The NCI Drug Information System. 3. The DIS Chemistry Module

The Chemistry Module of the Drug Information System (DIS) handles a database of 400,000 structures. New or modified records are created in this database on a daily basis and are merged into the file promptly. The Chemistry database is searchable in a wide variety of ways and provides novel methods for both input and output of chemical structures.     

Positive hole transfer between organic molecules of different classes in freon matrices

The processes of positive charge (hole) transfer between organic molecules of different classes occurring upon irradiation of their frozen freonic solutions at 77 K were studied by ESR spectroscopy. The efficiency of positive hole transfer was characterized by the “redistribution coefficient”, which was determined as ratio of concentrations of the radical cations of different compounds at equal concentrations of the parent neutral molecules. A very effective positive hole transfer to toluene was shown to occur for alkane-toluene pairs (K > 10). Meanwhile, much lower K values (～ 2 to 3) were observed for alkane-alkene pairs, in spite of a rather high difference in the ionization potentials (ca. 1 eV). This effect was explained by the conformation dispersion and distribution of environment configurations of ionized molecules for the alkane-alkene pairs. An effective positive hole transfer (K ～ 6) was observed for the pair dimethyl ether-acetone, where the conformation dispersion was absent, although the difference in ionization potentials for this pair was only 0.3 eV.     

THE QUENCHING OF TYROSINE AND TRYPTOPHAN FLUORESCENCE BY H2O AND D2O*

Abstract— The quantum yields and lifetimes of the fluorescence of tyrosine and tryptophan were determined in D₂O-H₂O and glycerol-H₂O solvent mixtures of varying composition from 10 vol.% to 100% H₂O at 15°C. Forboth amino acids the ratio of the quantum yields in D₂O and H₂O (i.e., q_D/q_H) was smaller than the ratio of the corresponding lifetimes (_D/_H). For tyrosine the ratio of the quantum yields in glycerol and H₂O (q_G/q_H) was also smaller than the corresponding _G/_H ratio, but for tryptophan q_G/g_HG/_H. The proximity of the q vs. plots for tyrosine in the two solvent mixtures indicates that at 15°C neither D₂O nor glycerol, in the pure state or when diluted with H₂O, quench tyrosine significantly. However, H₂O quenches tyrosine by a dynamic process, which increases both the radiative and the nonradiative rate constant. The quenching action is attributed to a tyrosine-H₂O exciplex, whose formation is independent of bulk viscosity and dielectric constant. Unlike tyrosine, tryptophan is quenched weakly by D₂O by a static process at 15°C (i.e., involving no change in), but H₂O quenches tryptophan much more efficiently by a dynamic process, which involves the nonradiative rate constant, but not the radiative constant. These results are explained on the basis of electrostatic complexation of the ammonium group to the carbon atom adjacent to the ring nitrogen with a lifetime which is longer thanin D₂O but shorter thanin H₂O, with solvent reorientation possibly also being an important factor in the quenching. This explanation is consistent with the fact that concentrated (8 M) urea increases q andof aqueous tryptophan ? 15–20%, while guanidine hydrochloride (6.4 M) has the opposite effect, i.e., it decreases q and t of tryptophan ? 15–20%, and with the fact that neither 8 M urea nor 6.4 M guanidine hydrochloride affects any fluorescence parameter of tyrosine at all.     

NewK-automorphisms and a problem of Kakutani

A property is introduced, for 1-1 measure-preserving transformations of probability spaces, calledloose Bernoulliness (LB), which is invariant under taking factors, inducing, and tower-building. It amounts to replacing, in Ornstein’s definition ofvery weak Bernoulli, the Hamming distance on strings by a coarser metric. The main result is the construction of a transformationT ₀ which is ergodic and of entropy 0 butnot LB. On the other hand, any irrational rotationis LB. Consequently, the equivalence relation generated by inducing and tower-building (which I callKakutani equivalence, and the Russians callmonotone equivalence) has at least two distinct equivalence classes among the ergodic entropy zero transformations. A similar situation exists for ergodic positive-entropy transformations: on the one hand, any Bernoulli shift is LB, while on the other hand a non LBK-automorphism can be made by skewingT ₀ over a Bernoulli base.     

Multiple harmonic conversion of pulsed CO2 laser radiation in Tl3AsSe3

The nonlinear material Tl₃AsSe₃ is used to convert pulsed 9.6 m CO₂ laser radiation into its second, third, fourth and fifth harmonic. Internal (external) conversion efficiencies of 28, 14(6.7), 3.6(3.1), and 0.5(0.3)% from the pump fundamental are achieved in 4.8, 3.2, 2.4, and 1.9 m generation respectively.