MOLECULAR AND M ACROMOLECU L AR STRUCTURE 



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which can only mean that the crystallites (or orientable units) contain 

 bundles of polypeptide chains running at right angles to the long axis of 

 the crystallites. While the actual molecular arrangements are still regarded 

 as uncertain, a proposal by Rudall (1946), to the effect that large transverse 

 folds (Fig. 85) such as were originally proposed by Astbury for the 

 supercontracted state, would be likely to lead to the cross-/? pattern, forms 

 the current working hypothesis. 



The discovery that highly-oriented " natural " cross-/? patterns are given 

 by the egg stalk of the green lace- wing fly Chrysopa (Parker and Rudall, 

 1957) places this pattern on a more definite basis. The polypeptide chains 



Fig. 85. Illustrating Rudall's proposal for a cross-/? crystallite which 

 could give rise to reflections from the 4-65 A spacing on the meridian. The 

 chains are extensively folded with their side spacings parallel to the fibre 

 axis. The crystallite is supposed to be longer in the side-chain direction 

 and stretching converts it directly to the parallel-/? form without rotating 

 the crystallite. 



here undoubtedly run at right angles to the length of the fibre and are 

 folded since a conversion to the ||/? form takes place on stretching. Long 

 spacings relatable to the transverse folds disappear on stretching. 



Astbury holds that a supercontraction resulting from a folding from the 

 a-form into a shorter configuration, such as that proposed in the cross-/? 

 structure, occurs in muscle contraction although it appears most clearly in 

 contracted muscle when this is produced by heating. Strength is given to 

 this opinion by a recent discovery (Astbury et al, 1955) that the cross-/? 

 configuration normally accompanies the a-pattern in bacterial flagella (see 

 Fig. 86) which appear from their helical shape in life to be in a state of 

 equilibrium between a contracted and an extended configuration. 



An entirely different proposal to explain the occurrence of the spacing 

 of 4-7 A on the meridian in the cross-/? pattern has been made recently by 

 Zubay (1959). He points out that an arc of spacing 4-7 A passing over the 

 meridian could be produced by small microcrystals formed by a-helices 



