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.
da Silva
Gabriel
G
Process Safety and Environment Protection Research Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia.
Kennedy
Eric M
EM
Dlugogorski
Bogdan Z
BZ
D000409:Alanine / Q000737:chemistry; D000596:Amino Acids / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry; D007700:Kinetics; D009589:Nitrogen Oxides / Q000737:chemistry; D015538:Nitrosation; D009603:Nitroso Compounds / Q000138:chemical synthesis*; D014633:Valine / Q000737:chemistry
D000596:Amino Acids; D009589:Nitrogen Oxides; D009603:Nitroso Compounds; C031701:nitrogen trioxide; D014633:Valine; D000409:Alanine; D005998:Glycine