Amino acids of the HA1 sequence involved in changes of charge state when comparing 1968, 1985 and 2000.

‘Group 2’ amino acids, as defined in the main text, are shown in italics on the diagonal – all three exhibited an increase in charge of +1 in 1985, followed by a reversion before 2000. All other amino acids belong to ‘group 1’. Note that those elements below the diagonal contribute to a positive overall charge, and vice versa for those above the diagonal.