Your second explanation most likely is true. I mean intra-molecular S-->N
shift of SO3.
Consider the following:
1. In strong acids RS-SO3-salts are known to undergo S-S bond cleavage
which give rise to SO3.
2. SO3 easily reacts with amines (in strong acids) producing sulfamic acids
RNHSO3H. The acid stability of N-S bond in these is similar to that of
peptide bond itself. (I recall the paper from G. Jung's lab which describes
isolation of peptides sulfated at guanidino function [---Arg(SO3-)---] as
final cleavage by-products).
3. The intra-molecular S-->N migration of SO3 should proceed via 5-membered
cyclic intermediate which may account for specific termination of Cys.
Otherwise, abnormal levels of termination are to be expected during all
pre-Cys steps.
Kindest regards.
Igor
------------------------------------------------
Igor L. Rodionov
Laboratory of Peptide Chemistry
Branch of Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry
Pushchino, Moscow Region
142292, Russian Federation
E-mail: <rodionov@fibkh.serpukhov.su> or <Igor_Rodionov@iname.com>
Fax: (0967)-79-05-27
Phones: (0967) 73 54 42; private: (0967) 73 58 26