The addition of the trivalent or tetravalent cations spermidine or spermine to a solution of T7 DNA in aqueous solution causes an alteration of the DNA from its extended coil form to a condensed form. If performed at low DNA concentration and at low ionic strengths, this transformation results in a monomolecular collapse to form a particle with a hydrodynamic radius of about 500 A. We have monitored this change using quasielastic and total intensity light scattering. In a solution of 50% methanol in water, the divalent cations Mg2+ and putrescine also can cause the condensation of DNA. Using Manning's (1978) counterion condensation theory, we calculate a striking unity among these disparate ions: the collapse occurs in each case when from 89 to 90% of the DNA phosphate charges are neutralized by condensed counterions.

Photodissociation of nitric acid and hydrogen peroxide via high-lying O−H overtone absorptions in the visible may act as a source of OH radicals in the atmosphere. We have used cavity ring down spectroscopy to measure the absorption cross sections for the third (4νOH) and fourth (5νOH) overtone transitions in these molecules. The integrated cross sections are (2.25 ± 0.15) × 10-21 and (2.57 ± 0.24) × 10-22 cm2 molecule-1 cm-1 for 4νOH and 5νOH in nitric acid, respectively, and (4.58 ± 0.39) × 10-21 and (5.67 ± 0.52) × 10-22 cm2 molecule-1 cm-1 for 4νOH and 5νOH in hydrogen peroxide. For both molecules, our report is the first direct intensity measurement for 5νOH, the lowest dissociative overtone transition. We compare our values for the lower overtones to those from previous studies, where available. Our measured cross sections suggest that the contribution of direct overtone excitation to the atmospheric photodissociation of HNO3 and H2O2 is small but not completely negligible.

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Abstract— Fluorescence, absorption and fluorescence excitation spectra, and quantum yields of 0.02 mM solutions of adenine, 7‐methyladenine (7‐MA), guanine and 7‐melhylguanine (7‐MG) are presented for excitation with240–300 nm light. The solvent is neutral ethylene glycol‐water (70:30 v/v) in the temperature range140–165 K. Phosphorescence spectra of adenine and 7‐MA at 140 K are also presented. The excitation spectrum of adenine shows vibrational structure, whereas the absorption does not. However, the fluorescence of adenine shows the vibrational structure, as do the absorption, fluorescence and excitation spectra of 7‐MA. The results confirm (and reinforce) the notion that luminescence from adenine under these conditions is from the N7–H tautomer, instead of the more abundant N9–H form. In a similar fashion, the data from guanine and 7‐MG strongly suggest that the luminescence from guanine is also mostly from the N7–H tautomer.

Absorption, fluorescence, and fluorescence polarization measuremts as a function of excitation and fluorescence wavelengths (\e and Xf) covering the spectral region 240-450 nm are reported for the nucleic acid constituents 5'CMP, (3' -5')CpC, poly(rC), 5'-TMP, (3' -5')dpTpT, 5'-GMP, (3' -5')GpG, 5'-AMP, (3' -5')ApA, and poly(rA) at ca. M solution in neutral ethylene glycol-water glasses at temperatures near -120 °C. AMP exhibits considerable mixed polarization in contrast to the free base adenine. CpC and ApA show evidence of stacking in their spectra, and their polarizations are dramatically different than for the monomers. The spectra of dpTpT and GpG are not detectably different from the monomers but evidence of interaction is seen in the polarizations. In all cases, the dinucleotide fluorescence showed very little depolarization when Xe and Xf were near the origin, in marked contrast to both vibronic exciton theory and Forster theory. In general, the polarization ratios decrease steeply as both Xe and Xf are varied away from the origin. For CpC and ApA the variation of polarization vs. Xf is apparently due to a transition moment directed perpendicular to the molecular planes indicating charge resonance character in the excited dimer state. Polarized fluorescence spectra calculated from vibronic exciton theory are shown. The poor agreement leads to the conclusion that, for these systems, the usual assumptions employed in stationary exciton and energy transfer theories regarding the states initially reached upon excitation are not valid. The spectra of poly A and poly C show only partial depolarization relative to the respective dinucleotides.