Browsing by Author "Santos, JG"

Now showing 1 - 4 of 4
  • No Thumbnail Available
    Item
    Determination of the rates of formation and hydrolysis of the Schiff bases formed by pyridoxal 5'-phosphate with L-tryptophan and its methyl and n-butyl esters
    (2002) Gorostidi, GRE; Santos, JG; Basagoitia, A; Blanco, FG
    The apparent rate constants of the formation (k(1)) and hydrolysis (k(2)) of the Schiff bases formed by pyridoxal 5'-phosphate with L-tryptophan and their methyl and n-butyl esters at a variable pH, 25 degreesC, and an ionic strength of 0.1 M were determined, along with the equilibrium constant (K-pH). The individual rate constants of formation and hydrolysis of the Schiff bases of systems corresponding to different chemical species present in the medium as a function of its acidity were also determined, as were the pK values for the Schiff bases. The influence of the a-carboxyl group on the formation and hydrolysis constants of the Schiff bases, and also on their pK values, is demonstrated.
  • Thumbnail Image
    Item
    Mechanism of the alkaline hydrolysis of O-ethyl S-(2,4,6-trinitrophenyl) thio- and dithiocarbonates
    (ROYAL SOC CHEMISTRY, 1999) Castro, EA; Cubillos, M; Santos, JG; Bujan, EI; Remedi, MV; Fernandez, MA; de Rossi, RH
    The alkaline hydrolysis of O-ethyl S-(2,4,6-trinitrophenyl) thio- and dithiocarbonates, 1 and 2, respectively, were studied kinetically in 5% dioxane in water, at 25.0 degrees C, and ionic strength 0.2 mol dm(-3) (KCl). Two kinetic processes, well separated in time, were detected. The fast process involves the formation of a sigma-complex by addition of HO- to one of the unsubstituted positions of the aromatic ring, followed by fast ionisation of this complex. The slow process leads to the formation of a mixture of 2,4,6-trinitrophenoxide and 2,4,6-trinitrobenzenethiolate ions in a 10 : 1 ratio, respectively, in the reaction of 2, and to a mixture in a 2. 1 ratio in the reaction of 1, independent of KOH concentration. Although the substrates show different kinetic behaviour with KOH concentration, the results can be discussed on the basis of a common reaction mechanism.
  • No Thumbnail Available
    Item
    On the L-DOPA and carbidopa reactivity against pyridoxal 5′-phosphate.: A kinetic study
    (2002) Gorostidi, GRE; Santos, JG; Figueroa, J; Blanco, FG
    The apparent rate constants of the formation (k(1))) and hydrolysis (k(2)) of Schiff bases formed by pyridoxal 5'-phosphate (PLP) with L-3,4-dihydroxyphenylalanine (L-DOPA) at a variable pH, 25 degreesC and an ionic strength of 0. 1 M was determined. The individual rate constants for the formation and hydrolysis of Schiff bases corresponding to the different chemical species present in the medium as a function of its acidity were also determined, as were the pK(a) values for the Schiff bases. The formation and hydrolysis rate constants of the Schiff bases were compared with those of the reaction of PLP with carbidopa (CD), showing that the reactivity Of L-DOPA and carbidopa on PLP are the same over the whole pH range studied, and that the hydrolysis rate is somewhat greater for the Schiff bases between PLP and CD than those between PLP and L-DOPA.
  • No Thumbnail Available
    Item
    Schiff's bases formed between pyridoxal 5′-phosphate and 4-aminobutanoic acid.: Kinetic and thermodynamic study
    (2003) Gorostidi, GRE; Castellanos, MG; Pérez, PM; Santos, JG; Blanco, FG
    The overall and individual kinetic constants of formation (k(1) and k(1)(i)) and hydrolysis (k(2), k(OH) and k(2)(i)) of the Schiff's bases formed between pyridoxal 5'-phosphate (PLP) and 4-aminobutanoic acid (GABA) at 10, 20, 25, 30, and 37degreesC, a variable pH and a constant ionic strength of 0.1 M (1 M=1 mol dm(-3)) were determined. The formation of a Schiff's base is an intramolecular acid-catalyzed process. The activation and thermodynamic parameters for the formation and hydrolysis of the Schiff's bases were also determined. DeltaH and DeltaS for the individual processes were all found to be negative.