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The (1)H NMR spectra of 10-benzyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (BnPh(2)()) and 10-methyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (MePh(2)()) decoalesce due to slowing inversion at nitrogen and to slowing isolated bridgehead phenyl rotation. The high nitrogen inversion barriers in MePh(2)() (DeltaG() = 12.2 +/- 0.1 kcal/mol at 250 K) and BnPh(2)() (DeltaG() = 10.6 +/- 0.1 kcal/mol at 215 K) are typical of tertiary amines in which at least one C-N-C bond angle is constrained to a small value. Compared to the minuscule rotation barriers about sp(2)-sp(3) carbon-carbon bonds in simple molecular systems, the bridgehead phenyl rotation barriers in MePh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) and BnPh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) are unusually high. Molecular mechanics calculations (MMX force field) suggest that the origin of the high phenyl rotation barriers lies in the close passage of an o-phenyl proton and a methyl (or benzylmethylene) proton in the transition state. BnPh(2)() crystallized from hexane as white needles in the monoclinic system Pn. Unit cell dimensions are as follows: a = 12.198(1) Å, b = 6.1399(6) Å, c = 14.938(2) Å, beta = 107.470(4) degrees, V = 1067.1(2) Å(3), Z = 2. In the crystal molecular structure, the imine bridge CNC bond angle in BnPh(2)() is constrained to a small value (96 degrees ). The benzylic phenyl group is oriented gauche to the nitrogen lone pair.

Bacteriochlorophyll c, the light-harvesting pigment from the green sulfur bacteria Chlorobium limicola, has been resolved into over a dozen chemically similar components by a new high pressure reversed-phase chromatographic procedure based on a stationary phase of polyethylene power. Detailed spectroscopic characterization of the resolved components has resulted in the identification of four different chlorins and six different esterifying alcohols. The major esterifying alcohol is trans, trans-farnesol, but smaller amounts of geranylgeraniol, tetrahydrogeranylgeraniol, phytol, cis-9-hexadecen-1-ol and 4-undecyl-2-furanmethanol are also observed. The high information content intrinsic in the compositional analyses of the complex pigment mixtures found in green sulfur bacteria appears to provide a new probe of the mechanisms of pigment biosynthesis in these organisms.

Determination of the crystal structure of ethyl chlorophyllide a.2H(2)O by single crystal x-ray diffraction techniques has provided the first detailed molecular structural data available for a magnesium-containing derivative of chlorophyll. At the same time the aggregation observed in this structure serves as the basis for a model of chlorophyll aggregation in vitro and in vivo. The magnesium atom in ethyl chlorophyllide is displaced 0.4 A from the plane of the chlorin ring in the same direction as the methyl ester substituent. A water molecule occupies the fifth coordination site. A second water molecule is hydrogen bonded to the coordinated water molecule and also to the carbonyl oxygen atom of the methyl ester. Hydrogen bonds between the water molecules and carbonyl oxygen atoms link the chlorophyllide molecules into a two-dimensional aggregate of crosslinked one-dimensional polymers. The relationship of this aggregate to aggregates of chlorophyll both in vitro and in vivo is discussed, as is the efficiency of exciton transport within the polymer.