Claude R. Jones

Spin‐orbit coupling effects on the zero field splitting of (π, π*) triplet states of conjugated enones

The optically detected paramagnetic resonance transitions of five cyclic conjugated enones have been observed in zero field. The |E|/h c parameters are small and nearly invariant for series, but the |D|/ h c parameters range from 0.22 to 0.30 cm−1. Simple theory suggests that both spin‐spin dipolar interactions and spin‐orbit interactions make significant contributions to the ZFS. The large variation in the ZFS of these enones is primarily due to the dependence of the spin‐orbit interaction on the energy gaps between the 3(π, π*) state and the 1(n, π*) and 3(n, π*) states.

Nuclear magnetic resonance of the base-pairing structure of the native and denatured conformers of Escherichia coli transfer RNATrp

The native and denatured forms of transfer RNATrp from Escherichia coli have been studied by high-resolution (300 MHz) proton nuclear magnetic resonance spectroscopy. The low-field nuclear magnetic resonance spectrum indicates that there are 22 ± 2 base-pairs present in the native form, and the observed resonance positions can be accounted for in terms of 19 base-pairs from the cloverleaf secondary structure, one protected U, and two tertiary structure base-pairs including A14-s4U8. The denatured conformer contains one or two less base-pairs than the native conformer, and the low-field spectrum of the denatured conformer can be accounted for in terms of a model which retains the amino acid acceptor stem, T ΨC stem, and the anticodon stem of the cloverleaf model, but which pairs some bases of the dihydrouridine stem with bases from the minor loop. The proposed model is consistent with the ease of interconversion of the native and denatured conformers, the pattern of kethoxalation of the native and denatured conformera, and the retention of a tertiary structure A14-s4U8 base-pair in the denatured conformer. Alternative models of the denatured form are considered, but they do not adequately explain the observed nuclear magnetic resonance spectrum. The binding of chloroquine to the native conformer has also been studied. A comparison of the binding of ethidium bromide and chloroquine to E. coli tRNAGlu indicates that the two drugs have very similar binding properties and this suggests a possible mechanism for the chloroquine-induced renaturation of tRNATrp.

ODMR determination of the zero field splitting parameters of a conjugated enone

Abstract We have observed the optically-detected ESR transitions of the steroidal enone testosterone acetate at zero field and found the zero-field splitting (ZFS) parameters to be | D | = 0.28 cm − and | E | = 0.01 cm − . Approximate theoretical calculations suggest that there are at least three important contributions to the ZFS; spin-spin dipolar interaction, spin-orbit coupling of the emitting 3 (π,π * ) state with the 3 (n,π * ) state, and spin-orbit coupling of the 3 (π,π * ) state with the 1 (n,π * ) state. Thus the calculated ZFS parameters of D = −0.46 cm − and E = −0.014 cm − represent a delicate balance of these three effects. Because of the large spin-orbit coupling effects the ZFS parameters of enone 3 (π,π * ) and 3 (n,π * ) states, and perhaps ketones in general, depend strongly on the separation between (n,π) and (π,π) states. This is a complicating feature which may make it difficult to use ZFS parameters to assign the orbital character of ketone triplet states.

Investigation of singlet‐triplet transitions by phosphorescence excitation spectroscopy. X A simple α, β‐unsaturated ketone

X‐ray diffraction techniques and Patterson methods have been used to determine the crystal structure of the simple conjugated enone, ketone I. The crystals studied belong to space group P1 with a = 9.365(6), b = 6.578(4), c = 8.321(5), α = 101.70(3), β = 101.55(3), and γ = 101.37(3). Least squares refinement of 836 counter‐collected, unique reflections having | F |2>2.5σ(| F |2) led to a final value of the conventional R1 factor (based on F) of 5.8%. The structure consists of rather loosely packed, strain free bicyclic molecules, in which the enone chromophore is planar. Phosphorescence excitation spectroscopy was used to obtain the polarized singlet‐singlet and singlet‐triplet absorption spectra of oriented crystals. The important results are: (i) The ordering of excited states in this molecule is 1(n, π*)>3(n, π*)>3(π, π*). (ii) Polarization measurements and Franck‐Condon considerations indicate that the difference in geometry between the 3(π, π*) state and the gound state is small. (iii) The 1(n, π*) s...