The paper by F. H. C. Crick explores the packing of α-helices and their implications for the structure of fibrous proteins. Key points include: 1. **α-Helix Packing**: α-helices of the same sense tend to pack about 20° away from parallel, leading to a coiled-coil structure. This packing is more challenging than that of opposite-sense helices, which can pack parallel. 2. **Simple Models**: The two-strand and three-strand rope models are described to illustrate the diffraction theory. These models predict diffuse α-keratin patterns and can explain the structure of long, thin molecules like tropomyosin. 3. **Diffraction Patterns**: The Fourier transform of a coiled-coil is used to calculate structure factors, showing that the two-strand rope model predicts strong reflections at 5.17 Å and 1.48 Å, while the three-strand rope model predicts similar patterns but with slightly different spacings. 4. **Additional Considerations**: Distortions in the simple structures, repeating sequences of residues, and different packing directions are discussed. These factors can alter the diffraction patterns but do not significantly affect the main features. 5. **Applications**: The models are applied to tropomyosin and α-keratin, suggesting that the observed diffraction patterns can be explained by α-helices packing side-by-side. The implications for globular proteins are also discussed, highlighting the potential for α-helices to pack at angles greater than 20°. 6. **Conclusion**: The paper concludes that the observed X-ray patterns of fibrous proteins can be explained by α-helices packing in a knobs-into-holes manner, without the need for repeating sequences of residues. This packing mechanism is likely the basis of the structure of fibrous proteins.The paper by F. H. C. Crick explores the packing of α-helices and their implications for the structure of fibrous proteins. Key points include: 1. **α-Helix Packing**: α-helices of the same sense tend to pack about 20° away from parallel, leading to a coiled-coil structure. This packing is more challenging than that of opposite-sense helices, which can pack parallel. 2. **Simple Models**: The two-strand and three-strand rope models are described to illustrate the diffraction theory. These models predict diffuse α-keratin patterns and can explain the structure of long, thin molecules like tropomyosin. 3. **Diffraction Patterns**: The Fourier transform of a coiled-coil is used to calculate structure factors, showing that the two-strand rope model predicts strong reflections at 5.17 Å and 1.48 Å, while the three-strand rope model predicts similar patterns but with slightly different spacings. 4. **Additional Considerations**: Distortions in the simple structures, repeating sequences of residues, and different packing directions are discussed. These factors can alter the diffraction patterns but do not significantly affect the main features. 5. **Applications**: The models are applied to tropomyosin and α-keratin, suggesting that the observed diffraction patterns can be explained by α-helices packing side-by-side. The implications for globular proteins are also discussed, highlighting the potential for α-helices to pack at angles greater than 20°. 6. **Conclusion**: The paper concludes that the observed X-ray patterns of fibrous proteins can be explained by α-helices packing in a knobs-into-holes manner, without the need for repeating sequences of residues. This packing mechanism is likely the basis of the structure of fibrous proteins.