J. Eisinger

Transition Metal Binding in DNA Solutions

Paramagnetic ions in water shorten the spin‐lattice relaxation time T1 of the water protons. The effectiveness with which the relaxation takes place depends not only on the ion concentration, but also on the environment of the paramagnetic ion. In some cases where the paramagnetic ion is bound to a large molecule such as DNA, it shortens the proton relaxation time to a greater extent than when it is in solution. By measuring T1 by a pulsed nuclear magnetic resonance method, we have studied the binding of transition metal ions to DNA. We have obtained estimates for the number of available binding sites as well as information on the type of site which the various ions are bound to.

Fluorescence Quenching and Isotope Effect of Tryptophan

The fluorescence spectra and quantum yields of trytophan (TRP) and several tryptophan derivatives in water and a polar glass have been measured over wide temperature ranges. For TRP, the wavelength of maximum emission shifts from 310 nm at 80°K to 355 nm at room temperature with almost all of the red shift occurring between 170° and 230°K, which is the temperature range where the glass softens. The successively more red‐shifted spectra have no isoemissive wavelength, which supports the view that reorientation of several solvent molecules in the solvent shell of the excited TRP molecule and not a 1:1 exciplex is responsible for the red shift. The quantum yield remains constant until a temperature is reached at which solvent reorientation is virtually complete. Above that temperature, the quenching of the 355‐nm emission can be fitted with a nonradiative de‐excitation having an activation energy of 7 kcal/mole. A model for these spectral changes and quenching mechanism will be offered. In deuterated solvents, a large isotope effect of fluorescence yield of TRP has been reported. This effect is clearly not caused by proton transfer in the excited state since it is found to be virtually the same for TRP and 1‐Me‐TRP. At temperatures below those at which solvent reorientation occurs, the isotope effect vanishes, and above them it approaches an asymptotic value, the quenching activation energy being independent of the isotopic constitution of the solvent. The fluorescence of most proteins and hormones originates in their tryptophan residues. The quantum yields, isotope effects, and emission spectra of such polypeptides are compared with the corresponding parameters for solvated TRP.

Triplet State of DNA

Additional proof has been obtained for the fact that the triplet state in DNA and poly dAT resides at the thymine residue by comparing the intensities of the triplet ESR and phosphorescence signals from DNAs from various sources having different (A+T)/(G+C) ratios. The TMP triplet which is not populated from the exited singlet state in dilute neutral solutions can be populated by having the TMP present in high concentrations or through energy transfer from the triplet state of acetone or acetophenone. The triplet state produced in this manner has been characterized by its Δm=2 ESR line, its phosphorescence spectrum, its decay time, and the effect of deuterium substitution on the latter. In all these respects the DNA and poly dAT triplet resembles the neutral TMP triplet (T) more closely than it resembles the ionized (pH 12) TMP triplet (T−). A determination of the acidity constants of the excited states of A and T gives strong evidence against single‐proton transfer from the excited singlet or triplet st...

BINDING OF MN2+ TO NUCLEIC ACIDS.

Measurements of the nuclear magnetic resonance relaxation times T1 and T2 of the water protons in aqueous solutions of nucleic acids and magnetic metal ions allow one to determine details about the metal‐ion binding. When Mn2+ is bound, the rotation of its hydration sphere becomes slower, so that its effectiveness in reducing the proton T1 from its value in pure water is enhanced. This relaxation enhancement has been measured for Mn2+ bound to DNA, RNA, synthetic polynucleotides, and E. coli ribosomes. Compared to water as unity, the enhancement factors range from 3.7 for polyuridylic acid to 16.7 for polyadenylic acid. The equilibrium constants for binding, and the concentration of binding sites have been calculated from the dependence of enhancement upon Mn2+ concentration.

Adsorption of Oxygen on Tungsten

The adsorption of oxygen on a single crystal tungsten ribbon at room temperature was studied. The maximum coverage was found to be 12.3×1014 atoms cm‐2 and the initial sticking probability is 0.24 approximately. The rate of formation of tungsten oxide at the surface of the hot filament was also studied. These results and earlier ones bearing on the adsorption of nitrogen on tungsten are discussed in the light of an idealized atomic model for the tungsten surface.

Properties of Hydrogen Chemisorbed on Tungsten

The sticking probability of hydrogen on a single crystal tungsten ribbon was measured as a function of the number of adsorbed atoms. The work function of the same ribbon was measured by a photoelectric technique. The results indicate that during the initial fast chemisorption characterized by an almost constant sticking probability of approximately, 0.2 the work function rose from 4.54 ev for the clean surface to a maximum value of 5.0 ev for the covered surface. The maximum coverage at room temperature was (7.5±1.0)×1014cm−2. Evidence for two states of adsorption is presented and the experimental techniques, used in these and earlier experiments, are described in some detail.

Luminescence and Electron Spin Resonance Studies of Adenine in Various Polynucleotides

The excited states of adenine in different polynucleotides dissolved in ethylene glycol: H2O glasses have been studied by optical emission and ESR at 77°K. In single‐stranded poly A the optical properties were very similar to the constituent AMP monomers except for a red shift of the luminescence. The ESR Δm=1 transitions were observed with D=0.116 cm−1 and E=0.027 cm−1 for poly A and D=0.119 cm−1 and E=0.027 cm−1 (all ±0.002 cm−1) for AMP. This means that the triplet transfer time is longer than 2×10−10 sec since more rapid triplet—triplet jumping would give a time averaging of E. In the ordered double‐helix form of poly A the excited states are completely quenched when one‐half of the bases are protonated. In poly (A+2U) and poly (A+2I) ordered copolymers the excited states are also quenched. However in random distributions of U, C, and I in single strands of poly A no quenching was observed. This leads to the conclusion that the hydrogen bonding rather than energy transfer is responsible for the quench...

Adsorption of Oxygen on Silicon

The sticking probability of oxygen on silicon has been measured as a function of coverage. At room temperature it has a maximum value between 0.01 and 0.02 for low coverage and decreases approximately exponentially as the coverage increases.

Triplet yield determination by E.S.R.

A practical method is given for measuring inter-system crossing yields. It is applicable to triplets with lifetimes larger than 10-2 s. By using Snyder and Kornegays programme, the intensity of the Δm = 2 spin resonance for a given number of randomly oriented triplets is computed as a function of zero-field-splitting and line width. The yield is then obtained by comparing the observed intensity to that of a standard with known yield.

Electrical Properties of Hydrogen Adsorbed on Silicon

The photoemission from a silicon surface has been studied as a function of the number of hydrogen atoms adsorbed on it. The electric dipole moment of an adsorbed H atom is found to be 0.15×10—18 esu approximately.