Nitrosation Of Amino Acids Research Articles

Process improvement in amino acid N-carboxyanhydride synthesis by N-carbamoyl amino acid nitrosation

Amino acid N-carboxyanhydrides (NCA), convenient monomer for polypeptide synthesis, are easily prepared in high purity as the result of N-carbamoyl amino acids (CAA) nitrosation by gaseous NOx (4:1 NO + O2 mixture, or NOCl) in toluene. Removal of polar side products is then efficiently carried out during subsequent work-up and crystallisation so that the resulting NCA obtained in good yield is suitable for controlled, primary amine-initiated polymerisation.

Effect of Added Nucleophilic Species on the Rate of Primary Amino Acid Nitrosation

The rate of primary amino acid nitrosation, both with and without the addition of nucleophilic species, has been studied using stopped-flow spectrophotometry. The rate of nitrosation in the presence of strong nucleophilic species such as thiocyanate and thiourea was shown to be much faster than nitrosation without the addition of a nucleophile. Rate constants were determined at 25 degrees C for reaction of the amino acids alanine, glycine, and valine with five common nitrosating agents. For the nitrosating agents nitrosyl chloride, nitrosyl bromide, and dinitrogen trioxide the rate of reaction was observed to approach the predicted encounter-controlled limit. However, for nitrosyl thiocyanate and S-nitrosothiourea nitrosation was found to be reaction-controlled. In the reaction-controlled regime, rate constants were found to increase with increasing electrophilic strength of the nitrosating agent, as measured by the parameter En, with a slope indicative of a product-like transition state. Activation energies were also measured, being around 10-30 kJ mol-1 for encounter-controlled rate constants, and 30-50 kJ mol-1 for reaction-controlled rate constants. Our results are discussed in the context of in vivo amino acid nitrosation, where it is proposed that the rate of nitrosation may be considerably greater than currently thought, due to the presence of nucleophilic species.

A method for the kinetic study of amino acid nitrosation reactions

A method for the kinetic study of amino acid nitrosation reactions whose products are unstable is described. The reactions of NaNO 2 with α-, β-, and γ-amino acids with a primary amino group in acidic media were studied spectrophotometrically by monitoring the absorbance of the nitrite. The experimental rate equation r= k 3exp[amino acid][nitrite] 2 was found. The second-order with respect to the nitrite concentration implies that dinitrogen trioxide would be the main nitrosating agent.

Reactivity of amino acids in nitrosation reactions and its relation to the alkylating potential of their products.

Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process.

Amino acid nitrosation products as alkylating agents.

Nitrosation reactions of alpha-, beta-, and gamma-amino acids whose reaction products can act as alkylating agents of DNA were investigated. To approach in vivo conditions for the two-step mechanism (nitrosation and alkylation), nitrosation reactions were carried out in aqueous acid conditions (mimicking the conditions of the stomach lumen) while the alkylating potential of the nitrosation products was investigated at neutral pH, as in the stomach lining cells into which such products can diffuse. These conclusions were drawn: (i) The alkylating species resulting from the nitrosation of amino acids with an -NH(2) group are the corresponding lactones; (ii) the sequence of alkylating power is: alpha-lactones > beta-lactones > gamma-lactones, coming respectively from the nitrosation of alpha-, beta-, and gamma-amino acids; and (iii) the results obtained may be useful in predicting the mutagenic effectiveness of the nitrosation products of amino acids.

Structural effects on the N‐nitrosation of amino acids

The relative importance of three different routes for the N- nitrosation of amino acids (nitrosation by N2O3, by NO+/NO2H2+ and by intramolecular migration of the nitroso group from the initially nitrosated carboxylate group) was investigated for methylaminobutyric acid, methylaminoisobutyric acid, azetidine-2-carboxylic acid, azetidine-3-carboxylic acid, indoline carboxylic acid, and phenylaminoacetic acid. Reaction kinetics were determined by the initial rate and Guggenheim methods, by spectrophotometric monitoring of the formation of nitroso amino acid. Kinetic parameters were calculated using a nonlinear optimization algorithm based on Marquardt's method. In the experimental rate equation the dominant term corresponds to nitrosation by dinitrogen trioxide, which experiments at various temperatures show to take place via an ordered transition state. Nitrosation by intramolecular migration is significant for substrates facilitating the formation of a transition state structure with a 5- or 6-membered ring. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 495–504, 1997.

The nitrosation of Amino Acids

AbstractWith a view to clarifying analogies and differences between the mechanisms involved in the nitrosation of amino acids and secondary amines, we studied the kinetics of the nitrosation of five imino acids (azetidine‐2‐carboxylic acid, pyrrolidine‐2‐carboxylic acid, piperidine‐2‐carboxylic acid, piperidine‐3‐carboxylic acid, and piperidine‐4‐carboxylic acid) and of the ethyl esters of three of them. Reaction kinetics were determined by the initial rate method, by spectrophotometric monitoring of the concentration of nitroso amino acid formed. The presence of the COO− group in the amino acids opens a new mechanistic route for the nitrosation of the secondary amino group: a nitrosyl carboxylate formed initially acts as an internal nitrosating agent, resulting in intramolecular migration of N  O from the carboxylate group to the secondary amino group. The observed order of the α−, β−, and γ‐amino acids as regards the ease of N‐nitrosation by this route is explained in terms of the relative energies of (a) the equatorial and axial orientations of the CringCcarboxyl bond, and (b) the chair and boat forms of the piperidine ring. © 1994 John Wiley & Sons, Inc.

Aminoacid nitrosation kinetics by alkyl nitrites in basic media

A kinetic study of the nitrosation of the aminoacids proline, sarcosine and 4-hydroxyproline using 2-ethoxyethyl nitrite as nitrosating agent has been carried out. The mechanism proposed is considerably different from that found when studying this kind of reactions in acid media.

Nitrosation of sarcosine, proline and 4-hydroxyproline by exposure to nitrogen oxides

The nitrosation of amino acids (sarcosine, proline and 4-hydroxyproline) was investigated in model experiments. The compounds were exposed in crystalline form and as aqueous solutions to defined concentrations of nitrogen oxides (in the range of 1–100 ppm) in the atmosphere of a reaction chamber. Nitrosated amino acids were analysed by GC/TEA after silylation. Generally the extent of nitrosation increased with NO x concentration, reaction time and air humidity. Nkrosation of aqueous amino acid solutions decreased markedly with increasing pH.

Stereospezifische 1,2‐Wanderungen von Hydroxyl‐, Aryl‐ und Alkyl‐ gruppen bei der Nitrosierung von Aminosäuren in Fluorwasserstoff/Pyridin. Vorläufige mitteilung

Stereospecific 1,2‐Migration of Hydroxyl, Aryl and Alkyl Groups during the Nitrosation of Amino Acids in Hydrogenfluoride/Pyridine Vorläufige MitteilungThe nitrosation of α‐amino acids in hydrogenfluoride/pyridine was reinvestigated. In contrast to recent reports [1] [2] it was found that this reaction takes place either with entire (phenylalanine, tyrosine, threonine) or partial (valine, isoleucine) rearrangement to yield β‐fluorocarboxylic acids. Glycine, alanine and α‐aminobutanoic acid were converted exclusively into the α‐fluorocarboxylic acids. The substitution of the amino group takes place with stereochemical retention as revealed by NMR. examination of the 2‐fluoro‐3‐methylpentanoic acids obtained from L‐isoleucine and D‐alloisoleucine, respectively.