Rudall’s crystalline fibrin
I have worked for several days on Rudall’s photograph of fibrin, and my results are
described in the following paragraph.
First, we note that there is what seems to be a powder line with six strong reflections
on it, forming a hexagon. The equatorial reflections correspond to spacing 2.84 A,
and the others to 2.81 A. These are close enough to the value for 200 of sodium chloride
to suggest that there are some oriented sodium chloride crystals in the preparation.
I have considered this possibility and rejected it. First, in order to give six spots
from the form 200, the sodium chloride crystals would have to be oriented with the
zone axis 220 vertical. Then the four spots not on the equator would be at angles
of close to 45º, whereas in fact these four spots are at the angle 57º with the equator.
Second, the difference between the spacings 2.84 and 2.81 A seems to be real. Third,
there is no indication of the reflections 111 and 220 of sodium chloride that would
be expected.
I have accordingly assumed that all of the spots on the photograph are to be attributed
to fibrin, except the ring identified by Rudall xx from 111 of copper, and a few round
spots (that is, not having the shape expected for a reflection from the fibrin sample),
that are also not duplicated in positions on the film where they should be because
of the symmetry of the film.
I have measured eight spots in the equator, which may be named A1 to A8. Their spacings
are 10.0, 4.89 A (marked on original inspection of the photograph as possibly double),
3.16, 2.84, 2.65, 2.47, 2.38, 2.24 A. Of these the strongest is A4. The remaining
spots lie approximately on two hyperbolas, duplicated also below the equator. These
hyperbolas correspond approximately to identity distance 7 A and 3.5 A along the fiber
axis.
J. R. Katz and A. deRooy reported fibrin to have the β-keratin structure. Later K.
Bailey, W.T. Astbury, and K.M. Rudall, Nature, 151, 716 (1943), stated that fibrinogen
and fibrin give an α-keratin pattern, and after extended stretching of the sample
gives a β-keratin pattern. However, W.T. Astbury, Proc. Roy. Soc. B 134, 303 (1946)
reproduces photographs of α-fibrin, β-fibrin, α-fibrinogen, and β-fibrinogen (his
designations). I note that the α photographs show a closer general resemblance to
the β-keratin pattern than to the α–keratin pattern. However, the strong 4.7 A reflection
of β-keratin is not present. I think that Astbury identified these as α photographs
because of the absence of this equatorial reflection, and that this was a mistake.
I think that it is likely that the α is really a special kind of β, such that the
4.7 A reflection is very weak.
When the off-equator reflections are measured it is found that they do not correspond
to a small fiber-axis identity distance. For example, spot B2 with d = 4.08 A, lies
on a hyperbola corresponding to identity distance 7.10 A, and a similar reflection
in the next hyperbola, C2, with d = 2.95 A, corresponds to 3.56 A. These two identity
distances are related by the factor 1/2. Similarly, two other reflections, B3 and
C3, with spacing 3.78 A and 2.81 A, respectively, lie on the hyperbolas corresponding
to 6.75 A and 3.39 A, respectively, which are again related by the factor 1/2.
We must decide between two/alternative explanations of these facts. One is that the
pattern is really the superposition of two fiber patterns, with fiber-axis identity
distances 7.10 A and 6.75 A, respectively. The second is that the fiber-axis distances
7.10 and 6.75 are respectively the 19th and 20th submultiples of a very large fiber-axis
identity distance, 135 A. several facts indicate the second of these to be correct.
Other reflections correspond to different fiber-axis identity distances, including
7.45 and 6.37, and half these values. These can be interpreted on the second alternative
as the 18th and 21st submultiples of 135 A, respectively, whereas there is no explanation
for them on the other basis. Also, the reflections C3 and B2 correspond to the same
equatorial reflection, A2 (which, however, appears possibly to be a double reflection,
one component of which might belong to one pattern and the other to the other). I
have accordingly proceeded on the assumption that the identity distance y has the
value 135 A.
We may now ask what the explanation of the non-equatorial reflections is. Fibrin is
made by the aggregation of fibrinogen molecules. Fibrinogen molecules are known to
be long and thin. They might consist of polypeptide chains in a beta sheet, with perhaps
two sheets, each containing 4 or 5 chains, constituting a molecule.
These molecules would line up beside one another in fibrin. They might line up in
such a way as to give essentially an aggregate of infinite sheets. However, it is
known that the about one residue in 25 is proline, which will cause a flaw in the
lattice. Other aspects of the distribution of amino acid residues might cause the
true identity distance to be 135 A or a multiple of this value, in the fiber-axis
direction. In a well-crystallized specimen of fibrin the observed reflections would
correspond to this value of y. However, only those orders of reflection would occur
with significant intensity that correspond approximately also to a small unit, with
y = to about 7 A, as in β-keratin and silk fibroin. The enhancement effect would be
expected to give rise to several closely adjacent hyperbolae, as is observed on Rudall’s
photograph.
I have found that all of the equatorial reflections A1 to A8 can be easily accounted
for by a monoclinic unit, such that the strong reflection A4 corresponds to the plane
203. The parameters of the unit area are a = 9.50 A, b = 135 A, c = 10.03 A, β = 85.1º.
The reflection A2 that appears to be double could have the indices 002 and 200, with
spacings 5.00 A and 4.75 A, respectively, the average spacing as measured being 4.89
A.
I am not sure what to do next. I think that it would be a good idea for us to have
our own photographs of fibrin. Would you have a fibrin preparation made and photographed.
Also, it occurs to me that it might be a good idea to get hold of some of the commercial
fibrin film that was made at Harvard, in order to see what sort of a diffraction pattern
it gives.
Linus Pauling
