I'm afraid that there's no easy way to lock Pro in a cis-conformation.
The two options you may consider are
i) to make an appropriately designed cyclopeptide and
ii) to incorporate conformationally rigid cis-Pro-mimetic, [e.g.
Curran, T.P., & McEnaney, P.M. (1995). A short synthesis of bicyclic
dipeptides corresponding to Xxx-L-Pro and Xxx-D-Pro having constrained
cis-proline amides. Tetrahedron Lett, 36(2), 191-194]. I recall at
least dozen of synthetic reports on this subject that appeared in the
last decade. At the moment some of this derivatives must be available
commercially.
You may be sure that these tricks will yield cis isomer only. BUT in
case of a 'very simple peptide' you will face the problem how 'to
filter off' the impact of structural modifications which you'll bargain
in this way. Most likely you will not be able to draw any clear-cut
answer from the results.
Regards
Igor
------------------------------------------------
Igor L. Rodionov
Laboratory of Peptide Chemistry
Branch of Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry
Science Avenue 6
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
http://www.fibkh.serpukhov.su/