Synthesis, physicochemical characterization and membrane interactions of a homologous series of N-acylserotonins: Bioactive, endogenous conjugates of serotonin with fatty acids.

N-Acylserotonins (NASTs), present in the mammalian gastro-intestinal tract and central nervous tissues, exhibit significant biological and pharmacological activities. In the present study, a homologous series of NASTs have been synthesized and characterized. Differential scanning calorimetric studies show that in the dry and hydrated states the transition temperatures, enthalpies, and entropies of NASTs exhibit odd-even alternation. Both odd and even chain length NASTs independently display linear dependence of the transition enthalpies and entropies on the chain length under dry as well as hydrated conditions, suggesting that the molecular packing and intermolecular interactions in each series (odd or even) are likely to be similar for NASTs with different acyl chain lengths in the dry state as well as in the hydrated state. Powder X-ray diffraction studies indicated that the incremental increase in the d-spacing per CH2group is 1.023 Å, suggesting that the lipid acyl chains are most likely packed in an interdigitated fashion. Results of computational studies are consistent with this and suggest that the acyl chains of the NASTs are tilted with respect to the bilayer normal. Incorporation of N-myristoylserotonin (NMST) into dimyristoylphosphatidylcholine (DMPC) membranes did not significantly affect the phase transition properties at low mole fractions (1-5 mol%), although distinct decrease in the chain-melting transition temperature and increase in the pretransition temperature were observed at higher contents (7.5-30 mol%), suggesting that NMST increases the stability of the tilted gel phase (L(ß)') but destabilizes the ripple phase (P(ß)'). These observations provide a thermodynamic basis for understanding the functional role of NASTs in their parent tissues.

Characterization of the molecular packing, thermotropic phase behaviour and critical micellar concentration of a homologous series of N-acyltaurines (n = 9-18). PXRD, DSC and fluorescence spectroscopic studies.

N-acyltaurines (NATs) are amides of fatty acids that can be structurally related to endocannabinoids. They show interesting physiological and pharmacological properties. We have synthesized a homologous series of NATs with saturated acyl chains (n = 9-18) and investigated their supramolecular structure and thermotropic phase transitions by powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC). The d-spacings obtained from PXRD increase linearly with chain length with an increment of ∼0.847 Å per additional CH2 moiety suggesting that NATs adopt a tilted bilayer structure with similar packing in crystal lattice. Results obtained from DSC studies indicate that the endothermic transition temperature (Tt) of NATs showed a gradually increasing trend with increasing acyl chain length. The enthalpy (ΔHt) and entropy (ΔSt) of transition show odd-even alternations with odd-chain compounds having higher values than the even-chain compounds. The critical micellar concentration (CMC) of NATs was determined in water at room temperature by fluorescence spectroscopy by monitoring the spectral changes of 8-anilinonaphthalene-1-sulfonic acid (ANS). The CMCs of NATs were found to decrease with increase in acyl chain length. The present results provide a thermodynamic and structural basis for investigating the interaction of NATs with other membrane lipids and proteins, which in turn can shed light in understanding how they function in vivo (in biological membranes).

Structure, supramolecular organization and phase behavior of N-acyl-ß-alanines: Structural homologues of mammalian brain constituents N-acylglycine and N-acyl-GABA.

N-Acyl-ß-alanines (NABAs) are structural homologues of N-acylglycines (NAGs) and N-acyl-γ-aminobutyric acids (NAGABAs), and achiral isomers of N-acylalanines, which are all present in mammalian brain and other tissues and modulate activity of biological receptors with various functions. In the present study, we synthesized and characterized a homologous series of NABAs bearing saturated acyl chains (n=8-20) and investigated their supramolecular organization and thermotropic phase behavior. In differential scanning calorimetric (DSC) studies, most of the NABAs gave one or two minor transitions before the main chain-melting phase transition in the dry state as well as upon hydration with water, but gave only a single transition when hydrated with buffer (pH7.6). Transition enthalpies (ΔHt) and entropies (ΔSt), obtained from the DSC studies showed linear dependence on the chain length in the dry state and upon hydration with buffer, whereas odd-even alteration was observed when hydrated with water. The crystal structures of N-lauroyl-ß-alanine (NLBA) and N-myristoyl-ß-alanine (NMBA) were solved in monoclinic system in the P21/c space group. Both NLBA and NMBA were packed in tilted bilayers with head-to-head (and tail-to-tail) arrangement with tilt angles of 33.28° and 34.42°, respectively. Strong hydrogen bonding interactions between COOH groups of the molecules from opposite leaflets as well as NH⋯O hydrogen bonds between the amide groups from adjacent molecules in the same leaflet as well as dispersion interactions between the acyl chains stabilize the bilayer structure. The d-spacings calculated from powder X-ray diffraction studies showed odd-even alteration with odd-chain length compounds exhibiting higher values as compared to the even-chain length ones and the tilt angles calculated from the PXRD data are higher for the even chain NABAs. These observations are relevant to developing structure-activity relationships for these amphiphiles and understand how NABAs differ from their homologues and isomers, namely NAGs, NAGABAs, and N-acylalanines.