est

July 19, 1949

Professor Josnua Lederberg
Department of Genetics

The University of Wisconsin
College of Agriculture
Madison 6, Wisconsin

Dear Professor Lederberg,

I want to thank you for your interest in my work and to apologize
for my delay in replying, occasioned by a modest post-doctoral vacation.
I shall be glad to give what assistance I can to your projected studies
of "specific adsorytion". Unfortunately, the publication which you
accurately describe as "sketchy" is nonetheless a oretty fair summary
of the information currently at hand. These experiments were carried
out with the sole objective of demonstrating that the underlying idea
is sound. A broad investigation of the vhenomenon has been started this
summer in our laboratories.

The following supplementary details of the procedure might help you
to repeat the experiments and answer your specific questions. The sodium
silicate solution diluted to a volume of 150 ml. was placed in a 600 ml.
beaker. The dye, acid, and the remainder of the water were mixed separ-
ately and added, all at once, with vigorous stirring. The beaker, was
covered with a watch glass and allowed to stand for $ days. At the end
of this time the firm gel was expelled onto paper towels, chovped up with
a spatula, and allowed to dry for 6 days longer when it had become quite

brittle and weighed i: the neighborhood of 30 g. fter grinding, sieving,

and extracting with methanol the weight was reduced to 10-411 ge
In measuring the adsorption I have always first equilibrated the
gel sample with the solvent alone in order to avoid the possibility that
the adsorbents would alter the pH of the dye solutions. or produce some
other random effect. This has been done by shaking the gel sample with
3 successive 10 ml. portions of solvent over a period of 2 or 3 hours.

The sample, contained in a glass stoopered cylinder, was. then mixed with
such a volume of solvent as to make the weight 9 g. greater than the

weight of the cylinder plus dry gel alone. Al g. portion of a 1.50 x 1074
formal solution of the dye was then added to give the described system.

Equilibrium is very closely approached on shaking for just a few
minutes. However, the specific adsorbents take up an aporeciably greater
‘ amount of dye over a period of hours and the published results were
obtained by shaking mechanically for 30 minutes, allowing to stand for
24 hours and again shaking for 30 minutes.

Cloudiness was minimized by centri®uging the supernatant solutions.
Readings in a Beckman spectrophotometer were taken at 5100 # (maximum for
the dyes in 5% acetic acid) and were corrected by subtracting 1.46 x the
extinction reading at 6000 8. At 6000 & the light absorption by the dyes
is negligible. The factor, 1.46, is empirical and approximate.

| {I can not be very positive about the reproducibility of the magnitudes
of the specific effects. Most of our experiments were done with methyl
orange and the "relative adsorption powers" have ranged from 2.5 to 5.5.
Small differences in the conditions of preparation may greatly influence
the characteristics of the adsorbents and for this reason it is important

to prepare a control gel at the same time and in exactly the same way as
the specific adsorbents. If this is done I am confident that you will
have no difficulty in obtaining specific effects large enough to be
visible before instrumental measurements.

Finally I must point out that these procedures are largely quite
arbitrary. Many obvious shortcuts might give just as good or even better
results. In particular, with regard to drying the gels, I believe that
they may be removed from the beakers as soon as they are firm enough to be
broken into discreet fragments and the grinding may be carried out as
soon as the material is sufficiently brittle.

I anticipate that our new investigation of specific adsorption may
turn up much useful information in the next two or three mont ‘hs and I
will try to advise you of important developments. However, don't let
our program deter you from starting your own work in this field. I feel
that the potentialities of specific adsorption fully justify the attention
of a large number of workers. I should be very interested to hear something
about the applications to the study of adaptive enzymes which you

contemplate.

Sincerely,

CM ihe,