A "Force Buffer" Protecting Immunoglobulin Titin**

), we show that the force ofapproximately 95 pN at which the I27 unfolding intermediateformsislargelyindependentoftheforceloadingrate(appliedforce versus time). Thus, stressing forces !95 pN hardlylower the energy barrier separating folded from intermediatestate. Because I27 must unfold via this intermediate this forceloadingrateindependencyprotectsI27 fromunfoldingatlow,physiologically relevant, stressing forces. Moreover, thecombined contributions of all Igs within the distal region ofthe I-band may constitute a “force buffer” that protects titinfrom mechanical stress.We applied SMFS to examine the unfolding of I27 inquasi-natural conditions. I27