C-DNA also known as C form DNA. It is one of the many possible double helical structures of DNA. This form of DNA can be observed at some conditions such as relatively low humidity and the presence of certain ions, such as Li+ or Mg2+.
Recent research suggests that both C-DNA and B-DNA consist of two distinct nucleotide conformations, B-I and B-II. The ratio of B-II conformation in C-DNA is more than 40%. However, the ratio of B-II conformation in B-DNA is only about 10%.
L van Dam, M H Levitt (2000). "BII nucleotides in the B and C forms of natural-sequence polymeric DNA: A new model for the C form of DNA". J Mol Biol304 (4): 541–61. doi:10.1006/jmbi.2000.4194. PMID11099379.