Brass Kendrew model of an Arg-Phe peptide, placed on top of a stack of glass sheets with contours for the 2Å resolution electron density map of Staphylococcal nuclease, a primitive comparison before crystallographic computer graphics and even before the "Richards box" half-silvered mirror method.  The arginine sidechain can be seen to hydrogen-bond between a backbone carbonyl O and the dark, triangular density of the nucleotide inhibitor of this enzyme.  Photographed in 1968.
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<span class="int-own-work" lang="en">Own work</span>