Amino acid interactions of spiropyran

A solution phase study was carried out to explore the interactions between the spiropyran molecule in its merocyanine zwitterionic merocyanine 'open' form and amino acids. The principal amino acids chosen for investigation were β-Alanine, L-Phenylalanine and 8-Aminocaprylic acid. Both single phase and two-phase switching experiments were undertaken to document the photochromic interconversion between spiropyran and merocyanine in the presence of these amino acids, compared to a water blank. A kinetics study was then carried out to examine how the rate of decay of the coloured merocyanine back to the closed spiropyran was affected by the presence of amino acids. Results show that there are significant differences in the decay rate depending on which amino acid was present. There are several factors which may be involved in how the different amino acids affect the decay rate of the merocyanine, such as the relative polarity of the solution. A significant stabilisation effect was also observed in the presence of water (i.e. no amino acid present), suggesting that hydrogen bonding may play an important role in stabilising the merocyanine isomer at the molecular level.