| Full Text: |
X-RAY STUDIES OF NUCLEIC ACIDS
13Y "T. T. ASTBURY
Department of Biornolecular Structure, University of Leeds
There is not a great deal that is recent in the experimental work reported in
this paper, for there has been little opportunity to go on with the nucleic
acids until the last few months, but our earlier publications (Astbury 5 Bell,
1938) included only part of our findings, and this is a very suitable occasion
to look at them all again from a more up-to-date vie«vpoint in order to see
how ,ve stand before renewing the attack.
In the eyes of the structure analyst at least, perhaps the most significant
physical fact about the nucleic acids is their high density. On drying in
vacao over Y.,O,, we have found the following values: Na thymonueleate,
1-63 g./c.c.; yeast nucleic acid and tobacco mosaic virus nucleic acid,
1-65 g.,,'c.c. Now if eve set out the chemical formulae for the various
nucleotides and estimate their average area from known atomic dimensions,
or better still, if the construct scale models, we see that this average area
amounts to about 13 by 8 A., or something of the order of ioo A'-'.' If
therefore the nucleotides lie on top of one another, or in some arrangement
that in terms of packing comes to the same thing, we have:
Tx (13 x 8) x 1-6-=1-65 x 330, i.e. 7'~ 3-1 A.,
where T is the effective thickness and 330 is the average weight of a sodium
nucleotide, and 1-65 x 10-'-4 g. is one-sixteenth of the mass of an oxygen
atom. By this single calculation we are at once informed of the essentials of
the problem; for, approximate as it is, it leaves no doubt that the distance of
separation of successive nucleotides must be uncommonly small, and it
reminds us of graphite, the flat networks of which are 3-4 A. apart, and
even more of ascorbic acid (Cox, 1932; Cox & Goodwin, 1936), the flat
rings of which are only 3'16 A. apart and whose density is as high as
1-74 g.;'c.c. The purine and pyrimidine components of the nucleotides
must be flat, or nearly so, because of their double-bond structure, and so it
follows with a high degree of probability- that (a) the sugar component also
must be flat, or nearly so, and parallel to the purine or pyrimidine, i.e. it
must be a furanose, not a pyranose, sugar; and (b) that the linkages between
base and sugar must be either all u. or all 9.
t'ulother small piece of preliminary arithmetic brings in the flow proper-
ties of Na thymonueleate solutions investigated by Signer, Caspersson C;
Hammersten (1938), who concluded that the units are rods about 300 times
|
![Page 01 [66]](astbury-pg01-xl.jpg "Page 01 [66]")