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The electronic spectra of 1,N-6-ethenoadenine (epsilon-adenine, epsilon-A), 3-methyl-epsilon-adenine (3-me-epsilon-A), and epsilon-adenosine (epsilon-Ado) have been examined with respect to the number of electronic transitions, their intensities, and their transition moments, on the basis of measurements of UV and IR linear dichroism (LD) on samples partially oriented in stretched poly(vinyl alcohol) films, fluorescence anisotropy (FA), and magnetic circular dichroism (MCD). The experimental results are compared with semiempirical molecular orbitaI calculations. Four pi-->pi(*) transitions were found to contribute to the UV spectrum of the epsilon-adenine chromophore with the following transition moment directions (relative to the ''adenine'' pseudosymmetry long axis, counted toward N-1): +27 degrees +/- 10 degrees (I at 32 500 cm(-1)), -47 degrees +/- 5 degrees (II at 37 000 cm(-1)), -30 degrees +/- 10 degrees (III at 32 000 cm(-1)), and -11 degrees +/- 10 degrees or -49 degrees +/- 10 degrees (IV at approximate to 46 000 cm(-1)). The orientation parameters in the stretched film experiments were determined from polarized IR measurements combined with UV LD and FA data. In addition, fluorescence emission and excitation spectra and fluorescence lifetimes of E-A in aqueous solutions indicate that epsilon-A does not behave as a single spectroscopic species. In contrast to 3-me-epsilon-A and epsilon-Ado, epsilon-A displays an emission wavelength dependent excitation spectrum and a biexponential fluorescence decay. This observation is attributed to a prototropic tautomeric equilibrium between the 1-H and 3-H forms of epsilon-A.

The photophysical properties of the optical DNA probe YOYO (homodimeric derivative of oxazole yellow) have been characterized in terms of the monomeric part, the YO chromophore. In aqueous solutions YO is virtually nonfluorescent but upon binding to DNA its fluorescence quantum yield is strongly increased. A similar enhancement of the fluorescence is observed for YO in the viscous solvent glycerol. The high fluorescence quantum yield of YO, when bound to DNA or in a viscous solution, is proposed to be a result of decreased rotational mobility around the internuclear bridge between the two aromatic ring systems. This hypothesis is based on similar values of the activation energies for the temperature-dependent nonradiative decay processes (E(A) = 53 kJ/mol) and viscous flow (E(A) = 63 kJ/mol), suggesting related rate-limiting mechanisms. A single electronic transition is found to be responsible for the intense visible absorption band. This conclusion is based on the observation of an essentially wavelength-independent reduced linear dichroism and similarly wavelength independent fluorescence anisotropy, and the fact that the emission spectrum is very nearly a mirror image of the absorption spectrum. The conclusion is further supported by quantum mechanical calculations (CNDO/S). By combination of measurements of fluorescence anisotropy of YO in glycerol and linear dichroism of YO in a stretched poly(vinyl alcohol) film, the transition moment of the strong visible absorption band was found to be nearly long axis polarized, in agreement with the CNDO/S calculations. The low-energy electronic transition and its polarization direction in the YO chromophore remain essentially unperturbed in the YOYO dye, suggesting that the results obtained for the excited state of the YO chromophore are applicable also to YOYO. One difference, though, is that in aqueous solutions the two YO chromophores of YOYO interact with each other, forming an internal dimer, resulting in a distorted absorption spectrum.

From measurements of UV and IR linear dichroism on molecules partially oriented in stretched polyethylene host the transition moment directions for the first four pi-pi* transitions of indole and some indole derivatives were determined. Relative to the pseudosymmetry long axis of indole, the transitions were normally found to be polarized at (angles counted away from the ring nitrogen): +42 +/- 50-degrees (1A1 --> 1L(by) at 287 nm), -46 +/- 5-degrees (1A1 --> 1L(a) at 265 nm), 0 +/- 15-degrees (1A1 --> 1B(by) at 220 nm), and for the 1A1 --> 1B(a) transition occurring around 200 nm, at least at +/- 30-degrees away from this axis. In addition, indication for a weak, essentially short axis polarized transition was found at 235 nm, possibly due to the 1A1 --> 1C transition. An ambiguity problem regarding the sign of the angles was resolved by exploiting the change of orientation properties upon introduction of substituents. Orientation parameters (including diagonalizing angle) were determined by consideration of a large number of in-plane as well as out-of-plane polarized vibrational transitions. The question regarding effects on the excited states by the presence of methyl and methoxy substituents, at varied positions in the indole chromophore, was addressed in terms of the perturbations they caused on the transition moments. Whereas none of the four transitions was found to be very sensitive in this respect to methyl or methoxy groups introduced in 2-, 3-, 5-, or 7-position of indole, the directions of the weak 1A1 --> 1L(by) but also the strong 1A1 --> 1B(by) transition were found to become significantly altered by a methoxy group in 4- as well as 6-position. The conclusions are consistent with recent fluorescence anisotropy data and semiempirical calculations.

Ordinary linear dichroism (LD) spectroscopy with rectangular optics is not applicable to thin uniaxial samples having the optical axis perpendicular to the plane of the sample. A method has been developed to study such samples at inclined optical incidence and a general relation derived between the LD (corrected for polarized reflection) and the average orientation of the absorbing transition dipole. For molecules of C2”, D2h, or higher symmetry the usual order parameters can be obtained. The method has been tested on anthracene in a polyethylene standard slab of perfectly axial symmetry. The estimated orientation agreed with previous reports on anthracene in stretched polyethylene sheets. A number of chromophoric molecules solubilized in macroscopically aligned lamellar mesophases of the systems sodium octanoate/decanol/water and sodium di- 2-ethylhexylsulfosuccinate (Aerosol OT)/water have also been studied. In the first system rodlike molecules (diphenylethyne, anthracene, retinal) are oriented with their long axes parallel to the hydrocarbon chains of the amphiphile, while an extremely long molecule (&carotene) as well as water-soluble planar cationic dyes are on the average oriented parallel to the lamellar planes. In the Aerosol OT system all molecules studied were oriented parallel to the lamellar planes.

A linear dichroism (LD) method using macroscopically aligned samples was recently developed for studying biological model membranes. We show here that for these systems the influence from multiple reflections must be considered when chromophore orientation is low. A comparison between order parameters obtained from LD and NMR has also been performed, by studying the 2H NMR quadrupole splittings of perdeuterated chromophores. The benzene plane was found to be preferentially oriented parallel to the amphiphilic surfaces of the lamellar phase of cetyltrimethylammonium bromide (CTAB)/l-hexanol/water and of the hexagonal phase of CTAB/water. In the lamellar liquid crystalline phase composed of monooctanoin/water, the molecular plane of benzene and naphthalene tended to be oriented perpendicular to the lamellar surface. The order parameters from LD were in satisfactory agreement with those obtained from NMR. Studies of benzene in aqueous micellar solutions of CTAB in Couette flow indicated that the rod-shaped micelles were inefficiently aligned. Lecithin liposomes oriented by flow showed distorted absorption spectra.

Moment directions and intensities of the two lowest electronic transitions of indole are important in many contexts but have been subject to controversy. We have measured UV and IR linear dichroism on indole, 5-methylindole, 5-methoxyindole, and 3-methylindole in an anisotropic polyethylene host and determined the transition moment directions and intensity profiles of the lowest, nearly degenerate transitions 1 L a ← 1 A 1 and 1 Lb← 1 A 1 . An ambiguity due to the two possible solutions for each transition moment angle can be removed by considering the effects of the changed alignment of the substituted indoles

Etude de l'interaction en solution aqueuse diluee. Guanosine et methyl-7 guanine forment des especes dimeres par interaction avec Ag + en solution neutre