I dare to add a few 'Russsian copekes' to the discussion on your difficult
SPPS (:-))).
1. From the fact that the problem coupling shifted from His to Asn during
second SPPS I would rather suspect your synthesizer malfunction (valve,
tubing or else). The suggestion that you are dealing with difficult
aggregating sequence seems unlikely for the following reasons.
Firstly, we have found that resin architecture does not influence the
point of aggregation onset - at least in the oligo-Ala model. You may see
it from the picture "ps-tg.pcx" which you will find in the VIRUS FREE
attachement to my next message ( four .pcx-files packed in the
pcxfiles.zip /Windows 95). TentaGel simply alleviates resin collapse.
Secondly, both H-> R A A K and N ->A H R A A K should not be truly
difficult since protected R and K exhibit only moderate aggregation
propensities while His(Trt) is a good 'aggregation breaker'. You may see
this from the figure 'aggrscal.pcx' which shows the aa aggregation scale
derived from swelling data recorded during SPPS of host-guest
Fmoc-(Ala)n-X-AlaAlaAlaNle-polystyrenes (for more dertails see aggrdegr.pcx
and 5lines.pcx).
Similar qualitative scale has been obtained earlier by Sheppard et al from
UV monitoring data. Slightly different aggregation propensities were
calculated by Krchnack et al via statistical approach to swelling data.
2. All chatropic inorganic salts are safe and useful when employed in
Boc-SPPS as was originally suggested by John Stewart. However, perhaps
almost all of them are mild Lewis acids and their usage in combination with
rather subtle side chain protecting and spacer groups in Fmoc chemistry
may need careful re-investigation. Even trifluorethanol may cause premature
PG cleavage (e.g. His(Trt) and the like), in the presence of HOBt/HOAt in
particular.
3. The proposal to change from NMP to DCM in case of visible resin collapse
sounds strange to me, since we routinely record peptide-resin swelling
behaviour over the last 10 years during all SPPS we undertake and
polystyrene-based resins shows invariably superior swelling in NMP as
compared to DCM. However this may not be absolutely true for TentaGels and
other PEG-resins, which show considerably greater swelling in DCM than in
DMF, DMA and NMP.
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