An expanded, highly dynamic denatured state of
staphylococcal nuclease exhibits a native-like topology
in the apparent absence of tight packing and fixed hydrogen
bonds (Gillespie JR, Shortle D, 1997, J Mol Biol 268:158–169,
170–184). To address the physical basis of the long-range
spatial ordering of this molecule, we probe the effects
of perturbations of the sequence and solution conditions
on the local chain dynamics of a denatured 101-residue
fragment that is missing the first three beta strands.
Structural interactions between chain segments are inferred
from correlated changes in the motional behavior of residues
monitored by 15N NMR relaxation measurements.
Restoration of the sequence corresponding to the first
three beta strands significantly increases the average
order of all chain segments that form the five strand beta
barrel including loops but has no effect on the carboxy
terminal 30 residues. Addition of the denaturing salt sodium
perchlorate enhances ordering over the entire sequence
of this fragment. Analysis of seven different substitution
mutants points to a complex set of interactions between
the hydrophobic segment corresponding to beta strand 5
and the remainder of the chain. General patterns in the
data suggest there is a hierarchy of native-like interactions
that occur transiently in the denatured state and are consistent
with the overall topology of the denatured state ensemble
being determined by many coupled local interactions rather
than a few highly specific long-range interactions.