We describe a large number of binding studies in aqueous media designed to provide new insights into noncovalent binding interactions, especially the cation-a interaction. The studies include 7 different hosts, over 70 guests, and over 150 new binding constants. In addition to the now standard NMR methods, circular dichroism has proven to be an especially useful tool for determining aqueous binding constants. We have found that, in addition to the alkyliminium and tetraalkylammonium guests we have studied previously, sulfonium and guanidinium guests also show substantial cation-r effects. Bromination of the host greatly enhances its binding ability in a general fashion, primarily as a result of hydrophobic interactions. Addition of methoxy groups did not enhance binding, apparently as a result of a collapse of the host into a conformation that is not suitable for binding. Replacement of two benzene rings of the host by furans or thiophenes also did not enhance binding. Ab initio calculations provide a rationalization for this effect and suggest a clearer model for the cation-a interaction.

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A light-controlled reversible binding switch based on photochromic 3H-naphtho [2,1-b]pyran is under development for studying cellular oscillatory calcium signals. The binding affinities of the closed and open forms of substituted naphthopyran 1 for Ca 2+ , Mg 2+ , and Sr 2+ in buffer were determined. The photochemically ring-opened form of the receptor exhibited increased affinity compared to the thermally stable closed form of the receptor. The binding affinity difference for Ca 2+ was ∼77-fold at pH 7.6. Calcium (Ca 2+ ) is a second messenger in many cell types, where it is used to translate extracellular signals into a wide variety of intracellular events. 1 Important cellular processes are controlled by Ca 2+ , and many disease states are associated with defects in the calcium signalosome. Manipulation of Ca 2+ concentration in cells through cage compounds is an important tool for learning about this widespread signaling system. 2 Caged Ca 2+ compounds undergo irreversible photochemical reactions that either release or take up Ca 2+ when triggered. However, cage compounds are less well suited to the examination of oscillatory calcium signals, which may encode information through both amplitude and frequency modulation. The origin of these oscillatory signals is known, but their effects at a molecular level are less well understood. There is a need to develop new methodologies to study the effects of spatiotemporal changes in Ca 2+ concentration. In the present work, we report the synthesis and characterization of compound 1, a new watersoluble 3H-naphtho[2,1-b]pyran with an iminodiacetic acid substituent at position 5 ( Figure 1). This compound is designed so that the open form of 1 will exhibit a higher affinity for Ca 2+ than the closed form. The binding affinities of closed and open forms of compound 1 were determined, and the effects of buffer composition and pH, on this reversible calcium binding photoswitch were also examined.Naphthopyran chelator 1 was obtained in five steps (Scheme 1 and Supporting Information) with an overall yield of 11%. Compound 1 is soluble up to ∼5.7 × 10 -4 M in aqueous solutions buffered at pH 8.7.Complexation of the closed form of 1 with metal ions was examined spectroscopically. The UV-vis spectra of 1 show minimal changes upon addition of excess Mg 2+ , Ca 2+ , and Sr 2+ ( Figure S1, Supporting Information). The addition of metal ions to the closed form of 1 did not result in any detectable thermal ring opening induced by metal complexation, unlike that observed for some photochromic chelators. 5d,7d,8 Small but significant and reproducible 1 H NMR shifts were observed upon addition of metal ions (Table S2, Supporting Information). One or more aromatic protons moved downfield upon addition of all metals to the closed form of 1. The methylene protons only shifted when calcium was added, and they moved upfield. The same pattern of complexation-induced shifting, but with smaller magnitudes, was also observed upon the addition of the same three metals to phenyliminodi...

[reaction: see text] A novel application of photochromic molecules is to mimic physiological oscillatory calcium signals by reversibly binding and releasing calcium ions in response to light. Substituent changes on the largely unexplored photochromic bisbenzospiropyran scaffold led to significant changes in thermal fading rates in several organic solvents. Excellent correlations have been found between fading rates and empirical Hammett constants as well as calculated ground-state energies. These correlations can be used to improve scaffold design.

Photochromic molecules have the potential to find utility in a wide variety of applications including photoswitchable binding and optical memory. This work explores the relationship between photochromism and structural parameters such as particular bond lengths for this class of compounds for which very few crystal structures have been published. Photochemical kinetics, Density Functional Theory (DFT) and X-ray crystallography were used to study the benzothiazolinic spiropyran 3-methyl-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline]. A second benzothiazolinic spiropyran 3-methyl-8-methoxy-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline] was synthesized and subjected to photochemical and computational studies. Selected structural and photochemical data for these, related benzothiazolinic spirooxazines and spiropyrans, and related thiazolidinic spiropyrans are compared. Both benzothiazolinic spiropyrans exhibit photochromic properties that are influenced by substituents, solvent, and temperature. The crystallographic C spiro -O bond distance of 3-methyl-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline] that has been shown to correlate with photochromic properties is 1.458 Å. The crystallographic C spiro -O bond distance matches that of the structure generated by DFT calculations exactly. The effect of substituents on calculated bond lengths and photochemical parameters was determined. For this class of compounds, both X-ray geometry and DFT optimized geometry may be used to predict photochromism, but not degree of photocolorability.

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