/. 05 
Methionine 
“ ‘ 
$0 
nm 
_ Net Beto Activity (counts per second) 
2. Treatment of the paper chro- 
matogram with a labeled reagent. 
The possibilities of this method have 
received little attention. We have 
made experiments on the possible 
location of certain amino acid groups 
by methylation with I'?!-labeled methyl 
iodide. No attempt has been made to 
apply this particular reaction as a 
quantitative tool, and its description 
here is merely included to illustrate the 
principles of the method which, it is be- 
lieved, has considerable potentialities. 
The important condition for the 
successful application of this method 
is that a labeled reagent be chosen 
which will selectively react with one or 
more of the compounds separated on 
the paper chromatogram but not with 
the paper material itself. 
A strip of Whatman No. 1 paper was 
spotted with solutions of different 
amino acids, dried, and exposed to 
methyl iodide-I'*! vapor at room tem- 
perature. Methylation of the amino 
acids resulted in the liberation of I! 
in the amino acid zones, particularly 
in the methionine zone, probably as a 
result of the formation of the methyl 
methionine sulphonium iodide. After 
+ The Ry value is the distance traveled 
by the individual band divided by the dis- 
tance traveled by the solvent front. 
Cysteine 
Lysine =| ha HCI 
acid 
ia 
02 03 04 O05 O06 07 O8 ¢ 
Me 
value 
pumping off the excess methyl iodide, 
the paper was scanned radiometrically. 
The plotted radiochromatogram is 
shown as B above. The high back- 
ground was apparently due to some 
methylation of the paper. 
3. Neutron activation of the paper 
chromatogram. In this method, the 
finished chromatogram undergoes neu- 
tron irradiation in a reactor after the 
residual solvent is dried off. For the 
success of this method, the separated 
components must contain or be asso- 
ciated with an element of suitable 
activation cross section so that it can 
be readily assayed against any back- 
ground radioactivity of the paper 
chromatogram itself. 
Fortunately, neither carbon, hydro- 
gen, nor oxygen of cellulose possesses 
significant neutron activation cross 
sections in this respect, and it was 
found that in washed Whatman No. 1 
paper the wide range of trace metals 
almost certainly present did not give 
rise to any serious effects under the 
moderate conditions of some experi- 
mental irradiations in the Harwell 
reactor. 
This method has been applied suc- 
cessfully to the radioactivation of 
bromine containing organic com- 
pounds by taking advantage of the 
A Chrematogram of protein 
hydrolysate from wheat grown | 
on gs eee 
B Strip spotted with known 
amino acids and exposed to 
U*'-labeled CH,!. 
C Neutron-activated chroma- 
togram of C,H,Ch, isomers 
D Neutron-activated chroma- 
togram of a bromine analog 
of DDT 
E Neutren-activated chroma- 
togram of bromine analogs of 
ODT derivatives 
Br®!(n,y)Br®? reaction. For example, 
an inactive bromine analog of DDT 
and three derivatives were separated 
on a reversed phase paper chromato- 
gram (3). The chromatogram was 
then irradiated for three days in the 
Harwell reactor at a flux of 10!° n/em?2/ 
sec and was scanned a few days later. 
The resulting radiochromatogram ob- 
tained is shown by E above. Only the 
DDT analog was present on the chro- 
matogram scanned in D. The small 
unidentified additional peaks may have 
been due to activated chromatographed 
impurities or to contamination. 
In the method of neutron activation, 
one important point must be borne in 
mind. The energy of recoil from the 
gamma emission in the (n,y) reaction 
is usually more than sufficient to rup- 
ture any chemical bond between the 
target atom and the rest of the mole- 
cule. The induced radioactivity, 
therefore, can only be associated with 
the component and cannot be used as a 
tracer in further tests with the eluted 
substance. 
Another complication may be due to 
the volatile nature of the recoil-freed 
isotope which can result in loss of 
activity and contamination of other 
parts of the chromatogram. For ex- 
ample, in the neutron activation of 
177 
