We report photochromic donor-acceptor Stenhouse adducts (DASAs) capable of fully reversible photoisomerization with visible light in organic solvents including chloroform, acetonitrile and benzene. The rates of photoisomerization and thermal reversion can be tuned by altering the electronics of the donor adduct. X-Ray crystallography and photo-NMR experiments unambiguously establish molecular structures.

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Alkane complexes of the type Cp'Re(CO)2(alkane) (Cp' = cyclopentadienyl or (isopropyl)cyclopentadienyl; alkane = isotopomers of n-pentane and cyclopentane) have been characterized using NMR spectroscopy following photolysis of Cp'Re(CO)3 in the appropriate alkane at 163-193 K. In the case of n-pentane, three different complexes are observed corresponding to binding of the three different types of carbon in this alkane. ROESY NMR experiments indicate that these isomeric complexes are slowly interconverting intramolecularly at 173 K. The order of the energetically preferred site of coordination is methylene (C2) approximately central methylene (C3) > methyl (C1) but with a spread of <0.2 kcal mol-1. Isotopic perturbation of resonance (IPR) experiments, conducted on several isotopomers of (i-PrCp)Re(CO)2(1-pentane), showed a large shielding of the 1H NMR chemical shift of the proton in a bound CHD2 moiety (delta -3.62) and CH2D (delta -2.64) compared with that of a bound CH3 moiety (delta -1.99). Likewise, the value of 1JCH for the coordinated methyl group of isotopomers of (i-PrCp)Re(CO)2(1-pentane) reduces in the order CH3 > CH2D > CHD2. This suggests that the alkane coordinates in an eta2-C,H fashion with a rapid exchange of bound hydrogen or deuterium within a methyl or methylene group, and that binding of a hydrogen atom is preferred over a deuterium by an amount of 0.23 +/- 0.03 kcal mol-1.

One-carbon metabolism in yeast is an essential process that relies on at least one of three one-carbon donor molecules: serine, glycine, or formate. By a combination of genetics and biochemistry we have shown how cells regulate the balance of one-carbon flow between the donors by regulating cytoplasmic serine hydroxymethyltransferase activity in a side reaction occurring in the presence of excess glycine. This control governs the level of 5,10-methylene tetrahydrofolate (5,10-CH 2 -H 4 folate) in the cytoplasm, which has a direct role in signaling transcriptional control of the expression of key genes, particularly those encoding the unique components of the glycine decarboxylase complex (GCV1, GCV2, and GCV3). Based on these and other observations, we propose a model for how cells balance the need to supplement their one-carbon pools when charged folates are limiting or when glycine is in excess. We also propose that under normal conditions, cytoplasmic 5,10-CH 2 -H 4 folate is mainly directed to generating methyl groups via methionine, whereas one-carbon units generated from glycine in mitochondria are more directed to purine biosynthesis. When glycine is in excess, 5,10-CH 2 -H 4 folate is decreased, and the regulation loop shifts the balance of generation of one-carbon units into the mitochondrion.