NOPR Collection:

13C Nuclear magnetic resonance studies of binding of thiocyanate to lactoperoxidase and horseradish peroxidase heme enzymes

Sun, 01 Apr 1990 00:00:00 GMT

Title: 13C Nuclear magnetic resonance studies of binding of thiocyanate to lactoperoxidase and horseradish peroxidase heme enzymes Authors: Modi, Sandeep; Behere, Digambar V.; Mitra, Samaresh Abstract: Interaction of thiocyanate with lactoperoxidase (LPO) and horseradish peroxidase (HRP) has been investigated by relaxation rate measurements (at 125.77 MHz) of ¹³C resonance of thiocyanate carbon. The apparent dissociation constant (K_D) for thiocyanate binding to LPO at pH = 6.1 and to HRP at pH = 4.0 has been deduced to be 85 mM and 160 mM respectively from the relaxation rate measurements. The pH dependence of K_D and ¹³C resonance line-width of thiocyanate has been used to calculate pK_a value of amino acid residue on these enzymes where the thiocyanate is shown to be binding. From the pH dependence of K_D and ¹³C resonance line-width, it is observed that thiocyanate binds to LPO and HRP only under acidic conditions (pH < 6.1 for LPO and pH < 4.0 for HRP). The binding is facilitated by protonation of an acid group on the enzyme with pK_a = 6.1 for LPO and 4.0 for HRP. The pH dependence of ¹³C resonance line-width of thiocyanate as well as K_Dhave been quantitatively analysed on the basis of a reaction scheme in which thiocyanate in deprotonated ionic form binds to the enzyme in protonated acidic form. K_D for thiocyanate binding to the enzyme has also been evaluated in the presence of excess of exogenous substrates such as resorcinol, cyanide and iodide. The presence of cyanide (which binds to heme iron of enzyme at sixth coordination position) does not have any effect on the binding of thiocyanate, indicating that binding site of thiocyanate ion is located away from the ferric centre of these enzymes. The presence of resorcinol, has significant effect on K_D for binding of thiocyanate to LPO but it has no effect on thiocyanate binding to HRP. The K D in the presence of iodide however shows that iodide competes with thiocyanate for binding at the same site in both the cases. Distance of the carbon atom of bound thiocyanate ion from ferric centre has been deduced from the ¹³C-TJ measurements and is found to be 8.4 Ǻ and 8.0 Ǻ for LPO and HRP respectively. This distance remains unchanged by the presence of cyanide ion at the sixth coordination site of the heme iron of these enzymes. Similarity in the mode of binding of iodide and thiocyanate suggests that the oxidation of thiocyanate ion by H₂O₂, like that of 1^- by H₂O₂ may also proceed via two-electron transfer pathway under acidic conditions. Page(s): 301-311

Interaction of water soluble metalloporphyrins with nucleic acid bases

Sun, 01 Apr 1990 00:00:00 GMT

Title: Interaction of water soluble metalloporphyrins with nucleic acid bases Authors: Reddy, Damodar; Mukherjee, Arka; Chandrashekar, T K Abstract: The interaction of metalloderivatives [Cu²⁺, Zn²⁺ and Co²⁺) of tetrakis (4-N-methylpyridyl)porphyrin (MTMpyP) with nucleic acid bases is described in terms of 1:1 molecular complexes. The binding constants evaluated from optical studies vary as CuTMpyP > ZnTMpyP with a given base and this variation is accounted in terms of structure of the macrocycle. The magnitude of binding constant for adenine complex is larger than that for cytosine complex with a given metal derivative. The Co^IITMpyP undergoes oxidation to CO^IIITMpyP in the presence of molecular oxygen and adenine. ESR studies of the complexes reveal that the π-complexation results in changes in the electronic structure of the central metal ion. Page(s): 312-315

Evidence for strengthening of hydrogen bonds in hydrophobic hydration-Hydrogen isotope effects in dehydration of tetraalkylammonium forms of Dowex -50 W resins

Sun, 01 Apr 1990 00:00:00 GMT

Title: Evidence for strengthening of hydrogen bonds in hydrophobic hydration-Hydrogen isotope effects in dehydration of tetraalkylammonium forms of Dowex -50 W resins Authors: Gupta, A R; Sarpal, S K; Majmudar, A A Abstract: Hydrogen isotope effects in dehydration of tetraalkylammonium [tetramethylammonium (TMA⁺), tetraethylammonium (TEA⁺) and tetrabutylammonium (TBA⁺)] forms of Dowex-50W resins at 353 K by a Rayleigh distillation type technique has been studied. The single stage separation factor, ɑ, is less than ɑ_w (the ɑ value for bulk water at 353 K) at the lowest n_w, (number of moles of water per equivalent of resin) for all the cationic forms. It increases gradually and crosses the ɑ_w at (n_w, at which ɑ = ɑ_w) value of 12, 17 and 22 for TMA⁺, TEA⁺ and TBA⁺ resins, respectively. For n_w > n, ɑ > ɑ_w for all resins. The value of ɑ reaches a maximum value, ɑ_max, of 1.049 at n ≈ 28 for TMA⁺ and then decreases slowly at still larger n_w values. For TEA⁺, the increase in ɑ in the region n_w> n is much greater, reaching a value of 1.067 at the highest value of n_w (=41) studied, whereas for TBA⁺, the ɑ at the highest studied n_w (= 39) is 1.045, indicating that ɑ_max for TEA⁺ and TBA⁺ would be observed at even higher n_w, values than for TMA⁺. These results show that the nearest neighbour water molecules around these ions form a clathrate-type structure, as predicted by the theory of hydrophobic effects, with n as the number of water molecules required to form a monomolecular layer on the cation surface, n as the number of water molecules in the hydrophobic hydration shell of the cation and ɑ_max as the average H-bond strength of water molecules in this shell. These data are also consistent with the general observation that the hydrophobic character of an ion increases with its size. The study provides the first experimental evidence for the strengthening of H-bonds (ɑ > ɑ_w for n_w > n) in the hydrophobic hydration shell. Page(s): 316-320

Structure-conductivity correlation in organic polymers: An MO approach

Sun, 01 Apr 1990 00:00:00 GMT

Title: Structure-conductivity correlation in organic polymers: An MO approach Authors: Ray, Arabinda; Makhija, K K Abstract: The poor electrical conductivity of polyphenylene sulphide (PPS) has been rationalised on the basis of Pariser-Parr-Pople MO method. This approach has been extended to explain the conductivity of polyphenylene oxide (PPO). The change in conductivity of polystilbene by replacing phenylene unit with thiadiazole and thiophene units has been discussed in the light of MO calculation. However, the reported conductivities could not be rationalised on the basis of this simple MO calculation. Page(s): 321-323