The free radicals in irradiated glycines. A comment

CHEMICAL PHYSICSLE’I’TERS

.Volume 8. numb& 2

THE

FREE

RADICBLS

IN IRRADIATED

GLYCINES.

15 January 1971

A COMMENT.

M. j. A. DE BIE * and R. BRAAMS ** ofOrganic Chemistry and Radiobiophysics,

Departments

Rijksuniverstteit

Utrecht.

Received

Utrecht,

The Netherlands

16 November 1970

In X- and Y-irAdiated polycrystalline glycines

the.radiolytic behaviour is dependent on the crystal and/or the extent of deuterium Iabelling. This is due to a selective transformation of the formed ‘CH-COO- radical and not to a selective decay of the ‘CH2 - COO’ radical. ‘CH-COO’ radical into the H$‘-

structure

In a previous letter [ 11 we discussed the importance of the crystal structure when cornpar-. ing EPR results in irradiated glycines with particuIar emphasis on deuterated compounds; conclusions were derived from a comparison of spectra of unlabeled and D-labeled glycines. We suggested that, contrary to the opinion of Weiner and Koski [S], the EPR spectrum in irradiated trideuteroglycine is caused bjr the radical %Q’CHCOz (1) only. We furthermore suggested that previous interpretations of glycine EPR spectra based on studies of the EPR spectra of deuterium labeled glycines are not correct because of the effect of the deuteration on the crystal structure. This led us to the conclusion that Morton’s assumption of selective hydrogen transfer processes 141 is superfluous. In his contiibution Mayer [2] does not challenge cur conclusions concerning the importance of the crystal- structure when comparing the EPR spectra of glycine pith those of deuterated glycities, but he suggests that our experimental results can be.explained by a selective decay of the ‘CH2COO- (II) radical; Then Morton’s interpretation of the results of other authors would not have to be challenged. ’ The selective decay of the ‘CH@O’ radical in yiglycine, as suggested by Mayer, is unlikely . because: (i) Our.exposure times for irradiaifon with 0.2 Mrad were about 25 min. Even after shorter, exposure times,. down to two minutes, the same EPR spectrum was always found. This spectrum did hot change with time and was !ndephdent of &e time between irradiation _*,Or&misch Chemisch Laboratbrium,‘ Croesestraat 79, ” : :Utrecht. the Netherlands. ** Fysisch Labor&oritim. Sorbh-pelaan 4,. Utrectt, .-. the Netherlands. ’ . -_I._. ‘. ‘. ,208: I,-_

and measurement. From our experimental EPR data we cannot find any indication that a relatively fast changing component could have been present in our samples. (ii) As stated in [l] the same dose leads in all modifications to the same total radical concentration (fig. 1) although the relative proportions of the two radical types differ. In our samples of y-glycine where only the type I radical is observed, the initial yield is the same as that of the combined initial yields of the two radical types observed in the other modifications. In y-glycine our initial yield of NH3’CHCOj is thus twice the initial yield of this radical in a-glycine. (iii) The initial total yield of radiolysis products is the same in different glycine crystal forms (fig. 2). If in gr-glycine a substantial contribution of the type II radicals would have decayed before ou’r measurements were undertaken, the initial total radical yield in y-glycine should have been‘higher than in uglycine and higher product yields would have been four@. This, however, was not the case. ____ The spectra given by Weiner and Koski 13J for glycine differing only in the isotopic make up of the methylene group are very similar but not ides-r tical. In all the examples given, the spectra from the glycines with the deuterated _methylene group‘are 5 - 12 gauss narrower than the corresponding ones with the protonated methylene group. . We have ascrided these dkfer&kes-in width to a different isotopic substitdtioti at.fhe or-carbon atom-in the radicals. This assumption issup: @rtqi by the chemical evidence obtained from qua&tative proton magnetic resoaance .studiis of ’ the ,rzdiolysis products’ in a!1 irk&a&d glycine ,_ .. , mdificatio*.[7];‘: -.: -,.I-SeZetitive hydrogen .._,. tkmsferprocesses, as kg‘. ‘, _-.. :-. .. .: : .. ; _. ‘. : : *.: ,. .:.,. _. ._ _.

103,

t

,

,

r

6 - Radical concentration (50’6 g-‘1

I

t

I

i

I

I

/.b*‘

I

. ..,*

6

i

I

1

_*_‘-:--s--.---.

4-

2102‘8 L 2 10 8 2

+ ~-glycinc I

glycine

-dj

Fig. 1. Rndicat concentration in X-irradiated (at 283oK, polycrystatline ix-gtycino. &-giycine. pgtycine and glpcined3 samples. Error in radical concentration is about 10% (for the complete curve. incidental experimental points may have a larger deviation from the curve): error in dose is abut. 10%. Both errors are largely system&c.

kg. 2. The metin G-valves for aU products in X- and gamma-irradiated samples of poIycz=ystaNineCY-glycine, ‘)c glycine aad gtycine-d3 as a function of the total irradiation dose; . _. 209 _ t_ .._-.,

’ ~olunid

8. humbcr

:

CHEMICAL PHYSICS LET+ERS

2

gested by Morton; .should in our opinion be .re-

fleeted in the isotopic composition of the major radiolysis products acetic acid and glyoxylic acid [5-71. Hence in &ideutero-glycine one should expect that the produced acetic acid is mainly in two .forms, viz. CD2Hdm’ and CDSCOO-. However, _,this is not the case, only CHQCOO- and _ CH2DCOO’ are actually formed [7,8]. A similar result is found for the glyoxylic acid. Our experimental

I;esults

indicate

that al-

though ‘CH2COO- has-been formed at an early stage in radiolysis, it is not observed in y-glytine after the irradiation because it is immediately transformed into the type I radical: ‘NH3’CHCO;. In our opinion the available

evidence indicates

that the primary processes during the radiolysis do not depend on the crystal form, but that sec-

ondary processes,

y

’ 15 Japuary 1971 .

such as radical abstraction

reactions etc.., appear to be quite sensitive to changes in crystal mbdifi&&ion, irradiation ternperature, and extent of labeling. ’ REFERENCES [lj M.2. A;de Bie ahd R.Braams, Chem. Phys. Letters 4 (l969) 331.

[2j.L Mayer, Chem. Phys. Letters 7 (1970) 117.

131R. P. Weiner and W. S.Koski. J.Am. Chem. Sot. 85 (1963) 873. [4] J.R. Morton, J.Am.Chem.Soc. 86 (1964)2325. [51 B. Rafewskp and K. Dose, 2. Naturforsch. 12b (1957) 384.

[6] G.Meshitsuka, K.Shindo, A;Minegishi. H.Suguro and Y.Shinozaki, Bull. Chem. Sot. Japsn 37 (1964)

* 928. [7] M..J.A.de Bie, Thesis, Utrecht (1968).

[S] M. J. A. de Bie, to be published.