To obtain Φ-values for core residues, and residues within elements of secondary structure, 64 mutations were made at 42 positions throughout the six helices: 23 of these were mutations of core residues, and 19 were residues where alanine to glycine “scanning” was performed, to report on the extent of secondary structure formation.23,24 Of the 23 core residues, most were non-disruptive replacements by alanine; however, in four cases this mutation was too destabilising and so non-conservative substitutions were made (Leu to Met and Trp to Phe). The residues to which the contacts are deleted and the number of heavy-atom side-chain contacts within 6 Å deleted on mutation are given in Table 1, which illustrates that truncation of core residues either reports mainly on contacts within one of the bundle cores or probes contacts in the central core. Thus, for example, in H4, R140 contacts only residues within B2, whereas L145 only makes long-range contacts with residues in H1 (in B1), through the central core.