Malcolm Daniels

Fluorescence of the Purine and Pyrimidine Bases of the Nucleic Acids in Neutral Aqueous Solution at 300°K

Fluorescence of adenine, guanine, cytosine, and uracil at room temperature in neutral aqueous solution has been detected by means of a digital signal accumulation technique. Corrected emission and excitation spectra are presented and compared with low-temperature data. The quantum yields are, respectively, 2.6 x 10-4, 3.0 x 10-4, 0.8 x 10-4, and 0.5 x 10-4 when the bases are excited at their low-energy absorption maxima.

Synchrotron excitation of DNA fluorescence: Decay time evidence for excimer emission at room temperature

The first lifetime measurements of DNA fluorescence are reported. Natural and synthetic DNA have been excited by 1.76 ns pulses of synchrotron ultraviolet radiation (270 nm) and the time profile of the fluorescence has been measured by synchronous single-photon counting. A post-pulse exponentially decaying emission has been observed with a lifetime of 2.9 +/- 0.4 ns for calf thymus DNA and 3.0 +/- 0.3 ns for poly(dA-T); this is most likely an excimer fluorescence.

EXCITED STATES OF DNA AND ITS COMPONENTS AT ROOM TEMPERATURE—III. SPECTRA, POLARISATION AND QUANTUM YIELDS OF EMISSIONS FROM ApA AND poly rA*

Abstract— Corrected normalised emission spectra from 300 to 480 nm and their polarisation are reported for neutral and poly rA at room temperature. Yields are 4 times (ApA) and 6 times (poly rA) greater than the monomer. Comparison of emission spectra and polarisation spectra demonstrates heterogeneity of emission and, following attempts at spectral synthesis, comparison with experimental reference spectra allows the observed total emission spectra to be resolved into four components assigned as I monomer‐like emission, II excimer fluorescence, HI monomer phosphorescence, IV excimer phosphorescence; the polarisation of the excimer fluorescences of ApA and poly rA are deduced. The observations can be understood quantitatively using a simple stacking model with excimer emissions originating in the stacked components. Quantitative differences between ApA and poly rA are due to their different hypochromism and extent of stacking. Differences of polarisation are attributed to different stacking geometries, and the directional properties of the transition moment suggest that the excimer fluorescence is largely charge‐resonance in nature.

FLUORESCENCE OF THYMINE IN AQUEOUS SOLUTION AT 300° K

Abstract— –Fluorescence of thymine in neutral aqueous solution at room temperature has been detected using the multiscaling operation of a multichannel analyzer. The emission maximum (2.96 μm‐1) and 0‐0 transition energy (3.37‐3.45 μm‐1) are close to those determined at liquid nitrogen temperature in mixed solvents. The quantum efficiency of fluorescence excited at 3.77 μm‐1 is calculated to be 1.04 × 10‐4.

Wavelength-resolved lifetime measurements of emissions from DNA components and poly rA at room temperature excited with synchrotron radiation

Abstract The wavelength-resolved luminescence decays of the very weak emission from DNA and of some components at room temperature have been investigated by using the luminescence equipment recently built at Orsay. It combines the LURE synchrotron source with a spectrophotofluorometer using a fast single photon counting detection operating in the nanosecond range. The high intensity of the synchrotron source together with the high pulse frequency have allowed for the first time the luminescence decays of such weak emitters (φf ⩽10-4) to be undertaken. It is shown that the room temperature fluorescence of the monomers (nucleosides) is indeed very short, τ

INTRINSIC POLARIZATION AND ROTATIONAL DEPOLARIZATION OF FLUORESCENCE OF DNA BASES IN AQUEOUS SOLUTION AT ROOM TEMPERATURE

Abstract— A counting procedure is described for determining the polarization characteristics of very weak emission. The degree of polarization of weak emission of DNA bases in aqueous solution at room temperature is found to he high and positive, and shows an inverse correlation with their emission quantum yields which can be understood in terms of a competition between emission lifetimes and rotational relaxation times. For 5‐methylcytosine the emission quantum yield can be changed by varying the acidity and the corresponding degrees of polarization show the behavior expected from Perrins relation. The ratio τo/τrot is discussed in terms of anisotropic relaxation and calculations of τ0.

Photolysis of the aqueous thymine system: I. Excitation at 1849 Å

Abstract 1. The 1849 A photolysis of thymine in aqueous solution occurs with a quantum efficiency varying from 8·10−2 to 0.2, depending on experimental conditions, in contrast to the photolysis at 2537 A which occurs with a quantum efficiency of 4·10−4. 2. Products of photolysis have been identified as thymine hydroxyhydroperoxides (4:5 and 5:4), thymine glycol (4:5) and hydrogen peroxide. These, together with thymine loss, have been determined quantitatively under a variety of conditions. 3. Mechanisms of photolysis are deduced. At low thymine concentration, reaction is due to water photolysis, and complete stoichiometry is obtained. Kinetic analysis shows that direct photolysis of thymine also occurs. Direct photolysis is accounted for by two processes, (a) photoionization, and (b) addition of O2 to an excited state. These primary processes are correlated with the far ultraviolet absorption spectrum of thymine.

60Co γ-ray induced peroxidation of DNA in aqueous solution

Abstract γ-Radiolysis of N 1 -substituted derivatives of cytosine gives hydroperoxides which are stable enough in neutral solution to be considered as models for the fast-decaying component of the DNA hydroperoxides. The decay kinetics of these derivatives and some of thymine have been investigated as a function of pH, temperature, buffer concentration and inert salt. Activation enegies for the decay of the hydroperoxides of cytidylic acid and thymine are found to be 10·3 kcal/ mol and 22·3 kcal/mol (45 and 93 kJ/mol) respectively. Decay kinetics of the DNA hydroperoxides have been measured from 12°C to 80°C; for the fast component Δ E ‡ = 10·2 kcal/mol) (42 · 6 kJ/mol) and for the slow component Δ E ‡ = 21 · 5 kcal/mol (89·8 kJ/mol) making their assignments to a location on the thymine residues quite reasonable. The decay kinetics of the slow component are strongly dependent on the presence of inert electrolytes and urea. This is discussed in terms of effects on the local water structure, which thus affects the hydrolytic transition state. Yields of the hydroperoxides, hydrogen peroxide and of base loss are reported for various DNA concentrations and are shown to be consistent with a simple free radical mechanism.

Radiationless transition rates of thymine and uracil in neutral aqueous solution at 300°K

Abstract Recent experiments in low-intensity fluorescence spectrometry, flash photolysis and continuous photolysis at 300°K in an aqueous medium have provided data for uracil and thymine from which all the pertinent phenomenological characteristics (Φ fl 0′ , Φ isc 0′ , Φ′ chem ) of the 0′ level can be evaluated. It is necessary to postulate considerable internal conversion 1 S → 0 S with τ ic ≈ 1–2 × 10 −12 sec, this process correlating with the dielectric relaxation time of water.

Time-resolved fluorescence emission and excitation spectroscopy of d(TA) and d(AT) using synchrotron radiation

The photophysics of the sequence isomers d(TA) and d(AT) has been investigated at room temperature in 5 x 10(-5) M neutral aqueous solution using pulsed ultraviolet excitation from the ACO synchrotron and detection by time correlation or gated single-photon counting. Decay profiles of the emissions at 350, 400 and 460 have been analyzed both independently and globally by reiterative non-linear least-squares fitting to models of two and three independently emitting species. No evidence has been observed for excited-state reaction. Time-windowed spectra, both emission and excitation, have been collected for three time windows and have been deconvoluted to give time-resolved spectra using the lifetimes resulting from the decay analyses. Spectra are separated into two classes, with picosecond and nanosecond lifetimes, respectively. The picosecond spectra have the emission and excitation spectral characteristics of mixed monomer (A and T) fluorescences and are assigned as originating from the unstacked fractions of d(TA) and d(AT). The nanosecond emission spectra from d(TA) and d(AT) are both two-component, with lambda max approximately 350 and approximately 425 nm and lifetimes of 2.3 and 6.1 ns, respectively. The time-resolved excitation spectra for the nanosecond emissions are quite different from the isotropic absorption spectra of d(TA) and d(AT) but correlate with the anisotropic absorption for out-of-plane transitions between stacked bases of co-crystals of 9-methyladenine and 1-methylthymine reported by Stewart and Davidson. The nanosecond spectra thus represent the direct excitation and emission of stacked pairs of bases. These results provide no evidence for energy transfer and are probably related to sequence-specific photo-adduct formation.