Sequence-specific DNA-binding proteins, such as transcription factors, predominantly interact with the edges of base pairs exposed in the major groove of B-form DNA rather than the minor groove. What is the principal chemical reason for this preference? Answer: The pattern of hydrogen bond donors, acceptors, and nonpolar groups presented by base pairs is more distinct and information-rich in the major groove.
While the major groove is wider than the minor groove (a contributing factor mentioned in A), the primary reason for the preference is the greater 'information content'. Each of the four possible base pairs (A-T, T-A, G-C, C-G) presents a unique pattern of chemical groups (hydrogen bond donors, hydrogen bond acceptors, methyl groups, etc.) in the major groove. This allows proteins to unambiguously distinguish between them. In the minor groove, the patterns are more degenerate; for example, A-T and T-A pairs present a very similar set of chemical groups, as do G-C and C-G pairs, making it much harder to read the sequence specifically. The charge on the backbone is uniform (B), and while flexibility exists, it is not the main driver of this specificity (D).
