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To study the effect of sequence on DNA structure, the two decamer crystal structures one alternating, d(GTACGCGTAC), and the other non-alternating, d(GGCCGCGGCC), were solved. Crystals of both decamers belong to the hexagonal space group P6 1 22, with one strand in the asymmetric unit. The unit cell constants of the alternating decamer are a = b = 39.26 A, c = 77.70 A. The structure was refined with 1,828 reflections from 8.0 to 2.0 A resolution to an R value of 21.3% with all DNA atoms and 63 water molecules. The isomorphous non-alternating decamer had unit cell dimensions of a = b = 39.05 A, c = 82.15 A. The structure was refined with 2,423 reflections from 8.0 to 2.0 A resolution to a final R value of 22.2% for all DNA atoms and 65 water molecules. Although the average helical parameters of the decamers are typical of A-DNAs, there are some minor differences between them. The helical twist, rise, x-displacement, inclination and roll alternate in the alternating decamer, but do not in the non-alternating decamer. The backbone conformations in both structures show some differences; the residue G(7) of the alternating decamer is trans for a and ywhile the trans conformations are observed at the residue G(8) of the non-alternating decamer.

Ground and low-lying electronic states of Cr atoms in the gas phase were generated from photolysis of Cr(CO) 6 vapor in He or Ar using an unfocussed weak UV laser pulse and their reactions with O 2 and N2O were studied. When 0.5-1.0 Torr of Cr(CO)6 /O2 /He or Ar mixtures were photolyzed using 295-300 nm laser pulses, broadband chemiluminescence peaked at ~420 and ~500 nm, respectively, was observed in addition to the atomic emissions from z 7 P o , z 5 P o , and y 7 P o states of Cr atoms. When N2O was used instead of O2, no chemiluminescence was observed. The chemiluminescence intensities as well as the LIF intensities for those three low-lying electronic states (a 7 S3, a 5 S2 and a 5 DJ) showed second-order dependence on the photolysis laser power. Also, the chemiluminescence intensities were first-order in O 2 pressure, but the presence of excess Ar showed a strong inhibition effect on them. Based on the experimental results, the chemiluminecent species in this work is attributed to CrO 2* generated from hot ground state Cr atoms with O 2. The apparent radiative lifetimes of the chemiluminescent species and collisional quenching rate constants by O 2 and Ar also were investigated.