SNGPAELCLKACLCALQALGGKS
lllHHHHHHHHHHHHHHHHllll
-------C123C-C---------

The first two cysteines (labeled C123C) stick out at the same side of the helix, and thus can be bridged. The third C sits at 'the other side', and thus cannot formn a cysteine bridge, and thus can be spinlabeled.

Additionally, the third C sits at the hydrophilic side...