Is awKaratin a Cdlod Coil ?
ALL reoent work'+ bee oonfirmed that the etruoture
of the rrgnthetio polypeptide poly-y-methyl-L-glut-
amate is bawd on the a-helix of Pa  *
         3 and Corey'.
!I'hia efruotm givea a strong 145-A.   exion on Dhe
meridian, and both MeoArthu@ and Peruts' have
shown thet this reflexion 8lao oaoura in a&e&in.
This eusgesfs forcibly that the a-helix forma an
important part of a-kemtin.


NO. 4334  November 22,1952   NATURE

`l'here ~MV certain di53cultiee in thin idea. One of
the m&u featurce of a-lsemti!! ia the strong reflexion
et 5.15 A. on, or very cloee~ to,. the meridi8n.  !l'he
l,orm81 a-helix givem sfrong reflexionn on the 54-A.
10~6~ line, but di@8ced Ram the meridi8n. The
diReren- in ep&ng ia not an in6uper8ble objection,
eince the a-helix might be more tightly coiled ; but
it is difklcult to see how one could obtain euch a
narrow meridional 8rc es ia obeerved for a-keretin,
even ellowing for our ignomnce of the det&h Of the
&lo-chain srmngement.
One wey to bring the reflexion nearer to the mer-
idian would be fo tilt the a-helix. Since the a-keretin
pattom is of wide occurrence, one would expect the
tilt to be 8 conetent festure of the efructure. The
moat likely general solution is th8t the a-helix ie bent
into e super-helix, or coiled-coil. If the u-helix is
twisted into a super-helix with a pitch engle of about
lSo, ita projection on to the exia would heve a periodic
variation in density et interv8b4 of 5.4 co8 18" 1
5-l A., and so might explain the observed meridiombl
reflexion which correeponda to this epecing. A pitch
angle of 18' could be obtained, for example, from a
super-helix of radius 104 A. (the probable distance
between helicee) and axial epecing 198 A. (the repeat
of African porcupine quill)D*g.
It is poeeible to meke a rough eetim8t.e of the
tbnergy required to deform the a-helix by this
amount by aesuming thst (1) the forces prevent-
ing Smell rotationa about eingle bonds 8re smell ;
(2) the hydrogen bond can be fairly eerily deformed
by sm8ll amounts in direction ; (3) the m8in restoring
force cornea from the reaiatance of the hydrogen bonds
to deformation in length.
AsmmGng 8 force constent for a hydrogen bond
of, sey, 3 x 104 dynee/cm. (eee, for erample, DevieP],
the evemge energy per residue to deform the a-helix
by the amount deearibed above ia about one-tenth of
8 kilo-c8lorie per residue, which ia sm8ll. Third energy
increa6ee roughly ae the fourth power of the pitoh
8ngle, 80 thet the a-helix csn eerily be bent through
small 8nglee, but re&ta hwge defonnetione.

So far, no reason ti been given why the a-helix
should be deformed. It EMMIM probable tlmt this ia
due to the difllculty of fitting together the side-ch8ine
of two 8dj8cent a-4elicae.
I should like to nuggeet that there may be e general
plan underlying the detailed packing of all the various
side-chains found in proteins. If the eide-cheina of
the a-helix 8re thought of echemetic8lly BB knobs on
the eurfsce of 8 cylinder, then it ia found that the
pettern on this eurfsce consista of kuok eltern8ting
with `holee', thet is, spscee into which the knobs
from a neighbouring a-helix could fit. The position
of theee hole8 is roughly independent of the exact
nhwe of the eurrounding side-cheina.
In poly-y-methyl-L-glutum8t43, the side-chsine 8re
long end flexible snd they can reech out tow8rda the
neercet hole without deforming the a-helix. In pro-
teins, on the other hand, the eidedheine are, on the
average, Qneller and lm flexible, end neighbour@
helicea are nearer together (lOi A. oomp8red with
12 A. for poly-y-methyl-~-glut), It is therefore
not unrenaonable to simplify the undoubtedly com-
@iceted packing of aide-chains into 8 @et of at8nd8rd
rigid knobs fitting into standad holee.
.:f is impossible to peak euch models of the a-helix
oloeely side by side, eince a good fit in one pl8ce.
producee a bad fit somewhere elee, due to the non-
in-1 nature of the helix. However, it c8n be
ahown that by deforming the helices into coiled-coils

the knoba can be m8de to interlock ayatematically.
The energy for deforming the helicee could come from
the cheer fitting together of the sidechains.
The objecta of this communic8tion 8re to &ret?8
that the a-helix can be deformed an appreciable
emount if the deformetion ie sy&em8tic, to euggeet
that there m8y be 8 geneml pl8n underlying the peck-
ing together of side-cheine, and to chow thet theee
two ideee leed to a coiledcoil which mey explrsin
Borne of the dstall better th8n 8 etreight a-helix.
                    F. H. C. CMOK

Medic81 Reeecuch Council Unit for Raeearoh on the
Moleculu Structure of Biologic81 By&ma,
      Cevendish Laboratory,
      Cambridge. Oot. 22.

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`Cocbrm. W.. nnd Crick. F. B. C.. Nature. 169. 224 (19521.
' Bmhi. C. it., Brom:L., EllIoti, A., EI&b&.W. E, and-Trotter,
  I. Fe. Nature. 109.367 (1952).

' YakI, II. L., P+ltn& L.. and Corey, R. B., Ndurs. 168,920 (1962).
@ Fmer, It. D. B., ad Prfce. W. C., Na!u?w. 170, 490 (1952).
`Cddaoj W.. Crick, F. II. C.. and Vand, V., AQ?U Cr@., 6, 681
`P.$;~j"., and CQRY, B. B., PWC. U.S. Nat. AC& Sci., m, 241
%cArth;r, I., NaMe, MB, 38 (1943).
*Bear, It. s., J. Amer. ch6m. SC% 65, 1784 (1942).
lTDwla. M., Ano. Rep. Chem. 13oc.. 48, 1 (1946).
"Crick. F. H. C., Aria C&.. & 381 (1952).