Naturally occurring cyclic amino acid. Organic osmolyte; synthesized by microorganisms to maintain osmotic equilibrium in saline environments and to protect enzymes and whole cells from denaturation. Isoln from the halophilic, phototrophic bacterium, Ectothiorhodospira halochloris, and characterization as a compatible solute: E. A. Galinski et al., Eur. J. Biochem. 149, 135 (1985). Biosynthesis: P. Peters et al., FEMS Microbiol. Lett. 71, 157 (1990). Calorimetric analysis of biosynthesis efficiency: T. Maskow, W. Babel, Biochim. Biophys. Acta 1527, 4 (2001). Chromatographic determn: V. Riis et al., Anal. Bioanal. Chem. 377, 203 (2003). Osmoprotection in E. coli: M. Jebbar et al., J. Bacteriol. 174, 5027 (1992). Mechanism of osmoprotection study: idem et al., ibid. 187, 1293 (2005). Enzyme stabilization study: K. Lippert, E. A. Galinski, Appl. Microbiol. Biotechnol. 37, 61 (1992). In vitro prevention of UVA-induced skin damage: J. Buenger, H. Driller, Skin Pharmacol. Physiol. 17, 232 (2004). Inhibition of β-amyloid peptide aggregation and neurotoxicity: M. Kanapathipillai et al., FEBS Lett. 579, 4775 (2005).