Highly reflective
crystals of the nucleotide base guanine are widely
distributed in animal coloration and visual systems. Organisms precisely
control the morphology and organization of the crystals to optimize
different optical effects, but little is known about how this is achieved.
Here we examine a fundamental question that has remained unanswered
after over 100 years of research on guanine:
what are the
crystals made of
? Using solution-state and solid-state chemical
techniques coupled with structural analysis by powder XRD and solid-state
NMR, we compare the purine compositions and the structures of seven
biogenic guanine crystals with different crystal morphologies, testing
the hypothesis that intracrystalline dopants influence the crystal
shape. We find that biogenic “guanine” crystals are
not pure crystals but
molecular alloys
(aka solid
solutions and mixed crystals) of guanine, hypoxanthine, and sometimes
xanthine. Guanine host crystals occlude homogeneous mixtures of other
purines, sometimes in remarkably large amounts (up to 20% of hypoxanthine),
without significantly altering the crystal structure of the guanine
host. We find no correlation between the biogenic crystal morphology
and dopant content and conclude that dopants do not dictate the crystal
morphology of the guanine host. The ability of guanine crystals to
host other molecules enables animals to build physiologically “cheaper”
crystals from mixtures of metabolically available purines, without
impeding optical functionality. The exceptional levels of doping in
biogenic guanine offer inspiration for the design of mixed molecular
crystals that incorporate multiple functionalities in a single material.
Nuclear magnetic resonance (NMR) properties of solvated
molecules
are significantly affected by the solvent. We, therefore, employ a
polarization consistent framework that efficiently addresses the solvent
polarizing environment effects. Toward this goal a dielectric screened
range separated hybrid (SRSH) functional is invoked with a polarizable
continuum model (PCM) to properly represent the orbital gap in the
condensed phase. We build on the success of range separated hybrid
(RSH) functionals to address the erroneous tendency of traditional
density functional theory (DFT) to collapse the orbital gap. Recently,
the impact of RSH that properly opens up the orbital gap in gas-phase
calculations on NMR properties has been assessed. Here, we report
the use of SRSH-PCM that produces properly solute orbital gaps in
calculating isotropic nuclear magnetic shielding and chemical shift
parameters of molecular systems in the condensed phase. We show that
in contrast to simpler DFT-PCM approaches, SRSH-PCM successfully follows
expected dielectric constant trends.
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