Gerardo R. Echevarría

Kinetic study of the Schiff-base formation between glycine and pyridoxal 5′-phosphate (PLP), pyridoxal (PL), and 5′-deoxypyridoxal (DPL)

The formation of Schiff bases between pyridoxal 5′-phosphate (PLP), pyridoxal (PL), 5′-deoxypyridoxal (DPL), and glycine has been investigated as a function of proton concentration at 25 °C and 0.1 mol dm–3 ionic strength. Ionization constants as well as equilibrium and microscopic kinetic constants of the ionic species present in solution have been determined.The results show that all the protonatable groups of the aldehyde molecule are involved in the intramolecular acidic catalysis of the dehydration process undergone by the α-hydroxyamine intermediate. Results also reveal that the formation of Schiff base with pyridoxal is greatly affected by the formation of the cyclic hemiacetal of this aldehyde.

Formation of Schiff bases of 5′-deoxypyridoxal and hexylamine in aqueous and non-aqueous media

We have studied the formation of Schiff bases of 5′-deoxypyridoxal (DPL) and hexylamine in various non-aqueous solvents (dioxane, pentanol/dioxane mixtures, pentanol, butan-2-ol, propanol, ethanol, methanol and ethane-1,2-diol) at 25 °C. The results are interpreted in terms of the tautomer forms of DPL present in the medium, and the rate constant of formation of the Schiff base has been obtained for the neutral species of DPL and the zwitterionic form (k1n= 1.7 × 102 dm3 mol–1 min–1 and k1z= 3.6 × 103 dm3 mol–1 min–1, respectively). We also determined the constant of the tautomeric equilibrium between the corresponding forms of the uncharged species of DPL in the different solvents. These results and those obtained in water/dioxane media indicate that the intramolecular acid catalysis involved in the formation of Schiff bases is also governed by the occurrence of a charged transition state.

Schiff bases of pyridoxal 5'-phosphate and polypeptides containing L-lysine : a kinetic study

Abstract The apparent rate constants of formation ( k 1 ) and hydrolysis ( k 2 ), and the apparent equilibrium constant ( K pH ) of the Schiff bases of pyridoxal 5′-phosphate and poly-L-lysine in different degrees of polymerization have been determined as a function of pH at 25 °C and a constant ionic strength of 0.1. The Schiff bases of PLP and L-lysine and L-alanine copolymers were also studied under the same conditions. The k 1 and K pH values obtained in every instance were larger than those reported for Schiff bases of PLP and primary amines. The absorption bands of the Schiff bases formed coincided with those obtained for the Schiff bases of PLP and primary amines in low-polarity media. These facts reveal the occurrence of effects that cannot be ascribed to the solvent and that influence intramolecular acid—base catalysis in the formation of the Schiff bases examined.

Kinetic and thermodynamic parameters for Schiff's base formation between pyridoxal 5′-phosphate and n-hexylamine

The kinetic and thermodynamic parameters for the reaction between pyridoxal 5′-phosphate and n-hexylamine in aqueous medium as a function of pH, at constant ionic strength (0.1) and temperatures of 10, 15, 20, and 30 °C have been calculated. The variation observed in the overall kinetic formation and hydrolysis constants of the corresponding Schiffs base can be accounted for on the basis of the protonation and reactivity of the substrates present (pyridoxal 5′-phosphate, n-hexylamine, and Schiffs base) in the medium as a function of pH. The first protonation, both of pyridoxal 5′-phosphate and of the Schiffs base, modifies the energies of activation corresponding to the formation and hydrolysis processes, respectively. The less negative charge the pyridoxal 5′-phosphate molecule bears, the more negative the ΔH and ΔS values become.

Determination of the rates of formation and hydrolysis of the Schiff bases formed by pyridoxal and poly-l-lysine

Abstract The rate constants of formation ( k 1ex ) and hydrolysis ( k 2 ) of the Schiff bases of pyridoxal (PL) with poly- l -lysine were determined as a function of pH at a temperature of 25°C and an ionic strength I =0.1 M. The k 1ex and k pH values obtained at every pH assayed were all smaller than those reported for the formation of Schiff bases between pyridoxal 5′-phosphate (PLP) and poly- l -lysine as a result of the occurrence of PL as a hemiacetal. The Schiff bases of PL with poly- l -lysine are more readily hydrolysed than those of PLP with the same polymer. The carbon-nitrogen double bond of the Schiff bases studied in this work lies in a hydrophobic environment similar to that provided by ethyl alcohol in terms of polarity.

Determination of the rates of formation and hydrolysis of the schiff bases formed by pyridoxal 5'-phosphate and hydrazinic compounds

The macroscopic rate constants of formation (k1) and hydrolysis (k2) for the reactions of pyridoxal 5′-phosphate (PLP) with hydrazine (PLP-HY system), carbidopa (α-hydrazino-α-methyl-β-(3,4-dihydroxyphenyl)propionic acid, PLP-CD system), hydralazine (1-hydrazinophthalazine, PLP-HL system) and isoniazid (4-pyridinecarboxylic acid hydrazide, PLP-ISO system) were fitted to a kinetic scheme that considers the different ionic species present in the medium, their protonation constants, and their individual rates of formation (k1i) and hydrolysis (k2i). The results obtained for the molecules bearing the hydrazine group are compared with those for the reactions of PLP with n-hexylamine (PLP-NHA system) and poly-l-lysine (PLP-LYS system). Some structural effects on the individual rate constants are also examined.

SOLVENT EFFECTS ON THE TAUTOMERIZATION OF 5′-DEOXYPYRIDOXAL. A PHOTOPHYSICAL STUDY

Abstract— Absorption and fluorescence spectra of 5′‐deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range10–65°C. The areas under the absorption bands were analyzed to obtain the mole fraction (fN, fz) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (Δv), fluorescence quantum yield of the neutral form (QN), fluorescence ratio of the neutral to the zwitterionic form (øN/øZ) and the rate constant of tautomerization (k1) from Z to N in the excited state. Some of these parameters (fN, Δv, QN, k1) were found to depend on the proton donor character of the solvent, whereas others (øN/øZ) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of –OH groups on its environment.

Kinetics of the formation and hydrolysis of the Schiff bases of pyridoxal 5′-phosphate and copolypeptides containing l-lysine and aromatic l-amino acids

Abstract We determined the apparent rate constants of formation ( k 1 ) and hydrolysis ( k 2 ) of the Schiff bases formed between pyridoxal 5′-phosphate and copolymers of l -lysine- l -tyrosine and l -lysine- l -phenylalanine, at different pH values, at temperature of 25°C and an ionic strength of 0.1 M. The values of the rate constant of formation obtained for the systems of the present work are greater than these for the PLP-poly- l -lysine system in all the range of pH studied, due to the presence of aromatic groups in the lateral chains of the polypeptide. The existence of these groups favors the presence of a few polar environment for the Schiff bases.

Photophysical Study of Pyridoxal 5′‐Phosphate and Its Schiff Base with n‐Hexylamine

Abstract— Phosphopyridoxal enzymes exhibit a wide variety of reactions. The activation mechanism proceeds with the formation of Schiff bases that undergo loss or transfer of one or more protons. Changes in the fluorescent properties of the pyridoxal 5′‐phosphate and its Schiff base may signal changes in the protonation state and/or in the microenvironment of the cofactor. In this paper the average fluorescence lifetime of pyridoxal 5′‐phosphate (PLP) as a function of pH was studied in a medium of 0.1 M ionic strength at 25°C. The ionic species of PLP with an unprotonated hydrogen atom were found to exhibit fluorescence lifetimes in the region of 100 ps on excitation at λ= 390 nm. The average fluorescence lifetimes for PLP in its Schiff bases with n‐hexylamine in media of low polarity are reported.

Determination of the rates of formation and hydrolysis of the Schiff bases formed by 5'-deoxypyridoxal and poly-L-lysine

Abstract The kinetics of the reaction between 5′-deoxypyridoxal (DPL) and poly- l -lysine in aqueous solutions at a variable pH and a constant ionic strength of 0.1 M was studied spectrophotometrically. The rate constants of formation and hydrolysis of the resulting Schiff base and its stability constant at a variable pH were also determined. A comparison of the formation rate constant for the Schiff base with those for the models of pyridoxal 5′-phosphate (PLP) with poly- l -lysine and DPL with n -hexylamine revealed the effect of both the phosphate group of PLP and of the polypeptide chain on intramolecular catalysis in the dehydration of the intermediate carbinolamine formed. The effects of the polypeptide and the phosphate group on the hydrolysis of the Schiff base and hence on the base stability are also demonstrated.