This tutorial review describes in a brief historical perspective the most important organic compounds that exhibit photochromism in the crystalline state since its discovery in 1867 up to now and considers in detail Schiff bases of salicylaldehyde with amines (anils). The latter comprise a chemical system undergoing hydrogen-atom tautomerism between enol and keto forms and show the phenomena of solid state photochromism and thermochromism. The system has been investigated extensively. Thus it has been shown that the photochromic property is a characteristic of the molecules but their chromobehaviour is influenced by the crystal structure of the compounds. Anils, apart from their fundamental interest, have potential for various applications.

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Intracellular aminopeptidases endoplasmic reticulum aminopeptidases 1 and 2 (ERAP1 and ERAP2), and as well as insulin-regulated aminopeptidase (IRAP) process antigenic epitope precursors for loading onto MHC class I molecules and regulate the adaptive immune response. Their activity greatly affects the antigenic peptide repertoire presented to cytotoxic T lymphocytes and as a result can regulate cytotoxic cellular responses contributing to autoimmunity or immune evasion by viruses and cancer cells. Therefore, pharmacological regulation of their activity is a promising avenue for modulating the adaptive immune response with possible applications in controlling autoimmunity, in boosting immune responses to pathogens, and in cancer immunotherapy. In this study we exploited recent structural and biochemical analysis of ERAP1 and ERAP2 to design and develop phosphinic pseudopeptide transition state analogs that can inhibit this family of enzymes with nM affinity. X-ray crystallographic analysis of one such inhibitor in complex with ERAP2 validated our design, revealing a canonical mode of binding in the active site of the enzyme, and highlighted the importance of the S2' pocket for achieving inhibitor potency. Antigen processing and presentation assays in HeLa and murine colon carcinoma (CT26) cells showed that these inhibitors induce increased cell-surface antigen presentation of transfected and endogenous antigens and enhance cytotoxic T-cell responses, indicating that these enzymes primarily destroy epitopes in those systems. This class of inhibitors constitutes a promising tool for controlling the cellular adaptive immune response in humans by modulating the antigen processing and presentation pathway. molecular structure | adaptive immunity | major histocompatibility molecules | specificity | kinetics

Endoplasmic reticulum aminopeptidases ERAP1 and ERAP2 cooperate to trim a vast variety of antigenic peptide precursors to generate mature epitopes for binding to major histocompatibility class I molecules. We report here the first structure of ERAP2 determined at 3.08 Å by X-ray crystallography. On the basis of residual electron density, a lysine residue has been modeled in the active site of the enzyme; thus, the structure corresponds to an enzyme-product complex. The overall domain organization is highly similar to that of the recently determined structure of ERAP1 in its closed conformation. A large internal cavity adjacent to the catalytic site can accommodate large peptide substrates. The ERAP2 structure provides a structural explanation for the different peptide N-terminal specificities between ERAP1 and ERAP2 and suggests that such differences extend throughout the whole peptide sequence. A noncrystallographic dimer observed may constitute a model for a proposed ERAP1-ERAP2 heterodimer. Overall, the structure helps explain how two homologous aminopeptidases cooperate to process a large variety of sequences, a key property of their biological role.

Background: ER aminopeptidases generate antigenic peptides, but how they recognize their substrates is unclear. Results: We solved crystal structures of ERAP2 in complex with a substrate analogue and a peptide product. Conclusion: The peptides were found trapped inside a large cavity adjacent to the catalytic site. Significance: Interactions of the substrate with the internal cavity can result in both substrate permissiveness and limited sequence bias.