Single substitutions closer to the N terminus (residues 14-18) did not significantly disrupt the helical motif, although some mutations (V14G, V14D, T16G) increased positional fluctuations of the extended segment (Figure ). Modifications of I19 that decreased the size of the side chain (I19G, I19A) significantly decreased helical propensity, and significantly increased positional fluctuations. This was explained by the loss of non-polar contacts with residues T16, L54, and Y100. Interestingly, replacement of I19 by E19 was tolerated as the helical motif was stabilized by compensating interactions between E19 and R97.