ABRF Electronic Discussion Group Archives viewed: pepsyn {post-cys couplings}

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Greetings, ABRFers;
I have been using a pioneer peptide synthesizer, doing FMOC peptide =
synthesis, with PEG-PAL-PS resins for amide synthesis. Many of the =
oligopeptides I make contain a C-terminal Cys, and I have noticed a =
consistent drop in absorbance after the Cys. I am using HATU, and =
standard cycles, with FMOC monitor on and extended coupling/deprotection =
if greater than 5% slope in deprotection. Is there a better coupling =
protocol? Also, I notice that the absorbance from FMOC release after =
deprotection of the PEG-PAL-PS resin is always much less than for any =
following deprotections. In theory, shouldn't the absorbances be at =
least equivalent? The resin starts out with the maximum number if FMOC =
groups available, and with less than perfect coupling the absolute =
number of FMOC groups released should decrease with increasing cycles. =
Instead, I see a 3-4X increase in absorbance. I assume this is due to =
non-specific binding of HATU to the resin, and tried the recommended =
neat DIPEA wash with little success. Any ideas?

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I have been using a pioneer peptide = synthesizer,=20 doing FMOC peptide synthesis, with PEG-PAL-PS resins for amide = synthesis. Many=20 of the oligopeptides I make contain a C-terminal Cys, and I have noticed = a=20 consistent drop in absorbance after the Cys. I am using HATU, and = standard=20 cycles, with FMOC monitor on and extended coupling/deprotection if = greater than=20 5% slope in deprotection. Is there a better coupling protocol? Also, I = notice=20 that the absorbance from FMOC release after deprotection of the = PEG-PAL-PS resin=20 is always much less than for any following deprotections. In theory, = shouldn't=20 the absorbances be at least equivalent? The resin starts out with the = maximum=20 number if FMOC groups available, and with less than perfect coupling the = absolute number of FMOC groups released should decrease with increasing = cycles.=20 Instead, I see a 3-4X increase in absorbance. I assume this is due to=20 non-specific binding of HATU to the resin, and tried the recommended = neat DIPEA=20 wash with little success. Any ideas?