NMRsolution structures are reported for two mutants (K16E, K16F) of the soluble amyloid β peptide Aβ(1–28). The structural effects of these mutations of a positively charged residue to anionic and hydrophobic residues at the α-secretase cleavage site (Lys16–Leu17) were examined in the membrane-simulating solvent aqueous SDS micelles. Overall the three-dimensional structures were similar to that for the native Aβ(1–28) sequence in that they contained an unstructured N-terminus and a helical C-terminus. These structural elements are similar to those seen in the corresponding regions of full-length Aβ peptides Aβ(1–40) and Aβ(1–42), showing that the shorter peptides are valid model systems. The K16E mutation, which might be expected to stabilize the macrodipole of the helix, slightly increased the helix length (residues 13–24) relative to the K16F mutation, which shortened the helix to between residues 16 and 24. The observed sequence-dependent control over conformation in this region provides an insight into possible conformational switching roles of mutations in the amyloid precursor protein from which Aβ peptides are derived. In addition, if conformational transitions from helix to random coil to sheet precede aggregation of Aβ peptides in vivo, as they do in vitro, the conformation-inducing effects of mutations at Lys16 may also influence aggregation and fibril formation.