Click reactions in chitosan chemistry

The review provides the first generalized and systematized information on the use of click reactions in chitosan chemistry for the preparation of novel polymers with attractive physicochemical and biological properties. The reactions of copper-catalyzed azide—alkyne cycloaddition and the click reactions of chitosan derivatives occurring in the absence of salts or metal complexes are discussed in detail. The data on the pre-click modification of chito-san (i.e., the introduction of azide function, alkyne fragment, highly dipolarophilic moieties, and thiol group into the polymer) are reviewed. Special attention is given to the application of new chitosan derivatives obtained by click modification.     

Coordination-driven inversion of handedness in ligand-modified PNA

Peptide nucleic acid (PNA) is a synthetic analogue of DNA, which has the same nucleobases as DNA but typically has a backbone based on aminoethyl glycine (Aeg). PNA forms duplexes by Watson Crick hybridization. The Aeg-based PNA duplexes adopt a chiral helical structure but do not have a preferred handedness because they do not contain a chiral center. An L-lysine situated at the C-end of one or both strands of a PNA duplex causes the duplex to preferably adopt a left-handed structure. We have introduced into the PNA duplexes both a C-terminal L-lysine and one or two PNA monomers that have a γ-(S)-methyl-aminoethyl glycine backbone, which is known to induce a preference for a right-handed structure. Indeed, we found that in these duplexes the γ-methyl monomer exerts the dominant chiral induction effect causing the duplexes to adopt a right-handed structure. The chiral PNA monomer had a 2,2':6',2'-terpyridine (Tpy) ligand instead of a nucleobase and PNA duplexes that contained one or two Tpys formed [Cu(Tpy)(2)](2+) complexes in the presence of Cu(2+). The CD spectroscopy studies showed that these metal-coordinated duplexes were right-handed due to the chiral induction effect exerted by the S-Tpy PNA monomer(s) except for the cases when the [Cu(Tpy)(2)](2+) complex was formed with Tpy ligands from two different PNA duplexes. In the latter case, the metal complex bridged the two PNA duplexes and the duplexes were left-handed. The results of this study show that the preferred handedness of a ligand-modified PNA can be switched as a consequence of metal coordination to the ligand. This finding could be used as a tool in the design of functional nucleic-acid based nanostructures.     

Relative Kinetic Stability of Cerium(IV) Complexes with Some Organic Compounds of the Aliphatic Series

A quantitative criterion of the kinetic stabilization of unstable metal oxidation states during complexation has been proposed on the basis of the of a comparative analysis of the stability constants β1yn and rate constants kn = 1 of intramolecular redox decomposition of cerium(IV) complexes [CeHyLn]4+y–nk with a number of aliphatic organic compounds (L = HkR, R(OH)k) in sulfate and nitrate media, as well as established correlations between log kn = 1 and log β1yn. This criterion is defined as the relative kinetic stability of the complexes to intramolecular redox decomposition –log (kn = 1/k0n = 1) determined from the log kn = 1–log β1yn correlations. The latter have been found to associeted with the Nernst equation.     

Thermodynamic and kinetic stability of cerium(IV) complexes with a series of aminopolyacetic acids

The compositions, stability constants, and rate constants of intramolecular redox decomposition of cerium(IV) complexes with anions of aminoacetic (H₂NCH₂COOH), iminodiacetic [HN(CH₂COOH)₂], nitrilotriacetic [N(CH₂COOH)₃], ethylenediaminetetraacetic [(CH₂COOH)₂N(CH₂)₂N(CH₂COOH)₂], and hexamethylenediaminetetraacetic [(CH₂COOH)₂N(CH₂)₆N(CH2COOH)₂] acids were determined by potentiometric, spectrophotometric, and kinetic methods at pH in the range 1.3?2.0 in perchlorate and nitrate media at an ionic strength I = 0.1 and a temperature of 298.15 K. Direct linear correlation between the logarithms of the stability constants of the complexes, log β₁₀₁, and logarithms of the cumulative protonation constants, log В _m+k (k = 1–2), of aminopolyacetic acid anions L ^m–, and inverse linear correlation between log β₁₀₁ and logarithms of the rate constants of intramolecular redox decomposition of the complexonates [CeL]^4–m (m = 1–4), log k _n=1, were found.     

The kinetics of cerium(IV) sulfate reaction with citrate and the thermodynamic characteristics of formation of intermediate complexes

Intermediate cerium(IV)-citrate complexes formed at the first stage of the oxidation of citric acid (Citr) with cerium(IV) were studied spectrophotometrically and pH-potentiometrically at ionic strength I = 2 (sulfate medium). Their composition and the form of the organic ligand present in them, the thermodynamic parameters of their formation, and the kinetic parameters of intramolecular redox decomposition were determined. A detailed scheme of the initial stages of the redox process in the Ce⁴⁺-SO ₄ ^2? -Citr system was considered, and the law of its initial rate and intermediate mechanism were determined. The results were compared with the corresponding data on several oxycarboxylic acids and polyhydric alcohols. The inverse linear correlation was found between the logarithms of stability constants and the logarithms of rate constants for intramolecular redox decomposition of [(CeOH)H_?2R]⁺ complexes with dibasic ligands of the type R = H₂L, H(OH)L, and L(OH)₂. The stabilizing role played by ligand oxy groups in these complexes was demonstrated.     

Azide pre-click modification of chitosan: <Emphasis Type="Italic">N</Emphasis>-(2-azidoethyl)chitosan

N-(2-Azidoethyl)chitosan was synthesized from low-molecular-weight chitosan (molecular weight 37 000 and degree of deacetylation 0.74) in three steps: (1) reaction of chitosan with chloroacetaldehyde to form a Schiffbase, (2) reduction of the Schiffbase with sodium borohydride, and (3) treatment of the reduction product with sodium azide. The course of the formation of the Schiffbase (this step is determining for the degree of substitution of the final product) was affected by the molar ratio of the reagents, the reaction time, and, primarily, the pH value of the medium. This method for selective to an amino group, does not require harsh conditions, and can be proposed for the azide pre-click modification of chitosan.