ABSTRACT
High-precision 1H(e,e'p)pi(0) measurements at Q2 = 0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma*N-->Delta transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I = 3/2) amplitude ratios, R(SM) = (-6.5+/-0.2(stat+sys)+/-2.5(mod))% and R(EM) = (-2.1+/-0.2(stat+sys)+/-2.0(mod))%, are dominated by model error. Previous R(SM) and R(EM) results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.
ABSTRACT
The parity-violating longitudinal analyzing power, A(z), has been measured in pvectorp elastic scattering at an incident proton energy of 221 MeV. The result obtained is A(z) = [0.84+/-0.29(stat)+/-0.17(syst)]x10(-7). This experiment is unique in that it selects a single parity violating transition amplitude (3P2 - 1D2) and consequently directly constrains the weak meson-nucleon coupling constant h(pp)(rho). When this result is taken together with the existing pvectorp parity violation data, the weak meson-nucleon coupling constants h(pp)(rho) and h(pp)(omega) can, for the first time, both be determined.
ABSTRACT
We report the 2.4 A resolution X-ray structure of a complex in which a small molecule flips a base out of a DNA helical stack. The small molecule is a metalloporphyrin, CuTMPyP4 [copper(II) meso-tetra(N-methyl-4-pyridyl)porphyrin], and the DNA is a hexamer duplex, [d(CGATCG)]2. The porphyrin system, with the copper atom near the helical axis, is located within the helical stack. The porphyrin binds by normal intercalation between the C and G of 5' TCG 3' and by extruding the C of 5' CGA 3'. The DNA forms a distorted right-handed helix with only four normal cross-strand Watson-Crick base pairs. Two pyridyl rings are located in each groove of the DNA. The complex appears to be extensively stabilized by electrostatic interactions between positively charged nitrogen atoms of the pyridyl rings and negatively charged phosphate oxygen atoms of the DNA. Favorable electrostatic interactions appear to draw the porphyrin into the duplex interior, offsetting unfavorable steric clashes between the pyridyl rings and the DNA backbone. These pyridyl-backbone clashes extend the DNA along its axis and preclude formation of van der Waals stacking contacts in the interior of the complex. Stacking contacts are the primary contributor to stability of DNA. The unusual lack of van der Waals stacking contacts in the porphyrin complex destabilizes the DNA duplex and decreases the energetic cost of local melting. Thus extrusion of a base appears to be facilitated by pyridyl-DNA steric clashes.
