A fluorescent phospholipid derivative, the fluoresceinthiocarbamyl adduct of a natural phosphatidylethanolamine, has been synthesized and incorporated into sonicated single-bilayer vesicles of egg lecithin and dipalmitoyllecithin. The surface location of this probe has been confirmed by using extrinsic fluorescence quenching studies together with steady-state emission anisotropy measurements. Electronic excitation energy transfer between 1,6-diphenyl-1,3,5-hexatriene incorporated within the hydrophobic core of the bilayer and the novel derivative has been investigated to estimate the depth within the bilayer at which the former is located. Efficiencies have been measured for two different phospholipids, egg lecithin and dipalmitoyllecithin, in the latter case both above and below the phospholipid phase transition, with and without added cholesterol. The observed dependence of the transfer efficiency on the acceptor concentration was compared with that calculated according to Förster theory applied to random two-dimensional distributions of donor and acceptor molecules in parallel planes for various interplanar separations, taking into account orientational effects. The Förster R0 of about 45 A for this donor-acceptor pair is particularly well suited to such studies since it is of the order of the width of the bilayer. The experiments showed that energy-transfer spectroscopy can provide useful quantitative information as to the transverse location of diphenylhexatriene in homogeneous phospholipid bilayers and may also reflect lateral partitioning of donor or of both donor and acceptor into different phases in systems exhibiting phase separations.

sites in the molecule also appear to be involved. Intramolecular electron transfer may be an important mechanistic pathway. The decay kinetics of the transients observed, although mainly first order, are dependent in a complex way on the solution environment. Even the presence of modest concentrations of solutes (phosphate buffer, formate, NaCl, for example) has a marked effect on the observed value of k. Future investigators of the decay kinetics of radicals derived from the one-electron reduction and oxidation of macromolecules are cautioned to control the nature of the reaction medium and recognize the range of possible interactions between the radical and its environment. References and Notes(1) Presented in part at the Fifth International Congress of Radiation Research, Seattle, Wash., July 1974. (2) G.

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Equilibrium vapour pressures of 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitro-rn-xylene (TNX), 2,4,6-trinitromesitylene (TNM), 2,4,6-trhitroaniline (TNA), 2,4,6trinitro-m-phenylene diamine (DATB), 2,4,6-N-tetranitro-N-methylaniline (Tetryl), 2,4,6-trinitrophenol (picric acid), 2,4,6-trinitroresorcinol (styphnic acid, TNR), 2,4,6-trinitro-m-cresol (TNC), pentaerythritoltetranitrate (F' ETN), cyclotrimethylenetrinitramine (RDX), sym-cyclotetramethylenetetramitramine (HMX) and nitroguanidine have been measured over a range of temperature using the Knudsen cell technique. The data were found, within the limits of experimental error, to follow the Clausius-Clapeyron equation and values for the constants A and B in the integrated form of the equation loglo p/N m -2 = A -B/T determined. The standard enthalpies, entropies and Gibbs energies for sublimation are quoted.

Oxidation of halide, thiocyanate, carbonate and selenite ions by hydroxyl radicals yields radicalanions which themselves oxidise amino-acids such as tryptophan and tyrosine. The free-radical product(s) of oxidation of tryptophan (TrpH) by Br;, (SCN);, CO; and a selenite free radical have similar spectral and kinetic properties in neutral and alkaline solutions. The spectral shifts and decay kinetics of the tryptophan free-radical observed at pH -3 show that oxidation of TrpH by Br; and (SCN), gives a radical which protonates (pKa = 4.3) and which is singly charged at pH -3.The different prototropic behaviour of the absorption of the product of oxidation of TrpH by the selenite radical suggests that in this case, more than one species is produced. The protonated free radical TrpH+ reacts with SCN-so that there is an apparent loss of reactivity of (SCN); towards TrpH, especially at low pH, high [SCN-] and low [TrpHl. A kinetic model for the (SCN);/TrpH reaction predicts a variation of the equilibrium concentration of (SCN); with [H+], [TrpHl and [SCN-I. The experimental data are in accord with the model and enable the equilibrium constants of the reactions to be obtained.