J. Frau | Universidad de las Islas Baleares (original) (raw)

Papers by J. Frau

Journal of Molecular Structure: THEOCHEM, 2009

is strictly decreasing, π 0 > π, H F 0 ∈ ϕ F (π 0), and H F ∈ ϕ F (π) imply H F 0 < H F , and for... more is strictly decreasing, π 0 > π, H F 0 ∈ ϕ F (π 0), and H F ∈ ϕ F (π) imply H F 0 < H F , and for every π > 0, every point in ϕ F (π) is reached by a unique equilibrium relative price path p(r, π) on [−S, S]. The correspondence ϕ I : R ++ → R is strictly increasing, π 0 > π, H I0 ∈ ϕ I (π 0), and H I ∈ ϕ I (π) imply H I0 < H I , and for every π > 0, every point in ϕ I (π) is reached by a unique equilibrium relative price path p(r, π) on [−S, S]. Proof: It is clear from the construction described in the text that the relative price paths satisfy p(r, π 0) > p(r, π) at all r if π 0 > π. Given the firms problems in (2.1) and (2.2), parts (i)-(iii) of Assumption A guarantee that a larger relative price strictly increases production of the intermediate good and strictly decreases the use of the intermediate good as an input in final good producing locations. Thus, by (2.3) and (2.4), if p(r, π 0) > p(r, π) for any r, the corresponding stocks satisfy H F (s, π 0) < H F (s, π) and H I (s, π 0) > H I (s, π) for all s > r. In the same way, if p(r, π) andp(r, π) are two paths both starting from π that satisfy p(r, π) >p(r, π) for any r, the corresponding stocks of excess supply satisfy H F (s, π) <Ĥ F (s, π) and H I (s, π) <Ĥ I (s, π) for all s > r. Conversely, no two terminal stocks can differ unless their associated relative price paths differ at some r.¤

Theoretica Chimica Acta, 1996

Summary The electrostatic forces within the active site of theβ-lactamaseStaphylococcus aureus P... more Summary The electrostatic forces within the active site of theβ-lactamaseStaphylococcus aureus PC1 have been used to predict structures for the precatalytic complex with ampicillin, methicillin, clavulanate and imipenem. There are significant differences in the orientation of theseβ-lactams within the binding site, which explains the differences in their resistance to the lactamase. The electrostatic forces were calculated using a distributed multipole analysis ofab initio wave functions for both the lactams and the binding site residues, to ensure a good representation of the orientation dependence of this dominant contribution. The predicted binding orientations are contrasted with those predicted by overlaying the electrostatic extrema around the ligands. The accuracy of the ligand-only-based predictions is limited in some cases because of the subtle steric requirements of the lactamase binding site.

Electronic Journal of Theoretical Chemistry, 1997

A complete study of the alkaline and acidic hydrolysis of the β-lactam ring of azetidin-2-one was... more A complete study of the alkaline and acidic hydrolysis of the β-lactam ring of azetidin-2-one was carried out using ab initio molecular orbital calculations at the RHF/6-31+G* and RHF/6-31G** levels, respectively. Alkaline hydrolysis has been studied through a BAC2 mechanism characterized by a nucleophilic attack on the β-lactam carbonyl group, formation of the tetrahedral intermediate and cleavage of the C-N bond until the formation of the final product of the reaction, this being the limiting step of the reaction. On the other hand, the acidic hydrolysis has been studied by means of a A-1 type unimolecular mechanism, characterized by a nitrogen-protonation followed by an opening of the ring and further addition of water to the carbonyl group. The system involving the azetidin-2-one ring, the H3O + ion and a water molecule has been considered. Three transition states have been identified; the barriers corresponding to the addition of H + to the nitrogen and the addition of water to the carbonyl group are practically negligible (first and third reaction steps, respectively).

Journal of Molecular Structure: THEOCHEM, 2009

Gas-phase deprotonation free energies and aqueous solvation free energies of 10 monoprotic and 6 ... more Gas-phase deprotonation free energies and aqueous solvation free energies of 10 monoprotic and 6 diprotic pyridines were calculated using CBS-QB3 and HF/CPCM or B3LYP/CPCM methods. The results have been processed considering three thermodynamic cycles for absolute and relative pK a calculations. In these cycles, different experimental values of solvation free energies for proton, water and hydronium were considered. Moreover, calculated ab initio and DFT values were used when possible. Results show that the inclusion of explicit single water molecule interacting with pyridine nitrogen improves predictions in 1.5 pK a units. Water molecule causes an increase in the solute-solvent surface interaction and allows the continuum method to reproduce correct solvation free energy differences between acids and bases. The correct combination of computational methodology and thermodynamic cycle leads to very accurate results, with mean absolute errors of 0.3-0.5 pK a units for monoprotic and 0.7-0.9 pK a units for diprotic pyridines.

Journal of the Chemical Society, Perkin Transactions 2, 2000

Thiolysis of benzylpenicillin has been investigated by HPLC and 1 H-NMR techniques. Thiols cataly... more Thiolysis of benzylpenicillin has been investigated by HPLC and 1 H-NMR techniques. Thiols catalyse the hydrolysis of benzylpenicillin through the formation of a thioester intermediate. The catalytically reactive form of the thiol has been demonstrated to be the thiolate anion. Variation of reactivity with changing basicity of the thiolate anion generates a Brønsted β nuc value of 0.96, indicating that the breakdown of the tetrahedral intermediate is the rate-limiting step, as occurs in aminolysis and alcoholysis. Solvent kinetic isotope effects of 2.2-2.4 indicate that the solvent, water, probably acts as a general acid catalyst in the breakdown of the tetrahedral intermediate. PM3 theoretical calculations support the proposal that breakdown of the tetrahedral intermediate is rate-limiting. The experimental activation energies for the thiolysis of benzylpenicillin vary from 6.9 to 10.4 kcal mol Ϫ1. †

The Journal of Physical Chemistry B, 2012

The origins of C−H activation in pyridoxal-5′-phosphate (PLP) Schiff bases and modulation of reac... more The origins of C−H activation in pyridoxal-5′-phosphate (PLP) Schiff bases and modulation of reaction specificity in PLPenzymes are still not completely understood. There are no available studies that compare the reactivity of C4′ carbons in ketimine Schiff bases with that of Cα carbons in their aldimine counterparts, which is essential to unravel the mechanisms that govern the evolution of their common carbanionic intermediates. Second-order rate constants for phosphate-catalyzed proton/deuterium exchange reactions in D 2 O of C4′ carbons suffer a 10 5-fold increase due to Schiff base formation (k B = 5.3 × 10 1 M −1 s −1) according to NMR measurements. The C4′ carbon acidity is also increased to pK a = 9.8, which is significantly higher than that of Cα in PLP-aldimines. DFT calculations reveal the role of each heteroatom in modulating the electrophilicity of C4′ and Cα carbons. Specifically, the protonation state of pyridine nitrogen is the main factor in determining the absolute carbon acidity in aldimines (pK a of Cα varies from ∼14 to ∼23) and ketimines (pK a of C4′ varies from ∼12 to ∼18), whereas the protonation state of both imine nitrogen and O3′ phenol oxygen modulates the relative acidities of Cα and C4′ from 1.5 to 7.5 pK a units. Our results provide an explanation to the modulation of reaction specificity observed in different PLP-enzymes based on the differences in the protonation state of the cofactor and Hbonding patterns in the active site.

The Journal of Physical Chemistry A, 2003

A comprehensive theoretical study based on density functional theory calculations (B3LYP and M06-... more A comprehensive theoretical study based on density functional theory calculations (B3LYP and M06-2X functionals) of the formation of Schiff bases of pyridoxamine analogues with two different aldehydes was conducted. The reaction mechanism was found to involve two steps, namely: (1) formation of a carbinolamine and (2) dehydration of the carbinolamine to give the final imine. Also, consistent with available experimental evidence, the carbinolamine dehydration was the rate-determining step of the process determined by means of M06-2X functional. Using an appropriate solvation method and reactant conformation ensures that all proton transfers involved will be intramolecular, which substantially reduces energy barriers and facilitates reaction in all cases. The formation of a Schiff base between pyridoxal 5-phosphate (PLP) and an amine or amino acid requires the contribution of an external water molecule in order to facilitate proton transfers. On the other hand, the formation of a Schiff base between pyridoxamine 5-phosphate (PMP) and a carbonyl compound requires no external aid since the spatial arrangement of the functional groups in PMP ensures that all proton transfers will be intramolecular.

Journal of Photochemistry and Photobiology A: Chemistry, 2010

4-Aminomethylpyridine (4-PAM) has been widely used as a model compound to elucidate the mechanism... more 4-Aminomethylpyridine (4-PAM) has been widely used as a model compound to elucidate the mechanisms of biological and biomedical action of the amino derivatives of vitamin B 6. By virtue of the presence of two ionizable groups (viz. a pyridine nitrogen and an amino function) in its structure, 4-PAM in solution occurs as various ionic and tautomeric forms in equilibrium. In this work, we optimized the geometries of such forms and found the protonation status of the ionizable groups in 4-PAM to affect the molecular geometry and frontier orbitals. In addition, we determined the experimental electronic excitation energies for each molecular species of 4-PAM from deconvoluted UV-vis spectra. The results thus obtained were compared with their theoretical counterparts as determined from TD-DFT calculations. Based on the outcome, the theoretical methodology used affords correct simulation of electronic excitation energies. The theoretical and experimental results showed that the deprotonation of the pyridine nitrogen has no effect on the energy of the first electronic transition, however it affects its intensity. Additionally, the deprotonation of both pyridine nitrogen and methylamino group increases the number of bands, by increasing the n-* transitions.

International Journal of Quantum Chemistry, 2010

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Chemical Physics Letters, 2008

Theoretical calculations based on pseudo-potentials were used to determine the structures of the ... more Theoretical calculations based on pseudo-potentials were used to determine the structures of the complexes formed between Cu 2+ ion and 2-(8-quinolinoxy)propionic acid (LR-74), which is a potent inhibitor of non-enzymatic glycation reactions. The most stable among such complexes consist of one atom of copper and two molecules of inhibitor, [Cu(LR-74) 2 ], and possess a distorted octahedral geometry. In the complex containing a single molecule of inhibitor, the square planar form, [Cu(LR-74)(H 2 O)] + , is slightly more stable than the octahedral form, [Cu(LR-74)(H 2 O) 3 ] + , in solution. All octahedral complexes exhibit strong Jahn-Teller distortion that results in substantially increased axial distances.

Chemical Physics Letters, 2009

Chemical Physics Letters, 2000

The alkaline hydrolysis of a thio-b-lactam in the gas phase was examined in the light of RHF and ... more The alkaline hydrolysis of a thio-b-lactam in the gas phase was examined in the light of RHF and DFT ab initio calculations. The solvent effect was considered via IPCM computations. The tetrahedral intermediate for the thio-b-lactam studied is unstable, so the compound evolves directly to the corresponding thio-azethidin-2-one open ring with cleavage of the C-S bond. The end-products obtained bear a carbamate group, which suggests that the thio-b-lactam might be an effective inhibitor for b-lactamases.

Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics, 2003

Aqueous solutions of pyridoxamine 5Vphosphate (PMP) at several pH conditions have been studied us... more Aqueous solutions of pyridoxamine 5Vphosphate (PMP) at several pH conditions have been studied using FT-IR spectroscopy using the attenuated total reflection (ATR) technique. In spite of the strong intense OH stretching and bending bands of water, most of the vibrational structure of solute can be observed from 900 to 1500 cm À 1. With increasing pH, very intense changes in the spectra have been observed due to concentration changes of the hydrogen bonded species. Spectra of the different ionic species have been calculated from the mathematical fitting of experimental absorption spectra as a function of pH. Spectra are characterized by the presence of broad band-like structures in the 2400-3500 cm À 1 region, with extended continua that indicate very large proton polarizability of hydrogen bonds. Contributions of the phosphate group to the total absorption have been analyzed by comparison with pyridoxamine spectra.

Biopolymers, 1998

In this article a conformational analysis of the D-alanyl-D-alanine dipeptide, both charged and n... more In this article a conformational analysis of the D-alanyl-D-alanine dipeptide, both charged and neutral, has been carried out. The preferred conformations were determined by means of ab initio and semiempirical quantum, together with empirical force field calculations. The AMBER* force field and the 6-31 / G** and 6-31G** ab initio levels give rise to a coincident minimum energy structure, which, on the other hand, differs from that determined by AM1, 3-21 / G, and 3-21G. The solvent effect on the different charged and neutral conformations have been considered through the AMSOL semiempirical method. A quantification regarding the structural similarities between the different dipeptide conformations and the ampicillin has been performed. The results show that the best overlay is attained by the minimum structure energy obtained by using the 6-31 / G** methodology, which presents a planar amidic nitrogen.

Pyridoxamine (PM) is a vitamin B6 derivative involved in a number of catalytic processes by virtu... more Pyridoxamine (PM) is a vitamin B6 derivative involved in a number of catalytic processes by virtue of its properties as enzyme cofactor1. Furthermore, it has been showed PM to be an inhibitor in non-enzymatic protein glycation2 (Maillard reaction). The first step in a non-enzymatic glycation process is a reversible reaction between the terminal amino residues in the protein and the carbonyl group of the sugar, in its extended form, that leads to the formation of a Schiff base. The Schiff base is stable for a limited

Journal of Molecular Structure: THEOCHEM, 2009

is strictly decreasing, π 0 > π, H F 0 ∈ ϕ F (π 0), and H F ∈ ϕ F (π) imply H F 0 < H F , and for... more is strictly decreasing, π 0 > π, H F 0 ∈ ϕ F (π 0), and H F ∈ ϕ F (π) imply H F 0 < H F , and for every π > 0, every point in ϕ F (π) is reached by a unique equilibrium relative price path p(r, π) on [−S, S]. The correspondence ϕ I : R ++ → R is strictly increasing, π 0 > π, H I0 ∈ ϕ I (π 0), and H I ∈ ϕ I (π) imply H I0 < H I , and for every π > 0, every point in ϕ I (π) is reached by a unique equilibrium relative price path p(r, π) on [−S, S]. Proof: It is clear from the construction described in the text that the relative price paths satisfy p(r, π 0) > p(r, π) at all r if π 0 > π. Given the firms problems in (2.1) and (2.2), parts (i)-(iii) of Assumption A guarantee that a larger relative price strictly increases production of the intermediate good and strictly decreases the use of the intermediate good as an input in final good producing locations. Thus, by (2.3) and (2.4), if p(r, π 0) > p(r, π) for any r, the corresponding stocks satisfy H F (s, π 0) < H F (s, π) and H I (s, π 0) > H I (s, π) for all s > r. In the same way, if p(r, π) andp(r, π) are two paths both starting from π that satisfy p(r, π) >p(r, π) for any r, the corresponding stocks of excess supply satisfy H F (s, π) <Ĥ F (s, π) and H I (s, π) <Ĥ I (s, π) for all s > r. Conversely, no two terminal stocks can differ unless their associated relative price paths differ at some r.¤

Theoretica Chimica Acta, 1996

Summary The electrostatic forces within the active site of theβ-lactamaseStaphylococcus aureus P... more Summary The electrostatic forces within the active site of theβ-lactamaseStaphylococcus aureus PC1 have been used to predict structures for the precatalytic complex with ampicillin, methicillin, clavulanate and imipenem. There are significant differences in the orientation of theseβ-lactams within the binding site, which explains the differences in their resistance to the lactamase. The electrostatic forces were calculated using a distributed multipole analysis ofab initio wave functions for both the lactams and the binding site residues, to ensure a good representation of the orientation dependence of this dominant contribution. The predicted binding orientations are contrasted with those predicted by overlaying the electrostatic extrema around the ligands. The accuracy of the ligand-only-based predictions is limited in some cases because of the subtle steric requirements of the lactamase binding site.

Electronic Journal of Theoretical Chemistry, 1997

A complete study of the alkaline and acidic hydrolysis of the β-lactam ring of azetidin-2-one was... more A complete study of the alkaline and acidic hydrolysis of the β-lactam ring of azetidin-2-one was carried out using ab initio molecular orbital calculations at the RHF/6-31+G* and RHF/6-31G** levels, respectively. Alkaline hydrolysis has been studied through a BAC2 mechanism characterized by a nucleophilic attack on the β-lactam carbonyl group, formation of the tetrahedral intermediate and cleavage of the C-N bond until the formation of the final product of the reaction, this being the limiting step of the reaction. On the other hand, the acidic hydrolysis has been studied by means of a A-1 type unimolecular mechanism, characterized by a nitrogen-protonation followed by an opening of the ring and further addition of water to the carbonyl group. The system involving the azetidin-2-one ring, the H3O + ion and a water molecule has been considered. Three transition states have been identified; the barriers corresponding to the addition of H + to the nitrogen and the addition of water to the carbonyl group are practically negligible (first and third reaction steps, respectively).

Journal of Molecular Structure: THEOCHEM, 2009

Gas-phase deprotonation free energies and aqueous solvation free energies of 10 monoprotic and 6 ... more Gas-phase deprotonation free energies and aqueous solvation free energies of 10 monoprotic and 6 diprotic pyridines were calculated using CBS-QB3 and HF/CPCM or B3LYP/CPCM methods. The results have been processed considering three thermodynamic cycles for absolute and relative pK a calculations. In these cycles, different experimental values of solvation free energies for proton, water and hydronium were considered. Moreover, calculated ab initio and DFT values were used when possible. Results show that the inclusion of explicit single water molecule interacting with pyridine nitrogen improves predictions in 1.5 pK a units. Water molecule causes an increase in the solute-solvent surface interaction and allows the continuum method to reproduce correct solvation free energy differences between acids and bases. The correct combination of computational methodology and thermodynamic cycle leads to very accurate results, with mean absolute errors of 0.3-0.5 pK a units for monoprotic and 0.7-0.9 pK a units for diprotic pyridines.

Journal of the Chemical Society, Perkin Transactions 2, 2000

Thiolysis of benzylpenicillin has been investigated by HPLC and 1 H-NMR techniques. Thiols cataly... more Thiolysis of benzylpenicillin has been investigated by HPLC and 1 H-NMR techniques. Thiols catalyse the hydrolysis of benzylpenicillin through the formation of a thioester intermediate. The catalytically reactive form of the thiol has been demonstrated to be the thiolate anion. Variation of reactivity with changing basicity of the thiolate anion generates a Brønsted β nuc value of 0.96, indicating that the breakdown of the tetrahedral intermediate is the rate-limiting step, as occurs in aminolysis and alcoholysis. Solvent kinetic isotope effects of 2.2-2.4 indicate that the solvent, water, probably acts as a general acid catalyst in the breakdown of the tetrahedral intermediate. PM3 theoretical calculations support the proposal that breakdown of the tetrahedral intermediate is rate-limiting. The experimental activation energies for the thiolysis of benzylpenicillin vary from 6.9 to 10.4 kcal mol Ϫ1. †

The Journal of Physical Chemistry B, 2012

The origins of C−H activation in pyridoxal-5′-phosphate (PLP) Schiff bases and modulation of reac... more The origins of C−H activation in pyridoxal-5′-phosphate (PLP) Schiff bases and modulation of reaction specificity in PLPenzymes are still not completely understood. There are no available studies that compare the reactivity of C4′ carbons in ketimine Schiff bases with that of Cα carbons in their aldimine counterparts, which is essential to unravel the mechanisms that govern the evolution of their common carbanionic intermediates. Second-order rate constants for phosphate-catalyzed proton/deuterium exchange reactions in D 2 O of C4′ carbons suffer a 10 5-fold increase due to Schiff base formation (k B = 5.3 × 10 1 M −1 s −1) according to NMR measurements. The C4′ carbon acidity is also increased to pK a = 9.8, which is significantly higher than that of Cα in PLP-aldimines. DFT calculations reveal the role of each heteroatom in modulating the electrophilicity of C4′ and Cα carbons. Specifically, the protonation state of pyridine nitrogen is the main factor in determining the absolute carbon acidity in aldimines (pK a of Cα varies from ∼14 to ∼23) and ketimines (pK a of C4′ varies from ∼12 to ∼18), whereas the protonation state of both imine nitrogen and O3′ phenol oxygen modulates the relative acidities of Cα and C4′ from 1.5 to 7.5 pK a units. Our results provide an explanation to the modulation of reaction specificity observed in different PLP-enzymes based on the differences in the protonation state of the cofactor and Hbonding patterns in the active site.

The Journal of Physical Chemistry A, 2003

A comprehensive theoretical study based on density functional theory calculations (B3LYP and M06-... more A comprehensive theoretical study based on density functional theory calculations (B3LYP and M06-2X functionals) of the formation of Schiff bases of pyridoxamine analogues with two different aldehydes was conducted. The reaction mechanism was found to involve two steps, namely: (1) formation of a carbinolamine and (2) dehydration of the carbinolamine to give the final imine. Also, consistent with available experimental evidence, the carbinolamine dehydration was the rate-determining step of the process determined by means of M06-2X functional. Using an appropriate solvation method and reactant conformation ensures that all proton transfers involved will be intramolecular, which substantially reduces energy barriers and facilitates reaction in all cases. The formation of a Schiff base between pyridoxal 5-phosphate (PLP) and an amine or amino acid requires the contribution of an external water molecule in order to facilitate proton transfers. On the other hand, the formation of a Schiff base between pyridoxamine 5-phosphate (PMP) and a carbonyl compound requires no external aid since the spatial arrangement of the functional groups in PMP ensures that all proton transfers will be intramolecular.

Journal of Photochemistry and Photobiology A: Chemistry, 2010

4-Aminomethylpyridine (4-PAM) has been widely used as a model compound to elucidate the mechanism... more 4-Aminomethylpyridine (4-PAM) has been widely used as a model compound to elucidate the mechanisms of biological and biomedical action of the amino derivatives of vitamin B 6. By virtue of the presence of two ionizable groups (viz. a pyridine nitrogen and an amino function) in its structure, 4-PAM in solution occurs as various ionic and tautomeric forms in equilibrium. In this work, we optimized the geometries of such forms and found the protonation status of the ionizable groups in 4-PAM to affect the molecular geometry and frontier orbitals. In addition, we determined the experimental electronic excitation energies for each molecular species of 4-PAM from deconvoluted UV-vis spectra. The results thus obtained were compared with their theoretical counterparts as determined from TD-DFT calculations. Based on the outcome, the theoretical methodology used affords correct simulation of electronic excitation energies. The theoretical and experimental results showed that the deprotonation of the pyridine nitrogen has no effect on the energy of the first electronic transition, however it affects its intensity. Additionally, the deprotonation of both pyridine nitrogen and methylamino group increases the number of bands, by increasing the n-* transitions.

International Journal of Quantum Chemistry, 2010

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Chemical Physics Letters, 2008

Theoretical calculations based on pseudo-potentials were used to determine the structures of the ... more Theoretical calculations based on pseudo-potentials were used to determine the structures of the complexes formed between Cu 2+ ion and 2-(8-quinolinoxy)propionic acid (LR-74), which is a potent inhibitor of non-enzymatic glycation reactions. The most stable among such complexes consist of one atom of copper and two molecules of inhibitor, [Cu(LR-74) 2 ], and possess a distorted octahedral geometry. In the complex containing a single molecule of inhibitor, the square planar form, [Cu(LR-74)(H 2 O)] + , is slightly more stable than the octahedral form, [Cu(LR-74)(H 2 O) 3 ] + , in solution. All octahedral complexes exhibit strong Jahn-Teller distortion that results in substantially increased axial distances.

Chemical Physics Letters, 2009

Chemical Physics Letters, 2000

The alkaline hydrolysis of a thio-b-lactam in the gas phase was examined in the light of RHF and ... more The alkaline hydrolysis of a thio-b-lactam in the gas phase was examined in the light of RHF and DFT ab initio calculations. The solvent effect was considered via IPCM computations. The tetrahedral intermediate for the thio-b-lactam studied is unstable, so the compound evolves directly to the corresponding thio-azethidin-2-one open ring with cleavage of the C-S bond. The end-products obtained bear a carbamate group, which suggests that the thio-b-lactam might be an effective inhibitor for b-lactamases.

Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics, 2003

Aqueous solutions of pyridoxamine 5Vphosphate (PMP) at several pH conditions have been studied us... more Aqueous solutions of pyridoxamine 5Vphosphate (PMP) at several pH conditions have been studied using FT-IR spectroscopy using the attenuated total reflection (ATR) technique. In spite of the strong intense OH stretching and bending bands of water, most of the vibrational structure of solute can be observed from 900 to 1500 cm À 1. With increasing pH, very intense changes in the spectra have been observed due to concentration changes of the hydrogen bonded species. Spectra of the different ionic species have been calculated from the mathematical fitting of experimental absorption spectra as a function of pH. Spectra are characterized by the presence of broad band-like structures in the 2400-3500 cm À 1 region, with extended continua that indicate very large proton polarizability of hydrogen bonds. Contributions of the phosphate group to the total absorption have been analyzed by comparison with pyridoxamine spectra.

Biopolymers, 1998

In this article a conformational analysis of the D-alanyl-D-alanine dipeptide, both charged and n... more In this article a conformational analysis of the D-alanyl-D-alanine dipeptide, both charged and neutral, has been carried out. The preferred conformations were determined by means of ab initio and semiempirical quantum, together with empirical force field calculations. The AMBER* force field and the 6-31 / G** and 6-31G** ab initio levels give rise to a coincident minimum energy structure, which, on the other hand, differs from that determined by AM1, 3-21 / G, and 3-21G. The solvent effect on the different charged and neutral conformations have been considered through the AMSOL semiempirical method. A quantification regarding the structural similarities between the different dipeptide conformations and the ampicillin has been performed. The results show that the best overlay is attained by the minimum structure energy obtained by using the 6-31 / G** methodology, which presents a planar amidic nitrogen.

Pyridoxamine (PM) is a vitamin B6 derivative involved in a number of catalytic processes by virtu... more Pyridoxamine (PM) is a vitamin B6 derivative involved in a number of catalytic processes by virtue of its properties as enzyme cofactor1. Furthermore, it has been showed PM to be an inhibitor in non-enzymatic protein glycation2 (Maillard reaction). The first step in a non-enzymatic glycation process is a reversible reaction between the terminal amino residues in the protein and the carbonyl group of the sugar, in its extended form, that leads to the formation of a Schiff base. The Schiff base is stable for a limited