The distant shielding effect in trinucleoside diposphates
A comparative study on the proton magnetic resonance of the dinucleoside monophosphates TpT, dApT, TpdA, the trinucleoside diphosphates dApTpT, TpTpdA, and poly T has been made. The purpose of this investigation was to establish whether in a trimer the proton chemical shifts of one terminal residue are influenced by the magnetic anistropy of the other terminal residue. The conformation of the trimers was first studied and shown to the similar to that of the corresponding dimer, except that of the percentage of the lefthanded conformers in the population of the trimers is probably lower than in the population of the dimers. The methylprotons, H‐6, and H‐1′ of the 3′‐terminal T residue in TpTpdA (or the 5′‐terminal T residue in dApTpT) are found to be more upfield than the same protons on the dimer TpT up to a magnitude of about 0.1 ppm. The shielding of these protons of the terminal T(s) in these trimers is even larger than those corresponding protons in the poly(T) at the same condition. From these results, it is concluded that a distant shielding effect is exerted by the 5′‐terminal. A residue (or by the 3′‐terminal A residue) on the other terminal residue in the trimers studied.
Determination of mutation rates in bacteriophage T4 by unneighborly base pairs: genetic analysis.
Abstract Earlier studies showed that the 2-aminopurine-induced mutation rate at a particular base pair can be influenced by the base adjacent to, or one additional base-pair removed from, the measured site (Koch, 1971). The present study extends to 0.3 map unit (about 30 base pairs) the distance at which a single base-pair substitution can exert such an effect. A particular base-pair substitution (defined as a ts mutation in the rIIA gene of bacteriophage T4) reduces the spontaneous, 2-aminopurine-induced and nitrous acid-induced reversions of an rIIA amber mutation approximately threefold. The ts mutation also reduces the 2-aminopurine-induced conversion of the corresponding ochre codon to amber (UAA → UAG) about twofold and to opal (UAA → UGA) about eightfold. The 2-aminopurine-induced reversion of the ochre codon to a glutamine codon (UAA → CAA), however, is not affected. Control experiments demonstrate that these observed reductions in mutation frequency do not result from unacceptable pathways of reversion in the presence of the ts allele.
Chemical Shifts, Ring Currents, and Magnetic Anisotropy in Aromatic Hydrocarbons
Both the chemical shifts of nuclear magnetic resonances and molecular diamagneticsusceptibilities have their origin in the electronic currents induced by an external magnetic field. For the aromatic hydrocarbons detailed chemical‐shift measurements and extensive measurements of the magnetic anisotropies are available to aid in a study of the validity of various theories connecting magnetic properties with magnetic electron currents. The available experimental data seem to present a definite paradox. The chemical shifts due to ring currents are theoretically directly proportional to the values of Δχ, the anisotropy in the diamagneticsusceptibility; yet the experimental values of Δχ are much larger than those calculated from theory, while the experimental chemical shifts are substantially smaller than the theoretical ones. Hoarau has suggested that only a part, χ L , of the total anisotropy Δχ arises from the π electrons involved in the ring current and that one should add Δχσ , a contribution from the electrons in the σ bonds, and Δχ p , the contribution due to the localized p electrons. The SCF—MO calculations of χ L for benzene give a value of approximately —30×10—6, which is only about half the experimentally observed value of —59.7×10—6. Rough calculations are presented which indicate that the contributions of the σ and localized p electrons are large enough to account for the observed values of Δχ and χ z for the aromatic hydrocarbons. An analysis of the chemical shifts of the aromatic hydrocarbons shows that they are in agreement with a Δχ value for benzene of approximately —31×10—6 in good agreement with the SCF—MO value of χ L . Calculations of the chemical shifts due to localized magnetic anisotropies show that they are quite small and can frequently be neglected. In calculating ring current contributions to the chemical shift in NMR spectroscopy it is recommended that the SCF—MO value of χ L , —30×10—6, be used in place of the classical value, —49.5×10—6.