intra-helical disulfide bonding?

[:-Peter]

I'm curious about the possibility that two cys residues separated by four
intervening amino acids in a putative alpha helix ( in a transmembrane
domain) may be covalently linked. 

1. Are the SH groups likely to be in a favorable conformation for this to
occur, or might they be linked to other TM domains.

2. Are cys residues in TM domains always part of disulfide-linked pairs?

Pointers to modelling sites on the web, references, or advice will be
greatly appreciated!

:-Peter