Probing the kinetic cooperativity of beta-sheet folding perpendicular to the strand direction.

In an attempt to determine how the folding dynamics of multistranded beta-sheets vary with the strand number, we have studied the temperature-induced relaxation kinetics of a four-stranded beta-sheet, DPDPDP. Our results show that the thermally induced relaxation of DPDPDP occurs on the nanosecond time scale; however, a comparison of the current results with those obtained on a sequence-related, three-stranded beta-sheet suggests that increasing the strand number from three to four increases the folding free energy barrier by a minimum of 0.8 kcal/mol, depending on the folding mechanism. Therefore, these results together suggest that the relaxation kinetics of DPDPDP can be analyzed according to a two-state model even though its folding may actually involve parallel (but degenerate or nearly degenerate) kinetic pathways. The apparent, two-state folding time of DPDPDP is determined to be approximately 0.44 micros at the thermal melting temperature, which makes it one of the fastest folders known to date.