J. C. Sutherland

Mechanisms of inhibition of pyrimidine dimer formation in deoxyribonucleic acid by acridine dyes.

The ultraviolet (UV)-induced formation of cyclobutyl pyrimidine dimers in Escherichia coli deoxyribonucleic acid (DNA) in vitro has been investigated in terms of the mechanism of inhibition by acridine dyes, the effect on dimer yield of specific singlet and triplet quenchers, and the mechanism of dimer formation. Our results indicate that (a) energy transfer is important in dimer reduction by acridines, (b) this transfer occurs from the singlet (S(1)) of DNA, and (c) at room temperature triplet quenchers do not reduce dimer yield in DNA.

Zeeman Effect in Porphyrins: Zero‐Field Splitting of the Excited Electronic States

Recent experimental studies have reported anomalous and contradictory results for the absorption of left and right circularly polarized light (LCPL and RCPL) by zinc and magnesium coproporphyrins in the presence of a magnetic field: for both compounds, the absorption bands for right and left circularly polarized light have distinctly different shapes at room temperature; at 77°K unexplained shoulders appear in the absorption spectra for both polarizations for the magnesium but not for the zinc compound; the value of the angular momentum of the lowest energy excited state computed from the separation of the peaks of the LCPL and RCPL absorption bands differs from the value obtained from magnetic circular dichroism (MCD) experiments by nearly 50%; no shape anomalies were observed in the MCD spectra. We derive a general solution for the mixing of two states by a magnetic field and show that all of the anomalous experimental data are explained if (a) the presumably degenerate pair of excited states are split ...

Emission and polarization spectrometer for biophysical spectroscopy

We have constructed a spectrometer which, using well established techniques, measures both fluorescence and circular dichroism (natural and magnetic) as well as several other parameters of interest in biophysical spectroscopy. This combination is feasible since the various measurements, aside from sharing many optical components, can all use a lock‐in amplifier to process electronic signals. Although fluorescence and circular dichroism provide greatly different information about the material under investigation, from the standpoint of instrumentation they are very similar. Combining several functions in a single spectrometer has the advantages of economy and flexibility.

Optical Properties of Sodium in the Vacuum Ultraviolet

The index of refraction, n, of sodium films evaporated onto quartz substrates has been determined in the spectral region from 420 to 2000 A from interference patterns observed in the reflected beam and from the critical angle marking the onset of “total reflection.” The growth of an oxide layer on the surface of a film is shown not to alter appreciably the position of interference maxima and minima. The real part of the dielectric constant was consistent with a nearly-free-electron model. The effective volume–plasmon energy was found to be 5.69±0.06 eV, in agreement with the values determined from electron-energy-loss experiments.

A spectrometer for the measurement of magnetic and natural circular dichroism

An instrument employing several novel features has been assembled for the measurement of magnetic and natural circular dichroism (MCD and CD) in the visible, ultraviolet and near infrared spectral regions. The instrument is fitted with two magnets: an electromagnet for routine use and a superconducting magnet for use when higher performance is required. The contributions of CD and MCD are separated by measuring the net spectrum twice; first with the magnetic field parallel to the Poynting vector of the optical beam and then with it antiparallel. The value of the net optical activity for each wavelength increment is converted to digital form and the manipulations required to separate CD and MCD and signal averaging are performed by a computer. The data can be plotted in a variety of formats on a Cal Comp plotter. The instrument is equipped with a Cary model 14 prism‐grating monochromator which, unlike double quartz prism monochromators, has good wavelength dispersion from the uv to the near ir spectral regions. Performance in the ultraviolet has been considerably improved by replacing the previously used electro‐optical modulator (Pockels cell) with a quartz strain‐birefringence modulator. The capabilities of this instrument are illustrated by spectra for the natural CD of ribulose‐1,5‐diphosphate carboxylase at 22° from 195 to 245 nm, the MCD of metmyoglobin cyanide at −196° and 22° from 340 to 470 nm, and the MCD of oxyhemoglobin at 4° from 700 to 1100 nm.

Photophysics and photochemistry of photoreactivation.

Abstract— Photoreactivating enzyme (PRE) monomerizes cyclobutyl pyrimidine dimers formed in DNA by UV light (Λ < 300 nm). The enzyme requires near UV and visible wavelengths (300 < Λ < 600 nm) for activity. Possible mechanisms of action of the PRE are suggested by non‐enzymatic processes in which pyrimidine dimers are monomerized by UV and visible light. Two such non‐enzymatic processes are (a) photolysis of dimers resulting from direct absorption of UV, and (b) sensitized monomerization involving charge transfer complexes. Several lines of evidence suggest that the mechanism of action of the PRE more closely resembles (b) than (a). Recent experiments on the PRE from E. coli reveal the presence of new long wavelength absorption which may indicate the presence of a ground state complex. The known ability of PRE to monomerize dimers of thymine, cytosine and uracil suggests that the carbonyl groups at 2 position of the pyrimidine ring may be important in the interaction between enzyme and dimer.

Inhibition of Pyrimidine Dimer Formation in DNA by Cationic Molecules: Role of Energy Transfer

In addition to the acridine dyes, acridine orange and proflavine, we find that three other cationic molecules which bind to DNA-ethidium bromide, chloroquine, and methyl green-inhibit the production of cyclobutyl pyrimidine dimers by ultraviolet radiation. Intercalation is not necessary for dimer inhibition. The long range nature of the inhibition implies that energy transfer is responsible. The transfer is between the lowest excited singlet state of DNA and the acceptor singlet, and seems to involve the Förster mechanism.

Extinction Coefficient and Imaginary Part of the Dielectric Constant for Sodium and Potassium above the Plasma Energy

The extinction coefficients, k, of sodium and potassium have been found by measuring the transmittance of vacuum-evaporated films 400–900 nm thick deposited on quartz or lithium fluoride substrates. Corrections were made for losses due to surface reflections, substrate absorption, and contamination of the exposed surface of the metal. Previously determined values of the refractive indices, n, of Na and K were used to compute the imaginary part, ∊2 = 2nk, of the dielectric constant. Above the interband energies, at 2–3 eV, ∊2 for both Na and K decreases rapidly with increasing photon energy up to the plasma energy. Beyond the plasma energy, ∊2 remains nearly constant within the limits of our measurements. The values of ∊2 for K are more than twice those for Na in the region above the plasma energy.

Optical Properties of Potassium for Photons of Energy 3.96 to 9.69 eV

The refractive index of potassium has been measured by the critical-angle method for photon energies from 3.96 to 9.69 eV. The potassium films were deposited on the flat surface of a quartz or calcium fluoride semicylinder so that, viewed through the semicylinder, the potassium surface in contact with the substrate is uncontaminated by oxidation. The geometry used facilitated the calculation of the refractive index from the observed reflectance as a function of angle of incidence. These data have been interpreted in terms of a nearly-free-electron model with a volume plasma energy of 3.72±0.05, an optical effective mass of 1.06±0.03, and an ion-core polarization of 0.20±0.03.

Universal computer interface for the Cary 118C spectrophotometer.

We have constructed a simple interface which allows a computer to collect spectral data from and control the wavelength setting of a Cary 118C spectrophotometer. The interface is compatible with most types of computers; the computer need not be located near the spectrophotometer. Programs to operate the spectrophotometer can be written in high-level languages such as fortran or basic.