Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-t-butylphenyl) group show low fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes (∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60°with respect to the dipyrrin framework whereas the angle is ∼80°for mesityl or o-tolyl groups. The calculated potential energy surface for the phenylsubstituted complex indicates that the excited state has a second, lower energy minimum in which the non-hindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a Supporting Information Available: Theoretical analysis of the excited-state surfaces and Franck-Condon-active modes for selected compounds, static absorption and emission spectra, time-resolved absorption and emission spectra, and ORTEP diagrams of the structures. Crystallographic data is available as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access

Mitochondria are central organelles in cellular energy metabolism, apoptosis, and aging processes. A signaling network regulating these functions was recently shown to include soluble adenylyl cyclase as a local source of the second messenger cAMP in the mitochondrial matrix. However, a mitochondrial cAMPdegrading phosphodiesterase (PDE) necessary for switching off this cAMP signal has not yet been identified. Here, we describe the identification and characterization of a PDE2A isoform in mitochondria from rodent liver and brain. We find that mitochondrial PDE2A is located in the matrix and that the unique N terminus of PDE2A isoform 2 specifically leads to mitochondrial localization of this isoform. Functional assays show that mitochondrial PDE2A forms a local signaling system with soluble adenylyl cyclase in the matrix, which regulates the activity of the respiratory chain. Our findings complete a cAMP signaling cascade in mitochondria and have implications for understanding the regulation of mitochondrial processes and for their pharmacological modulation.Mitochondria play central roles in cellular energy metabolism, as well as in the regulation of cell cycle progression, apoptosis, and aging processes (1, 2). Despite their importance, signaling into, from, and within mitochondria is still not well understood. Emerging signaling mechanisms in mitochondria and between the organelle and its environment include reversible protein deacetylation (3, 4), redox regulation and reactive oxygen species formation (5-7), and cyclic adenosine monophosphate (cAMP) signaling (8, 9).cAMP-dependent effects and proteins of cAMP signaling systems, such as cAMP-responsive element-binding protein (CREB), protein kinase A (PKA), and A-kinase anchoring proteins (AKAPs), 3 have been described in mitochondria (10 -12). In addition to these effector proteins, a complete cAMP signaling microdomain requires enzymes for synthesis and degradation of the second messenger. Although an intramitochondrial cAMP source has been identified recently (8), there is no known cAMP-degrading enzyme in this organelle. Cyclic AMP is formed inside mitochondria by soluble adenylyl cyclase (sAC) (8), a member of Class III of the nucleotidyl cyclase family, which also comprises the G-protein-regulated transmembrane adenylyl cyclases (13). Unique from transmembrane adenylyl cyclases, sAC is activated by bicarbonate (14), and it appears to act as a metabolic sensor (15), whose mitochondrial form(s) seems to modulate PKA-mediated regulation of respiration (8) and apoptosis (16).The opponents of the cyclic nucleotide-forming cyclases are cyclic nucleotide monophosphate (cNMP)-degrading phosphodiesterases (PDEs). Mammalian cells contain a varying subset of members of the classical PDE family, which comprises 11 PDE gene families (PDE1-11) (17, 18) and non-generic PDEs such as the protein human Prune (19,20). The isoforms of the generic PDEs comprise homologous catalytic domains, fused to varying regulatory domains, making them sensitive to a variety of signals s...