Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

Spectroscopic studies of the biologically active compound 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), have been performed. Absorption studies in the UV-Vis region for FABT in polar solvents, like water or ethanol, exhibit the domination of the enol form over its keto counterpart, with a broad absorption band centered around 340 nm. In non-polar solvents such as n-heptane or heavier alkanes the 340 nm absorption band disappears and an increase of the band related to the keto form (approximately 270 nm) is observed. Fluorescence spectra (with 270 nm and 340 nm excitation energies used) show a similar dependence: for FABT in 2-propanol a peak at about 400 nm dominates over that at 330 nm while in n-heptane this relation is reversed. The solvent dependent equilibrium between the keto and enol forms is further confirmed by FTIR and Raman spectroscopies. As can be expected, this equilibrium also shows some temperature dependences. We note that the changes between the two tautomeric forms of FABT are not related to the permanent dipole moment of the solvent but rather to its dipole polarizability

4-Hydroxy-1-naphthaldehydes: proton transfer or deprotonation

A series of naphthaldehydes, including a Mannich base, have been investigated by UV-Vis spectroscopy, NMR and theoretical methods to explore their potential tautomerism. In the case of 4-hydroxy-1-naphthaldehyde concentration dependent deprotonation has been detected in methanol and acetonitrile. For 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde (a Mannich base) an intramolecular proton transfer involving the OH group and the piperidine nitrogen occurs. In acetonitrile the equilibrium is predominantly at the OH-form, whereas in methanol the proton transferred tautomer is the preferred form. In chloroform and toluene, the OH form is completely dominant. Both 4-hydroxy-1-naphthaldehyde and 4-methoxy-1-naphthaldehyde (fixed enol form) show dimerization in the investigated solvents and the crystallographic data, obtained for the latter, confirm the existence of a cyclic dimer

Association equilibria of alkyl derivatives of urea and thiourea

The studies and comparison of a series of molecular mono- and di-substituted derivatives of urea and thiourea in solvents of increasing polarity are presented [1–4]. These substances are characterized by a high tendency to self-associate through the formation of intermolecular hydrogen bonds due to the presence in their structure both groups as donors (NH) as well as proton acceptors (C=O) or (C=S). Studies were performed by using IR spectroscopy, method of measuring the average molecular weight and the dipole moments. The experimental data were verified by DFT quantum chemical calculations with B3PW91 correlation functional. Simultaneous use of these techniques alowed establishing not only the efficiency of aggregation, but also the structure and polarity of formed aggregates. It was shown, that in solvents with weak acidic C-H groups the aggregation was strongly limited because of molecular interactions between solute and solvent. The theoretical DFT calculations which included the impact of the environment on the nature of interactions in the complex were carried out [e.g. Scheme 4.1.4]. A combination of geometry optimization in polarizable continuum model (PCM) with the connection of chloroform molecules (1,2-dichloroethane) with urea dimers enabled to obtain the expected theoretical simulation compliance with the experiment. The equilibrium constants were calculated on the basis of data obtained in two independent methods of measurement: IR spectroscopy and measurements of average molecular weights. Good agreement of experimental data of both research techniques were found up to concentration of 0.03 mol/dm3 [Fig. 2.5]. The type of associates have been assessed following the dipole moments measured as a function of concentration, and on the results of density-functional theory (DFT) calculations on the structure and energy of particular species. All of the urea derivatives demonstrated an increase in dipole moment with increased concentration, suggesting linear-type aggregation [Fig. 4.1.3]. Contrastingly, the dipole moments of the N,N-dimethylthiourea and mono-N-alkyl-substituted thioureas decreased with concentration and suggest that cyclic dimers or trimers are formed by C=S…(HR)2N-C=S interactions [Fig. 4.2.2]. The efficiency of self-aggregation was described by use of two equilibrium constants. The first constant, K1, was describing dimer formation and the second constant, K2, the subsequent multimer formation. In N,N’-thioureas aggregation was lower than for the related urea compounds [Table 4.1.1 and Table 4.2.1]. Differences between urea and thiourea derivatives result from the fact that the ureas are stronger bases and, therefore, more active in aggregation

Structure and function of protein-lipid systems

Biomembranes play many structural and functional roles in both prokaryotic and eukaryotic cells [10]. They define compartments, the communication between the inside and outside of the cell. The main components of biomembranes are lipids and proteins, which form protein-lipid bilayer systems [10]. A structure and physicochemical properties of protein-lipid membranes, which determines biological activities of biomembranes, are strongly dependent on interactions between lipid and protein components and external agents such as a temperature, pH, and a membrane hydration [4]. A lipid bilayer matrix serves as a perfect environment for membrane proteins (Fig. 1), and it assures activities of these proteins. Because biomembranes are composed of many different groups of lipids and proteins and have a complex structure, it is difficult to study in details their physicochemical properties using physicochemical methods. For these reason, lipid membranes of liposomes are used in many scientific laboratories for studding processes associated with a lipid phase transition, a membrane hydration, or protein-membrane interactions. The structure of liposomes (Fig. 5), and an influence of pH and an ionic strength on a lipid bilayer structure are discussed in the presented work. The role of membrane proteins in determination of biological activities of biomembranes is highlighted. A high variety of a structure and an enzymatic activity of membrane proteins is responsible for a high diversity of biological functions of cell membranes [2]. α-Lactalbumin (α-LA) is a peripheral membrane protein (Figs 8 and 9), its biological function is strongly related to its conformational structure and interaction with lipid membranes [49]. The complex of α-LA in a molten globule conformational state with oleic acid, termed as a HAMLET complex, are disused in a context of its anti-tumor activity

FTIR-ATR and fluorescence studies of protein-lipid systems

Lipid-protein systems paly curtail roles in living systems [49]. Hence, a determination of their structure at different levels of organization is still one of the most important tasks in many research projects. A study of lipid-protein systems is based on many physicochemical techniques, such as spectroscopy of FTIR, Raman, fluorescence, NMR, EPR, as well as DLS, DSC and TEM methods. In the presented paper tow of the most frequently used methods, that is FTIR and fluorescence spectroscopy, will be discussed in details. They are characterized by a relatively low cost of sample preparation, a short measuring time, and they give a huge number of structural and physicochemical information about lipid-protein systems. In the FTIR-ATR spectroscopy many of vibrational bands are commonly used as very precise vibrational indicators of structural changes in lipids and proteins (Fig. 1) [1–6]. They allows to characterize lipid and protein components separately in mixed systems. Additionally, structural changes in lipid membranes can be monitored in one FTIR-ATR experiment simultaneously in a region of hydrophilic lipid head-groups (Fig. 5) [17, 18], in a hydrophobic part composed of hydrocarbon lipid chains (see Figures 2 and 3) [7–9], and in a lipid membrane interface represented by ester lipid groups (Fig. 4) [4, 6, 11, 12]. A secondary structure of proteins and peptides in different experimental conditions can be defined in the FTIR-ATR spectroscopy on the base of amide I bands (Fig. 6 and Tabs 1, 2 and 3) [20–22]. A fluorescence spectroscopy is a complementary methods to FTIR spectroscopy in a study of lipid-protein systems. It competes information about time-dependent and very fast (in a scale of femtoseconds) structural processes in both lipids [41–45] and proteins [23, 27, 48]. The folding, denaturation, and aggregation of proteins and lipid membranes accompanied by changes in an order, packing and hydration of the system under study [23, 27, 41–45, 48]