Research Article

Eye patches: Protein assembly of index-gradient squid lenses

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Science  11 Aug 2017:
Vol. 357, Issue 6351, pp. 564-569
DOI: 10.1126/science.aal2674

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Squid lenses beat spherical aberration

When light rays pass through a curved lens, greater refraction at the edges can distort the resulting image. This problem can be overcome if the refractive index of the lens is varied according to the curvature. Cai et al. show that the lenses of squid eyes have an internal structure containing a set of globular proteins that form a gradient of colloidal particles to counter spherical aberration (see the Perspective by Madl). Thus, the evolutionary process has used the principles of patchy colloid theory to construct a self-assembling, complex optical device.

Science, this issue p. 564; see also p. 546

Abstract

A parabolic relationship between lens radius and refractive index allows spherical lenses to avoid spherical aberration. We show that in squid, patchy colloidal physics resulted from an evolutionary radiation of globular S-crystallin proteins. Small-angle x-ray scattering experiments on lens tissue show colloidal gels of S-crystallins at all radial positions. Sparse lens materials form via low-valence linkages between disordered loops protruding from the protein surface. The loops are polydisperse and bind via a set of hydrogen bonds between disordered side chains. Peripheral lens regions with low particle valence form stable, volume-spanning gels at low density, whereas central regions with higher average valence gel at higher densities. The proteins demonstrate an evolved set of linkers for self-assembly of nanoparticles into volumetric materials.

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