Crystalline cages held together by hydrogen bonds can trap a wide range of molecules based on their size.

Classical crystal growth models posit that crystallization outcomes are determined by nuclei that resemble mature crystal phases, but at a critical size where the volume free energy of nuclei begins to offset the unfavorable surface free energy arising from the interface with the growth medium. Crystallization under nanoscale confinement offers an opportunity to examine nucleation and phase transformations at length scales corresponding to the critical size, at which kinetics and thermodynamics of nucleation and growth intersect and dramatic departures in stability compared to bulk crystals can appear. This tutorial review focuses on recent investigations of the crystallization of organic compounds in nanoporous matrices that effectively provide millions of nanoscale reactors in a single sample, ranging from controlled porous glass (CPG) beads to nanoporous block-copolymer monoliths to anodic aluminum oxide (AAO) membranes. Confinement of crystal growth in this manner provides a snapshot of the earliest stages of crystal growth, with insights into nucleation, size-dependent polymorphism, and thermotropic behavior of nanoscale crystals. Moreover, these matrices can be used to screen for crystal polymorphs and assess their stability as nanocrystals. The well-aligned cylindrical nanoscale pores of polymer monoliths or AAO also allow determination of preferred orientation of embedded nanocrystals, affording insight into the competitive nature of nucleation, critical sizes, and phase transition mechanisms. Collectively, these investigations have increased our understanding of crystallization at length scales that are deterministic while suggesting strategies for controlling crystallization outcomes.

Taking the Cystine: Kidney stones that form from l -cystine are much less common than those forming from calcium oxalate monohydrate, but are more likely to cause chronic kidney disease. Rimer et al. (p. 337 ; see the cover; see the Perspective by Coe and Asplin ) designed two structural mimics for l -cystine. Atomic force microscopy showed that at low concentrations, the mimics could change the l -cystine crystal habit and inhibit overall crystal growth. These structural mimics may thus offer hope for treating cystinuria.

Many naturally occurring biopolymers (i.e., proteins, RNA, DNA) owe their unique properties to their well-defined three-dimensional structures. These attributes have inspired the design and synthesis of folded architectures with functions ranging from molecular recognition to asymmetric catalysis. Among these are synthetic oligomeric peptide (“foldamer”) mimics, which can display conformational ordering at short chain lengths. Foldamers, however, have not been explored as platforms for asymmetric catalysis. This report describes a library of synthetic helical “peptoid” oligomers that enable enantioselective transformations at an embedded achiral catalytic center, as illustrated by the oxidative kinetic resolution of 1-phenylethanol. In an investigation aimed at elucidating key structure–function relationships, we have discovered that the enantioselectivity of the catalytic peptoids depends on the handedness of the asymmetric environment derived from the helical scaffold, the position of the catalytic center along the peptoid backbone, and the degree of conformational ordering of the peptoid scaffold. The transfer of chiral information from a folded scaffold can enable the use of a diverse assortment of embedded achiral catalytic centers, promising a generation of synthetic foldamer catalysts for enantioselective transformations that can be performed under a broad range of reaction environments.