To investigate potential structures of d(CGG/CCG)n that might relate to their biological function and association with triplet repeat expansion diseases (TREDs), the structure of a single-stranded (ss) oligonucleotide containing d(CCG)15 [ss(CCG)15] was examined by studies of the pH and temperature dependence of electrophoretic mobility, UV absorbance, circular dichroism, chemical modification, and P1 nuclease digestion. ss(CCG)15 had an unusually high pKa (7.7 +/- 0.2). At pH 8.5, ss(CCG)15 formed a relatively unstable (Tm = 30 degrees C in 1 mM Na+) hairpin containing CpG base-pair steps. At pH 7.5, the hairpin contained protonated cytosines but no detectable C x +C base pairs, increased thermal stability (Tm = 37 degrees C), increased stacking of the CpG base-pair steps, and a single cytosine that was flipped away from the central portion of the helix. Examination of ss(CCG)18 and ss(CCG)20, which were designed to adopt hairpins containing alternative GpC base-pair steps, revealed hairpins containing CpG base-pair steps, pKas of approximately 8.2 and approximately 8.4, respectively, and distorted helices. The results suggest that DNA sequences containing (CCG)(n > or = 15) adopt hairpin conformations that contain CpG rather than GpC base-pair steps; the mismatched cytosines are protonated at physiological pH but are not H-bonded. We propose that protonation arises from the stacking of two cytosines in the minor groove of a distorted helix.

Objectives. To assess the effectiveness of assisted problem-based learning (PBL) compared to a didactic approach in a pharmaceutics course. Design. Data were collected over 7 offerings of the course. In the first half of the semester about half of the students (PBL1) learned in a PBL format while the other students (PBL2) received didactic lectures. In the second half of the semester, the teaching methods were reversed. Assessment. Performance on the midterm examination and a comprehensive final examination was used to assess the effect of PBL. Over the 7-year period, PBL1 students scored significantly higher on the midterm examinations. Scores on the final examinations did not differ significantly, but PBL2 students had a higher mean score on questions based on material from the second half of the semester. Conclusions. PBL produced a short-term (weeks) improvement in learning and our results suggest that the effect may persist in the medium term (months).

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