We report in this work the isotherms of cholesterol and stearic acid at the air-water interface modified by different chitosans (chitosan chloride, hydrophobic modified chitosan, and medium and high molecular weight chitosans) in the aqueous subphase. The Langmuir-Blodgett films of the complexes cholesterol-chitosan and stearic acid-chitosan are analyzed by atomic force microscopy (AFM), and a molecular simulation was performed to visualize the chitosan-lipid interactions. Strong modifications are obtained in the isotherms as a result of the chitosan interactions with cholesterol and stearic acid at the air-water interface. These modifications were dependent on the type and concentration of chitosan. Severe modifications of all phases were noticed with larger molecular areas, and the observed changes in the compressional modulus were dependent on the type of chitosan used. The complexes of chitosan-stearic acid were more flexible than the ones of chitosan-cholesterol. The AFM images demonstrated that chitosan was disaggregated by the cholesterol and stearic acid interactions producing more homogeneous surfaces in some cases. The hydrophobic chitosan showed more affinity with stearic acid, while both medium and high molecular weight chitosans produced homogeneous surfaces with cholesterol. The simulated chitosan chains interacting with cholesterol and stearic acid demonstrated the possibility of specific sites of electrostatic bonds between these molecules. Adsorption of cholesterol on the different powdered chitosans, performed by HPLC, showed that the medium and high molecular weight chitosans could retain higher proportions of cholesterol compared with the other analyzed samples.

Five metal complexes have been prepared by reacting 3- and 4-pyridineboronic acid (3- and 4-pba) with potassium tetrachloroplatinate (K2PtCl4) and hexachloroplatinic(IV) acid hydrate (H2PtCl6·aq): [4-pbaH]2[PtCl4], [3-pbaH]2[PtCl4], [4-pbaH][Pt(4-pba)Cl3], cis-[Pt(4-pba)2Cl2]·H2O, and cis-[PtIV(3-pba)2Cl4]·2H2O. All compounds have been characterized by single-crystal X-ray diffraction analysis, showing that the primary hydrogen bonding interactions of the resulting 1D, 2D, and 3D networks contain at least one of the following synthons: X-H···Cl2Pt− (X = C, N+), B(OH)2···Cl2Pt−, and B(OH)2···(HO)2B. The dimensions are enhanced further by secondary +N−H···ClPt, O−H···O, and O−H···ClPt hydrogen bonding interactions between donor and acceptor atoms located at the periphery of these synthons. Additional weak C−H···O, C−H···Cl, B···N, and π···π stacking interactions stabilize the crystal structures further.

A series of cyclic nucleotide analogues to HepDirect prodrugs were prepared by a three-component reaction of protected thymine, phosphoryl chloride, and 5-aryl-alpha-D-xylofuranoses derivatives. One of the cyclic nucleotides showed NMR data that suggest a predominant twisted conformation; however, in spite of having an aryl group at the C4 position within the crystal lattice, the cyclic nucleotide had a chair conformation with the aryl group axially oriented. By analyzing the unprecedented X-ray structure, it was observed that the oxygen atom from the phoshoryl group (P=O) is found in close proximity to the o-hydrogen atom of the aryl group (2.51 A), suggesting thus an attractive nonbonding electrostatic interaction, which might be the driving force that overcomes the steric diaxial interactions imposed by the aryl group. Theoretical studies (NBO) for two model compounds showed that there are indeed interactions between filled (donor) Lewis-type NBOs and empty (acceptor) non-Lewis NBOs corresponding to the nO-->sigma*(C-H) interaction. Additionally, conversion of a diastereomeric mixture of cyclic nucleotides into the more stable diastereomeric cyclic nucleotide was observed and explained by spontaneous isomerization in the phosphorinane ring. This finding supports the recently established hypothesis for the mode of action of prodrug cleavage, for which the anomeric effect plays an important role.

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