Molecular relaxation dynamics and self-association of dexamethasone sodium phosphate solutions

Kinetic and Structural Insights into β-Cyclodextrin Complexation with Asparagine Enantiomers: An Experimental and Theoretical Study

We report on the dynamic interactions between β-cyclodextrin (β-CD) and each one of the two enantiomers of asparagine (d-Asp, l-Asp). Molecular docking methodologies were applied to elucidate the formation of the β-CD-d-Asp and β-CD-l-Asp inclusion complexes. Ultrasonic relaxation spectra revealed a single relaxation process in the frequency range studied that is attributed to the complexation between β-CD and asparagine enantiomers. Kinetic parameters and thermodynamic properties for each system were determined directly from the concentration- and temperature-dependent acoustic measurements, respectively. Both β-CD-d-Asp and β-CD-l-Asp systems revealed subtle differences in their thermodynamic and kinetic properties. The infrared absorption spectra of the host molecule, the guest enantiomers, and both inclusion complexes were recorded to verify and further elucidate the complexation mechanism. DFT methodologies were performed to calculate the theoretical IR spectra of the inclusion complexes and compared with the corresponding experimental spectra. The close resemblance between the experimental and theoretically predicted IR spectra is supportive of the formation of inclusion complexes. The encapsulation of asparagine enantiomers in β-cyclodextrin enables not only applications in drug delivery but also the detection and separation of chimeric molecules.

The Identification of Structural Changes in the Lithium Hexamethyldisilazide-Toluene System via Ultrasonic Relaxation Spectroscopy and Theoretical Calculations

Ultrasonic absorption measurements were carried out over a wide concentration and temperature range by means of a pulse technique to examine the structural mechanisms and the dynamical properties in lithium hexamethyldisilazide (LiHMDS)-toluene solutions. Acoustic spectra revealed two distinct Debye-type relaxational absorptions attributed to the formation of trimers from dimeric and monomer units and to the formation of aggregates between a LiHMDS dimer and one toluene molecule in low and high frequencies, respectively. The formation of aggregates was clarified by means of molecular docking and DFT methodologies. The aggregation number, the rate constants and the thermodynamic properties of these structural changes were determined by analyzing in detail the concentration-dependent relaxation parameters. The low-frequency relaxation mechanism dominates the acoustic spectra in the high LiHMDS mole fractions, while the high-frequency relaxation influences the spectra in the low LiHMDS mole fractions. In the intermediate mole fraction region (0.25 to 0.46), both relaxations prevail in the spectra. The adiabatic compressibility, the excess adiabatic compressibility and the theoretically estimated mean free length revealed a crossover in the 0.25 to 0.46 LiHMDS mole fractions that signified the transition from one structural mechanism related with the hetero-association of LiHMDS dimers with toluene molecules to the other structural mechanism assigned to the formation of LiHMDS trimers. The combined use of acoustic spectroscopy with theoretical calculations permitted us to disentangle the underlying structural mechanisms and evaluate the volume changes associated with each reaction. The results were compared with the corresponding theoretically predicted volume changes and discussed in the context of the concentration effect on intermolecular bonding.

Evidence of Self-Association and Conformational Change in Nisin Antimicrobial Polypeptide Solutions: A Combined Raman and Ultrasonic Relaxation Spectroscopic and Theoretical Study

The polypeptide Nisin is characterized by antibacterial properties, making it a compound with many applications, mainly in the food industry. As a result, a deeper understanding of its behaviour, especially after its dissolution in water, is of the utmost importance. This could be possible through the study of aqueous solutions of Nisin by combining vibrational and acoustic spectroscopic techniques. The velocity and attenuation of ultrasonic waves propagating in aqueous solutions of the polypeptide Nisin were measured as a function of concentration and temperature. The computational investigation of the molecular docking between Nisin monomeric units revealed the formation of dimeric units. The main chemical changes occurring in Nisin structure in the aqueous environment were tracked using Raman spectroscopy, and special spectral markers were used to establish the underlying structural mechanism. Spectral changes evidenced the presence of the dimerization reaction between Nisin monomeric species. The UV/Vis absorption spectra were dominated by the presence of π → π* transitions in the peptide bonds attributed to secondary structural elements such as α-helix, β-sheets and random coils. The analysis of the acoustic spectra revealed that the processes primarily responsible for the observed chemical relaxations are probably the conformational change between possible conformers of Nisin and its self-aggregation mechanism, namely, the dimerization reaction. The activation enthalpy and the enthalpy difference between the two isomeric forms were estimated to be equal to ΔH₁* = 0.354 ± 0.028 kcal/mol and ΔH₁⁰ = 3.008 ± 0.367 kcal/mol, respectively. The corresponding thermodynamic parameters of the self-aggregation mechanism were found to be ΔH₂* = 0.261 ± 0.004 kcal/mol and ΔH₂⁰ = 3.340 ± 0.364 kcal/mol. The effect of frequency on the excess sound absorption of Nisin solutions enabled us to estimate the rate constants of the self-aggregation mechanism and evaluate the isentropic and isothermal volume changes associated with the relaxation processes occurring in this system. The results are discussed in relation to theoretical and experimental findings.