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Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Groups.

Edited by Saul Patai Copyright 1996 John Wiley & Sons, Ltd.

ISBN: 0-471-95171-4

CHAPTER 17

Radiation chemistry of amines, nitro and nitroso compounds

WILLIAM M. HORSPOOL

Department of Chemistry, The University of Dundee, Dundee DD1 4HN, Scotland Fax: +44 (0)382 34 5517; e-mail: W.M.HORSPOOL@DJMDEE.AC.UK

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

823

II. RADIOLYSIS OF AMINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

824

A. Radiolysis of Aliphatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . .

824

B. Radiolysis of Aromatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . .

826

III. RADIOLYSIS OF AMINO ACIDS . . . . . . . . . . . . . . . . . . . . . . . . . . .

828

IV. RADIOLYSIS OF NITRO AND NITROSO COMPOUNDS . . . . . . . . . .

832

A. Aliphatic Nitro Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

832

B. Aromatic Nitro Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

832

C. Nitroso Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

834

V. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

835

I. INTRODUCTION

A detailed review1 of this area was published thirteen years ago as part of this series. Within that article a comprehensive coverage of the various methods for irradiating compounds was given. This included energy absorption, early events, dosimetry and reaction kinetics including free-radical reactions, radical scavenging and pulse radiolysis. That review suggested that the radiation chemistry of the species covered by this review was becoming an increasingly important area of study. There are indeed many citations dealing with radiation-induced reactions where amines, nitro and nitroso compounds are used as secondary reagents. More often than not the radiation process is utilized to produce a primary oxidant that then reacts with the substrate under discussion. The radiation process in these instances is not directly reacting on the amine or nitro compound. Thus a reviewer has many problems to wrestle with regarding inclusion or omission of articles. This review of the area has tried to provide a flavour of what is happening currently but is by no means exhaustive for the reasons given above.

823

824

William M. Horspool

II.RADIOLYSIS OF AMINES

A.Radiolysis of Aliphatic Amines

Some studies have focused on the generation of the corresponding radical cations of methylamine, dimethylamine and N-methylpiperidine2,3 by -irradiation at low temperature. The radical cation of t-BuNH2 can be formed k D 3.4 ð 106 M 1 s 1 by oxidation with DMSOÐCl4.

Trimethylamine has been subjected to radiolysis studies. Thus the trimethylamine radical cation 1 can be produced by -irradiation (60Co source) at 77 K in trichlorofluoromethane. This technique utilizes the facile generation of the radical cation of the trichlorofluoromethane. This radical cation then transfers an electron from the substrate to produce the radical cation of the amine. The EPR spectra of the radical cations were recorded. The cations produced under these conditions can be trapped indefinitely and do not undergo proton loss to give the radical 25. In a pulse radiolysis examination in basic aqueous solutions saturated with N2O and O2 two radicals 2 and 3 are seen in a ratio of 9:1. The radical 1 reacts readily with oxygen k D 3.5 ð 109 M 1 s 1 to yield the iminium ion 4. The electron transfer to yield 4 is preferred to the path that would yield the peroxy radical 5 followed by fission to 4. Trapping of 4 by hydroxide and hydrolysis of the hydroxymethyl dimethylamine accounts for the principal products, dimethylamine and formaldehyde, of the reaction6. However, further study showed that the formation of a nitrogen-centred radical was the important first step. Thus attack by the hydroxy radical on the amine affords the radical cation 1 and hydroxide. The former species can be transformed into each of the others by hydrolytic processes yielding the alkyl radical 2 or, on protonation, the conjugate acid 6. The radical cation 1 is the stable species in acid7. This reaction has also been discussed in a review article8.

Me3 N +

Me2 NCH2

O2 2

 

Me2 NCH2 O2

(1)

(2)

(3)

 

(5)

 

+

 

 

+

 

 

 

Me2 N

 

CH2

Me2 N

 

CH2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

 

(4)

 

(6)

 

 

Triethylamine can also be converted into the corresponding radical cation by - irradiation at low temperature in trichlorofluoromethane5 or by oxidation with the

carbonate radical CO3ž 9 produced by pulse radiolysis. The radical cation of triethylamine has also been studied using time-resolved fluorescence detected magnetic resonance (FDMR). This work showed that the rate of formation of the radical cation was the same in n-hexane or in cyclohexane10. Triethylamine has, of course, been used in many studies as a sacrificial electron donor. This is a common use in photochemical systems as well as in pulse radiolysis. A typical example of this is the use of triethylamine as an electron donor in the photoreduction of carbon dioxide to formic acid in nonaqueous polar solvents using oligo(p-phenylenes) as the photocatalyst11, or the formation of hydrogen from water12. The sacrificial use of triethylamine is also seen in the pulse radiolysis generation of captive electrons in glasses at low temperature. The glasses are ethanol/triethylamine and radiolysis brings about the ejection of an electron from the triethylamine thus producing the corresponding radical cation13. Other studies have examined the temperature dependence of electron trapping in such glasses14,15.

17. Radiation chemistry of amines, nitro and nitroso compounds

825

Silicon derivatives of these simple amines have also been studied using -irradiation in CFCl3 solution at 77 K. The radical cations 7 and 8 are formed in each of the cases. The EPR study showed that the singly filled MO of the radical cation was delocalized and extended into the silyl groups. The hydrazine derivative 9 also affords a radical cation

within which a twisted geometry exists with the two nitrogen tensors at an angle of 24°16 .

+

n

m

R

+

(Me3 Si)nNRm

 

 

 

(Me2 SiH)2 NH

1

2

Me

(7)

(8)

 

 

 

 

2

1

Me

 

 

3

0

 

 

2

1

H

 

 

 

 

 

 

Dopamine (10) has also been the subject of some study. Maity and coworkers17 have studied the pulse radiolysis or -irradiation induced reduction of the protonated form. In this instance the addition of an electron affords the radical anion 11 with a bimolecular rate constant for the reaction of 2.5 ð 108 M 1 s 1.

HO

 

 

HO

 

+

 

NH2

+ NH3

(Me3 Si)2 N N(SiMe3 )2 HO

 

 

HO

 

(9)

(10)

 

(11)

H

 

H

 

 

 

+

H

 

N +

 

 

N

 

 

 

 

 

 

N

 

 

 

+

 

 

(12)

(13)

 

(14)

 

The cyclic amines, aziridine and azetidine, can be converted to their radical cations 12 and 13, respectively, by -irradiation at 77 K in trichlorofluoromethane18. In the case of the aziridine radical cation there is evidence that it opens to afford 14. There is a slight solvent dependency in the reaction of these systems and when the radical cation 13 of azetidine is irradiated in a matrix of CFCl2CF2Cl the radical cation converts into the neutral radical 15. With aziridine in the same matrix the radical 16 is obtained without the generation of the radical cation. Hindered secondary amines such as 2,2,6,6-tetramethylpiperidine are of interest as antioxidants. However, -irradiation (at 25 °C with 72 rad min 1) of well oxygenated dilute solutions of the piperidine 17 in 2,4-dimethylpentane has shown that these amines are not primary antioxidants19 22. Studies have also examined the use of secondary amines for the protection of polymers against damage from -radiation23. The stabilizers undergo oxidative transformations in the process. Hindered amine stabilizers, in combination with trivalent phosphorus melt processing stabilizers, are the stabilizers of choice. These are better than phenolic stabilizers since there is less discolouration of the polymer. Specifically polypropylene can also be protected against -initiated oxidation24.

826

William M. Horspool

FDMR has also been used to detect the transient radical cations formed from secondary amines by pulse radiolysis. As mentioned earlier this technique has been used to study a variety of systems such as the radical cation of triethylamine. The radical cations of diethylamine, n-propyl amine and t-butylamine, have also been studied25. The results have shown that the FDMR signal is enhanced with increasing alkyl substitution of the amine as in the pyrrolidines (18) and the piperidines (19)25.

N

 

 

 

R

 

R

 

 

 

( )n

Me

N

Me

R

N

R

R

N

R

Me

Me

 

H

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H

 

 

H

 

(15) n = 2

 

 

 

 

 

 

 

 

 

(16) n = 1

 

(17)

 

(18) R = H or R = Me

(19) R = H or R = N

-Irradiation at 77 K in trichlorofluoromethane of cyclic tertiary amines also affords radical cations that can be trapped indefinitely. In these systems there apparently is no reaction between the radical cations and free amine. The EPR spectra of the radical cations were recorded. The cations produced under these conditions can be trapped indefinitely and do not undergo proton loss to give the corresponding carbon-centred radical. Several systems (20 24) were examined in this way and all were found to be stable. In the bis amine (22, n D 1) evidence was obtained from the EPR study that there was weak N N interaction4. The influence of silicon in 25 was also examined16.

( )n

 

 

 

 

+

 

 

 

 

 

+

 

+

Me

N

N Me

 

 

 

 

N

 

Me2 NCH2 NMe2

 

 

 

Me n = 1 or 2

 

 

 

 

(20)

 

(21)

 

(22)

 

 

 

 

N

 

 

+

N

 

 

 

+

N

 

 

 

N SiMe3

 

 

 

N

 

 

 

 

 

+

 

 

 

(23)

 

(24)

 

(25)

B. Radiolysis of Aromatic Amines

Triphenylamine (TPA), N,N,N0 ,N0 -tetramethyl-p-phenylenediamine (TMPD) and dimethylaniline (DMA) have been popular substrates for reaction under pulse radiolysis conditions. One of the earlier reports dealt with the formation of the radical cation of TMPD by reaction k D 3 ð 108 M 1 s 1 with the peroxy radical derived from oxidation of methylene chloride CHCl2O2 by pulse radiolysis26. DMA is also oxidized to its radical cation by the same reagent k D 2.5 ð 107 M 1 s 1 . Since then it has been

17. Radiation chemistry of amines, nitro and nitroso compounds

827

 

TABLE 1. Absolute rate constants for reaction of

 

 

peroxyl radicals RO2

ž with TPMD

 

 

 

 

 

R

k M 1 s 1

 

 

 

 

ð

7

 

 

 

Me

4.3

107

 

 

 

Et

3.3 ð

107

 

 

 

Bu

2.9 ð

106

 

 

 

i-Pr

9.2 ð

106

 

 

 

t-Bu

1.1 ð

108

 

 

 

ClCH2

4.2 ð

108

 

 

 

Cl2CH

7.4

ð

109

 

 

 

Cl3C

1.7

ð

10

 

 

demonstrated that a variety of oxidants can be used to convert TMPD into the radical cation TMPDžC . The absolute rate constants for the formation of this species by reaction with alkyl and haloalkylperoxyl radicals RO2ž have been determined27,28 (Table 1).

Similar oxidation of amines occurs with pulse radiolysis of an aerated DMSO solution containing 5% CCl4 where the reactant species is DMSO Ð Cl. The effective production of the radical cations was concentration dependent but reached a plateau at 0.39 mmol/J. - Radiolysis gave a higher plateau value of 0.42 mmol/J4. Amines have also been oxidized by the use of sulphate radical anion obtained by pulse radiolysis in 95% acetonitrile solution. The rate constants were in the range of 107 109 mol 1 s 1 for the formation of the radical cation. Apparently these values are lower than those obtained from other solvent systems such as water29. A publication has collated data re absolute rate constants for the reaction of peroxy radicals with organic solvents30. The same amine substrates TPA and TPMD can also be converted into their radical cations k D 1010 M 1 s 1 by the use of bromine atoms generated by pulse radiolysis31.

TMPD k D 5.2 ð 108 M 1 s 1 , p-diaminobenzene k D 5 ð 107 M 1 s 1 and diphenylamine k D 1 ð 107 M 1 s 1 can all be readily converted into the corresponding radical cation by oxidation with pulse radiolysis generated SO3ž . With higher redox potential amines such as aniline and N,N-dimethylaniline the oxidation to the radical cation fails32. Rate constants have also been measured for conversion of the same amines into their radical cations by reaction with SO4ž 33 .

Indoles can be also be converted into their radical cations by the use of ClO2ž as the oxidant produced by pulse radiolysis. From the reactivity of the resultant cation it was possible to establish the one-electron reduction potential of the indole in question. Typical results from this are illustrated in Table 234. As can be seen, the one-electron reduction potential is influenced by alkyl substitution.

A measure of the one-electron reduction potential of tryptophan at pH 7 has shown it to be 0.093 V more positive than the tyrosine radical35. Further details on tryptophan and peptides, within which this moiety is present, have also been obtained using pulse- radiolysis-generated N3ž . This oxidant converts tryptophan into its radical cation with a

second-order rate constant of 3 ð 109 M 1 s 136 . Interestingly the indolyl radical cation is capable of oxidizing tyrosine to the phenoxy radical37. Using this result and N3ž as the oxidant it has been possible to examine the through-bond electron transfer in peptides where the tryptophan unit is separated by an insulator from a tyrosine. Again the formation of the radical cation of the tryptophan was the first event38. Other studies39 have shown that the redox potential of the tryptophan unit is only slightly lowered from the value for tryptophan (Em D 1.05 V at pH 7) when it is incorporated into peptides40. The

828

William M. Horspool

 

 

TABLE 2. One-electron reduction potentials of some

 

indoles

 

 

 

 

 

Indole

E° (V)

 

Indole

1.24

 

N-Methylindole

1.23

 

2-Methylindole

1.10

 

3-Methylindole

1.07

 

2.3-Dimethylindole

0.93

 

Tryptophan

1.24

 

 

 

TABLE 3. Absolute rate constants for the reaction of chloroalkylperoxy radicals with chlorpromazine

Radical

Solvent

Rate constant M 1 s 1

CCl3O2ž

CHCl2O2ž

CHCl2O2ž

CH2ClO2ž

CH2ClO2ž

H2O/i-PrOH/CCl4 ratio 90:10:0.06

5.2

8

ð 108

H2O/i-PrOH/CHCl3

ratio 90:10:0.1

3.7 ð 106

H2O/i-PrOH/CHCl3

ratio 9:81:10

3.4

ð 107

H2O/i-PrOH/CH2Cl2

ratio 90:10:0.5

2.8

ð 106

H2O/i-PrOH/CH2Cl2

ratio 66:33:1

1.7

ð 10

reduction potential determinations of biochemically important free radicals have been reviewed41.

Other important aromatic amines such as chlorpromazine (26) have also been subjected to oxidation studies using oxidants produced by pulse radiolysis. Typical among these is the use of chloroalkylperoxyl radicals formed by pulse radiolysis in a variety of solvents. These oxidants yield the corresponding radical cation. The rate constants (Table 3) for these reactions were determined42. Other studies have determined the reactivity between chlorpromazine and Br2ž in H2O/DMSO in varying proportions. The rate constants for the formation of the radical cation of chlorpromazine were similar in value to those obtained from the peroxy radical reactions4.

S

Cl

N

NMe2

(26)

III. RADIOLYSIS OF AMINO ACIDS

An excellent, fairly recent, review of this subject was published in 1987 as part of a text book dealing with radiation biology43. Since that time several advances have been reported. Much of the work reported in earlier reviews draws on material published in the 1960s and 1970s and, of course, most of this was reviewed (up to 1982) in the earlier volume1 in this series. As with the other studies reviewed in this chapter, much of the work deals with the radiolytic generation of an oxidizing species that subsequently reacts with the substrate, in this section with amino acids. Typical of this is the description by Monig¨ and coworkers44 of the reaction of hydroxy radicals, generated by -irradiation,

17. Radiation chemistry of amines, nitro and nitroso compounds

829

 

TABLE 4. Efficiency of decarboxylation of amino acids

 

 

at pH 10.1 by hydroxy radicals

 

 

 

 

 

 

 

 

 

 

G(CO2) CO2 yield versus

 

 

Amino acid

radiation dose

 

 

 

 

 

 

 

Glycine

4.0

 

 

 

˛-Alanine

5.2

 

 

 

Valine

3.6

 

 

 

Leucine

3.9

 

 

 

Serine

4.5

 

 

 

N,N-Dimethylglycine

5.4

 

 

 

 

 

 

 

with amino acids. This brings about an efficient decarboxylation of the acid provided that the reaction is carried out in basic aqueous solution. Under these conditions the lone pair of electrons on the nitrogen is not protonated and it is at this site that the hydroxy radical attacks via a three-electron bonded system as illustrated in 27. This intermediate collapses to hydroxide, carbon dioxide and the strongly reducing carbon-centred radical 28. The detailed study (some examples are shown in Table 4) showed that decarboxylation occurs only when the amino and the carboxyl groups are on the same carbon. Furthermore, the yield of carbon dioxide amounts to 60 100% of the hydroxy radicals present.

RCHCOORCHNH2

NH2

OH

(27)(28)

The introduction of phenyl groups affords another reaction path via the aryl group. Thus a typical example is that of phenylalanine (29) that can be hydroxylated radiolytically at the aryl group45. Another study has examined the reaction of pulse-radiolysis-generated hydroxyl radicals with the same substrate. Hydroxy radical attack in this instance leads to the formation of hydroxycyclohexadienyl radicals that can be oxidized to tyrosines (30). The attack by the hydroxy radical is quite random, however, although the meta position appears to be disfavoured. With sulphate radicals decarboxylation is again an important process along with tyrosine formation46. Apparently, sulphate radical attack generates a radical cation which either reacts with water to afford tyrosines or undergoes intramolecular electron transfer that results in decarboxylation. Conversion of the carboxylate into the corresponding radical occurs by electron transfer in situations where the ˛-amino group is protonated. These processes have been studied using Fe(VI) and Fe(V) employing pulse radiolysis. The rate constants for the reaction of Fe(VI) with the carboxylates is in the range of 10 103 M 1 s 1 while those for the reaction of Fe(V) are orders of magnitude greater47. Other studies with metal ions have examined the decarboxylation and deamination of 2-methylalanine 31. This pulse study identified that the reaction of the carbon-centred radical 32 with Cu2C or CuC formed a transient. In the case of Cu2C the transient is suggested as 33, with a copper carbon bond. This decomposes by a ˇ-carboxyl elimination reaction yielding CuC , carbon dioxide and the salt 34 that hydrolysed into acetone. This mechanism is thought to describe a new pathway for biological damage induced by free radicals48. The yield of radiation-induced radical formation in short peptides has also been measured and compared with the amino acids present49.

830

 

 

 

 

William M. Horspool

 

 

 

 

 

 

 

 

 

OH

 

 

CH3

 

 

 

 

 

 

 

 

CCO

PhCH2 CHCO2 H

 

 

 

 

 

CH2 CHCO2 H

CH

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

 

2

 

 

 

 

 

 

 

 

 

NH2

 

+

 

NH

 

NH2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

(29)

(30)

 

 

 

(31)

 

 

 

CH3

 

 

 

2 +

 

 

 

CH3

 

 

 

 

 

CH3

 

 

 

 

CH2

 

CCO2

 

CuIII

 

CH2

 

CCO2

 

 

 

 

 

 

 

 

 

CH2

 

 

C

+

 

 

 

 

 

 

 

 

 

 

 

+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NH3

 

 

 

NH

 

 

 

 

 

NH3

 

 

+ 3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(32)

(33)

 

 

 

 

 

(34)

 

Methionine (35) also undergoes decarboxylation affording the radical 36 by attack of hydroxy radicals at pH 350. In this system the hydroxy radical attacks at sulphur in the first instance. This transient, with a three-electron S O bond, is converted into the transient 37 and it is from this, the five-membered transition state, that decarboxylation takes place. The free acid is essential since no decarboxylation occurs with the ester 38. A study of the transients in such systems has been carried out in frozen aqueous solutions at 77 K51. With S-methylcysteine (39) there is no interaction between the sulphur and the nitrogen. The transient formed on hydroxy radical oxidation was proposed as 40 and no decarboxylation takes place52. In the constrained methionine derivative 41 decarboxylation induced by hydroxy radicals is pH dependent53. Again the oxidation takes place at the sulphur. However, the intermediate 42 has a constant lifetime in the pH range 2.5 8. At higher pH the key intermediate is 43 and it is this that undergoes decarboxylation into the radical 44.

 

 

 

 

 

+

 

 

 

 

 

CO2 R

 

S NH2

 

COOH

 

 

 

 

CH3 SCH2 CH2

 

CHNH2

CH3 SCH2 CH2 CHNH2

CO2 CH3 SCH2

 

CHNH2

 

 

 

(35) R = H

(36)

(37)

 

(39)

 

(38) R = Et

 

 

 

 

 

 

CH3

 

 

H

 

 

 

 

 

 

 

+

 

 

 

 

 

 

 

 

 

S O

 

 

 

 

 

 

 

 

 

 

 

+

 

+

 

 

 

 

 

 

 

 

NH3

 

 

 

O

 

NH3

 

 

 

 

 

 

 

 

 

 

 

 

+ NH3

S

COO

 

C

 

 

 

 

Me

 

S O

 

 

O

 

(40)

 

 

 

(41)

Me

 

 

 

 

 

 

 

(42)

 

 

 

17. Radiation chemistry of amines, nitro and nitroso compounds

831

 

 

 

+

 

 

 

 

NH3

NH2

 

S +

CO2

Me S

Me

..

. OH

 

 

 

 

 

 

 

 

 

(43)

(44)

Gly-Met

Met-Glu

Met-Gly-Gly

Gly-Met-Gly

Pro-Met

Glu-Met

+

O

 

 

SMe

 

 

 

 

 

 

H3 N

 

 

 

 

 

 

NH

CO

 

 

 

 

2

 

 

 

CO

 

 

 

 

 

 

2

 

 

 

 

 

 

NH

 

 

CO

 

 

 

 

 

 

2

 

 

 

CO

 

+

 

SMe

 

O

 

NH

 

 

 

 

2

 

3

 

 

CO2

O

 

 

 

 

 

NH

 

 

 

CO2

 

 

 

 

 

NH

 

 

O

 

 

 

 

SMe

 

 

 

 

 

 

+

O

CO2

 

 

 

 

 

 

 

 

NH

C

 

NH

 

CO

3

 

 

 

 

2

 

 

NH

 

 

 

 

 

 

 

O

 

 

 

 

 

SMe

 

 

 

 

 

O

 

 

SMe

NH2

 

 

 

 

 

 

 

 

 

 

 

NH

CO

 

 

 

 

 

2

 

 

CO

 

 

 

 

 

 

2

 

 

 

 

 

+

 

 

NH

 

CO

 

 

 

 

2

 

H3 N

O

SMe

SCHEME 1

832

William M.

Horspool

 

 

 

Other studies have

examined the effect

of pulse-generated hydroxy radicals on

short peptides containing the methione unit. Transients

involving S

 

N interaction

 

have been detected

in L-methionyl-L-methionine54. In

-glutamylmethionine and

the S-alkylglutathione derivatives decarboxylation is also observed. However, the decarboxylation is thought to proceed by two different routes involving either (i) electron transfer between the oxidized sulphur and the carboxyl group on the terminal C-atom when both reactants are within the same peptide unit or (ii) interaction between a hydroxy radical adduct and a protonated amino group sited ˛ to a carboxyl group. This results in a process referred to as N-terminal decarboxylation55. The influence of peptide sequence has also been studied56. Mechanistically the decarboxylation, when it occurs, is the same in these systems as in the shorter peptides or in methionine itself and involves electron transfer from the methionine carboxylate function to the oxidized sulphur function. The effect of the make-up of the peptide is seen with the following systems: Met-Gly, Met-Glu, Met-Gly-Gly, Gly-Met-Gly and Pro-Met where decarboxylation does not occur. However, 80% decarboxylation occurs with -Glu-Met (Scheme 1)56.

Amino acids and proteins have also been shown to undergo oxidation when exposed to oxygen free radicals generated by -irradiation. This treatment results in the formation of hydroperoxide groups in, e.g., bovine serum albumin (BSA) or lysozyme. Common amino acids such as glutamate, isoleucine, leucine, lysine, proline and valine also undergo this peroxidation with similar efficiency57. The oxidation of BSA has also been studied

using a variety of pulse-radiolysis-generated oxidants such as Br2ž 58 . Other research has examined the reaction of hydroxy radicals, generated by 60Co irradiation, with proteins either under an atmosphere of N2O or of oxygen59 62.

IV. RADIOLYSIS OF NITRO AND NITROSO COMPOUNDS

A. Aliphatic Nitro Compounds

Pulse-radiolysis-induced electron transfer to nitro groups has been studied in some detail. Thus 1,1-dinitrocyclohexane undergoes conversion to the corresponding radical anion 45 on radiolysis in t-BuOH/H2O (20:80) at pH 7. This species collapses to the radical 46 with loss of nitrite63. The products of the reaction result from the subsequent reactions of the radical 46 or of the nitronate 47 formed by addition of another electron to the radical. A later pulse-radiolysis study has examined64 the reactivity of 2,2-dinitropropane and 1,1-dinitrocyclopentane and has shown that a similar reaction path is followed as that for 1,1-dinitrocyclohexane. However, the loss of nitrite from the radical anions of 2,2-dinitropropane and 1,1-dinitrocyclopentane was shown to be faster than that for 1,1-dinitrocyclohexane. The radical cation of 1-cyano-1-nitrocyclohexane (48), formed by radiolysis, also decomposes rapidly by elimination of nitrite. The subsequent cyanocyclohexyl radical undergoes hydrogen abstraction reactions from solvent64.

NO2

 

NO2

 

NO2

NO2

NO2

 

 

CN

(45)

(46)

(47)

(48)

B. Aromatic Nitro Compounds

Aromatic nitro compounds are also prone to undergo one-electron reduction on pulse radiolysis. The behaviour of the radical cation so formed is dependent upon the type of

Соседние файлы в папке Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups. Part 2