Rationalizing\nthe Influence of Small-Molecule Dopants\non Guanine Crystal Morphology

Many spectacular optical phenomena in animals are produced\nby reflective\nassemblies of guanine crystals. The crystals comprise planar H-bonded\nlayers of π-stacked molecules with a high in-plane refractive\nindex. By preferentially expressing the highly reflective π-stacked\n(100) crystal face and controlling its cross-sectional shape, organisms\ngenerate a diverse array of photonic superstructures. <i>How\nis this precise control over crystal morphology achieved?</i> Recently, it was found that biogenic guanine crystals are composites,\ncontaining high quantities of hypoxanthine and xanthine in a molecular\nalloy. Here, we crystallized guanine in the presence of these dopants\nand used computations to rationalize their influence on the crystal\nmorphology and energy. Exceptional quantities of hypoxanthine are\nincorporated into kinetically favored solid solutions, indicating\nthat fast crystallization kinetics underlies the heterogeneous compositions\nof biogenic guanine crystals. We find that weakening of H-bonding\ninteractions by additive incorporation elongates guanine crystals\nalong the stacking directionthe opposite morphology of biogenic\ncrystals. However, by modulation of the strength of competing in-plane\nH-bonding interactions, additive incorporation strongly influences\nthe cross-sectional shape of the crystals. Our results suggest that\nsmall-molecule H-bond disrupting additives may be simultaneously employed\nwith π-stack blocking additives to generate reflective platelet\ncrystal morphologies exhibited by organisms.