CD1 lacks a 3-Methyl group. This group twists the resulting dye bond out of alignment and serves two purposes.
1. The twist adjusts the hues of the dyes, and the couplers were chosen to optimize the dyes with the twist. Without the twist, the dye hues are just a bit off.
2. The Methy group, combined with the twist blocks attack by oxygen and thus adds dye stability to the final dye that forms.
3. CD-4 is far less likely to cause allergic reactions and is as active as CD1. CD3 and CD6 are also unlikely to cause skin problems, but CD3 is much less reactive. CD6 is about as reactive as CD4.