., by water, thus leading to a strong quenching effect as observed in the present study. In future studies, ultrafast time‐resolved spectroscopy may be used to evaluate the hydrogen‐mediated interaction dynamics, and thus elucidate the quenching pathway of these interesting fluorophores by hydrogen‐bond donors . Regardless, our results have shown that the emission of N ‐amidinated ciprofloxacin derivatives is strongly responsive to hydrogen‐bond donors in nonpolar solvents.…”

Interplay of Multiple Interaction Forces: Binding of Norfloxacin to Human Serum Albumin

“…The spectral properties of NOF are highly sensitive to the environment due to the pH-dependent prototropic equilibrium (Figure a). − Representative examples of pH-dependence of the spectral properties of the drug are illustrated in Figure b. The absorption profile of NOF at neutral pH (pH 7.40) exhibits a strong absorption at λ abs ≈ 270 nm attributable to the aromatic nucleus and a long wavelength absorption in the region 300–350 nm with two subpeaks (λ abs of ∼325 and 335 nm) emanating from n → π* electronic transition. − Decrease in medium pH from neutral (pH 7.40) to acidic (pH 4.0) regime is found to accompany (Figure b inset) a discernible red-shift of the short wavelength absorption by ∼5 nm (from λ abs of ∼270 to ∼275 nm), with concomitant appearance of a tail at the long wavelength region (beyond 350 nm).…”

“…The spectral properties of NOF are highly sensitive to the environment due to the pH-dependent prototropic equilibrium (Figure a). − Representative examples of pH-dependence of the spectral properties of the drug are illustrated in Figure b. The absorption profile of NOF at neutral pH (pH 7.40) exhibits a strong absorption at λ abs ≈ 270 nm attributable to the aromatic nucleus and a long wavelength absorption in the region 300–350 nm with two subpeaks (λ abs of ∼325 and 335 nm) emanating from n → π* electronic transition. − Decrease in medium pH from neutral (pH 7.40) to acidic (pH 4.0) regime is found to accompany (Figure b inset) a discernible red-shift of the short wavelength absorption by ∼5 nm (from λ abs of ∼270 to ∼275 nm), with concomitant appearance of a tail at the long wavelength region (beyond 350 nm). This conforms to the signature of prototropic transformation of NOF from zwitterionic (in neutral pH) to cationic (in acidic pH) form, which complies well with reported literature suggesting that the prototropic equilibrium of NOF in the pH range 2.5–7.5 is restricted within the zwitterionic and cationic species only. − Concurrently, the remarkable sensitivity of the emission spectral properties of NOF on medium pH is also displayed in Figure b.…”

“…This is principally driven by the underlying complex nature of the mechanism of action of fluoroquinolones that entails hydrophobic as well as hydrophilic pathways, though the latter is often argued to provide the main route via which they enter the cytoplasm. , Norfloxacin (NOF, Figure a), a benchmark molecular system from the fluoroquinolone family, has been reported to display a vast range of antibacterial activity that leads to their applications in the treatment of a number of diseases, e.g., sexually transmitted disease such as prostatitis, skin infections, acute cystitis and nosocomial pneumonia, complicated urinary tract infection, − chronic bacterial infections, − and so forth. The chemical structure of NOF accounts for a pH-dependent prototropism characterized by two p K a values, p K 1 = 6.23 and p K 2 = 8.55, respectively, at the carboxyl group and a piperazinyl moiety. − The study of interaction of drug molecules with relevant biological and/or biomimicking receptors has formed a dynamic research avenue for years, particularly in relation to deciphering important issues, e.g., drug availability, drug efficacy, and drug transport. − Nevertheless, this field of research is still far from being completely understood and hence triggers intriguing inquisitions. The present study is focused on an endeavor to explore the important and pertinent issue of binding interaction of this important chemotherapeutic antibacterial drug NOF with the transport protein, human serum albumin (HSA).…”

Free-Standing Membrane Liquid-State Platform for SERS-Based Determination of Norfloxacin in Environmental Samples

“…The pK a values are approximately 6.2 and 8.5 [23,24]. Therefore, it indicated that the structure of its aqueous solution is displayed in four forms, namely, neutral form, tautomeric form, anionic form and cationic form [25]. We suspect that the neutral form of NOR has better chance to adsorb on the surface of Ag 7 Au 3 [26].…”