As a further check on the accuracy of the A-form structure, we compared the principal direction of alignment (S zz ) determined experimentally using RDCs assuming a ssRNA A-form structure with the orientation predicted by PALES (71) using a ssRNA A-form structure. Surprisingly, we find that the experimentally determined S zz deviates from the helix axis by $19.8 ( Figure 3C ). Interestingly, PALES predicts a principal direction of order that deviates from the helix axis by 14.4 ; the S zz orientation predicted using PALES is in good agreement from that measured experimentally (deviation $ 5 ). The deviation from the helix axis can be attributed to the absence of the complementary strand, resulting in an overall shape with a long axis that is not coincident with the helical axis, as reported previously for a quadruplex DNA topology (72) .