Proton-transfer in 1,1,3,3 tetramethyl guanidine by means of ultrasonic relaxation and Raman spectroscopies and molecular orbital calculations

Bicyclic guanidine superbase carboxylate salts for cellulose dissolution

Bicyclic guanidines are utilized in organic synthesis as base catalysts or reagents. They also offer a platform for coordination chemistry, for example in CO2 activation, and their carboxylate salts offer an efficient media for cellulose dissolution. We have studied a series of bicyclic guanidines with varying ring sizes and with varying methyl substituents with a specific aim to find hydrolytically stable acetate salts for dissolution and processing of cellulose. Different superbase synthesis pathways were tested, followed by hydrolytic stability and cellulose dissolution capacity tests. The synthesis pathways were designed to enable the scale up of the production of the superbases considering the availability of the starting molecules and the feasibility of the synthesis. As a result, we found several hydrolytically stable bicyclic guanidine structures, which can overcome many of the reoccurring problems as carboxylate salts or free bases.

Guanidine dicycloamine-based analogs: green chemistry synthesis, biological investigation, and molecular docking studies as promising antibacterial and antiglycation leads

Dicyandiamide (DCD) reacted with amino acids 1a-f to produce biguanides 2 and 4 and guanidine pyrazolones 3, 5, 6, 7, and 8, according to the reaction. DCD exhibited the following reactions: imidodicarbonimidicdiamide 9, diazocan-2-ylguanidine 10, methyl biguanidylthion 11, N-carbamothioylimidodicarbonimidicdiamide 12, 2-guanidinebenzoimidazole 13a, 2-guanidinylbenzoxazole 13b, and 2-guanidinylbenzothiazol 13c. These reactions were triggered by 6-amino caproic acid, thioacetamide, thiourea, o-aminophenol, o-aminothiophenol, and anthranilic acid, respectively. Compound 2 had the least antimicrobial activity, while compound 13c demonstrated the most antibacterial impact against all bacterial strains. Furthermore, in terms of antiglycation efficacy (AGEs), 12, 11, and 7 were the most effective AGE cross-linking inhibitors. Eight and ten, which showed a considerable inhibition on cross-linking AGEs, come next. Compounds 4 and 6 on the other hand have shown the least suppression of AGE production. The most promising antiglycation scaffolds 8, 11, and 12 in the Human serum albumin (HAS) active site were shown to be able to adopt crucial binding interactions with important amino acids based on the results of in silico molecular docking. The most promising antiglycation compounds 8, 11, and 12 were also shown to have better hydrophilicity, acceptable lipophilicity, gastrointestinal tract absorption (GIT), and blood-brain barrier penetration qualities when their physicochemical properties were examined using the egg-boiled method.

Non-Steroidal Anti-Inflammatory Drugs Loaded to Micelles for the Modulation of Their Water Solubility

The low water solubility of aspirin (ASPH) is well known, creating research challenges regarding both its composition and its delivery. Therefore, the development of new aspirin-based formulations that are water soluble is a research, technological, and financial issue. With the aim to improve the water solubility of ASPH, the micelle of formula SLS@ASPH (SLS = Sodium Lauryl Sulfate) was formed. The Critical Micelle Concentration (CMC) of SLS in the presence of ASPH was determined by ultrasonic velocity, complementary, and transient birefringence measurements. The SLS@ASPH was characterized by the melting point (m.p.), attenuated total reflection spectroscopy (FT-IR-ATR), and X-ray fluorescence spectroscopy (XRF) in a solid state and in a solution by ultraviolet-visible (UV-Vis) and 1H NMR spectroscopies. The SLS/ASPH molar ratio was determined to be 5/1 in SLS@ASPH. The inhibitory activity of SLS@ASPH towards lipoxygenase (LOX), an enzyme that takes part in the inflammation mechanism, was studied. The inhibitory activity of SLS@ASPH against LOX is 3.5-fold stronger than that of free SLS. The in vitro toxicity of the SLS@ASPH was tested on immortalized human keratinocyte (HaCaT) cells.

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.

Structure and dynamics of aqueous norspermidine solutions: anin situultrasonic relaxation spectroscopic study

Anin situultrasonic relaxation spectroscopic study is presented in an effort to determine the structural changes and the dynamics involved when norspermidine (NSpd) is dissolved in water. Our aim is to elucidate the mechanism responsible for the observed relaxation mechanism in acoustic spectra and estimate the corresponding thermodynamic parameters and the associated volume change. The experimental spectra of aqueous NSpd solutions revealed a single Debye-type relaxation mechanism attributed to proton-transfer reaction. The concentration and temperature dependence of the acoustic parameters supports this assignment. The activation enthalpy and entropy were estimated equal to ΔH^*= 1.79 ± 0.20 kcal mol^-1and ΔS^*= -18.31 ± 0.73 cal mol^-1 K^-1, respectively. The concentration and temperature dependence of the sound velocity and absorption in the solutions exhibit characteristic features that are related to alterations in the network rigidity due to variations in hydrogen-bonding interactions at molecular level. The volume change associated to proton-transfer reaction for NSpd has been estimated and compared with the volume change observed for an analogous guanidine, the 1,1,3,3 tetramethyl guanidine. The obtained results are discussed in the framework of an existing theoretical structural model highlighting the strong molecular association in these liquid mixtures leading to complementary information on the structure and dynamics of guanidine amines. A comprehensive model of the whole relaxation processes is presented and discussed in detail.

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

Detailed concentration-dependent measurements of sound absorption and velocity have been performed in dexamethasone sodium phosphate (DSP) aqueous solutions in the MHz frequency range. A single well-resolved relaxation process dominates the experimental acoustic spectra following a Debye-type distribution function. The analysis of the temperature-dependent ultrasonic relaxation data also revealed analogous effect with concentration on the relaxation spectra. All acoustic parameters were estimated by means of a fitting procedure. The behavior of the relaxation frequency and amplitude with concentration allowed us to assign the observed process to self-association mechanism. Combining the ultrasonic and electric conductivity data, the self-association scheme has been established. The thermodynamic constants and the rate of the aggregation due to hydrophobic interactions have been estimated in view of the Eyring’s theory. The concentration dependence of relaxation amplitude and characteristic frequency revealed that the presence of additional relaxation processes in the spectra related to additional mechanisms, such as conformational changes and proton-transfer reaction is excluded and the self-association process considered here was found to dominate in this frequency range. The results have been discussed in view of the fair ability of DSP for hydrophobic interactions and aggregate formation in aqueous environment.