  We study hairpin folding dynamics by means of extensive computer simulations,
with particular attention paid to the influence of helicity on the folding time
$\tau$. We find that the dynamical exponent $\alpha$ of the anomalous scaling
$\tau \sim N^\alpha$ for a hairpin with length N changes from 1.6 ($1+\nu$) to
1.2 ($2\nu$) in three dimensions, when duplex helicity is removed. The relation
$\alpha = 2\nu$ in rotationless hairpin folding is further verified in two
dimensions ($\nu = 0.75$), and for a ghost-chain ($\nu = 0.5$). This, to our
knowledge, is the first observation of the theoretical lower bound on $\alpha$,
which was predicted earlier on the basis of energy conservation for polymer
translocation through a pore. Our findings suggest that the folding dynamics in
long helical chains is governed by the duplex dynamics, contrasting the earlier
understanding based on the stem-flower picture of unpaired segments. We propose
a scaling argument for $\alpha = 1+\nu$ in helical chains, assuming that duplex
relaxation required for orientational positioning of the next pair of bases is
the rate-limiting process.
