Re: Pepsyn

John Stewart (John.Stewart@UCHSC.edu)
Fri, 13 Mar 1998 09:06:27 -0700 (MST)

Frank:
I'll add my grey-haired two cents to the colorful discussion of
colored peptides. The color is most likely due to acid-catalyzed
condensation of trifluoroacetaldehyde with the Trp residue (couples at
position 2 of the indole ring) in your peptide. The decomposition of TFA
to the aldehyde is reported to be spontaneous.
When we use TFA for deprotection of Boc groups in SPPS, we store
the TFA at least overnight with indole (1 mg/ml) before using the reagent.
The solution turns a purple color, the intensity depending on the amount
of aldehyde in the TFA. The purple reagent is used as is. The indole
scavenges the aldehyde from the TFA so it cannot react with the Trp in
your peptide. This is discussed in Stewart and Young, SPPS, 2nd edition
1984, pages 28 and 68.

John M. Stewart, Department of Biochemistry
Univ. of Colorado Medical School, Denver, CO 80262
Phone: 303-315-7534; FAX: 303-315-8215
Email: John.Stewart@UCHSC.edu

On Wed, 11 Mar 1998, Frank wrote:

> Dear ABRF
> I have a question concerning an unusual peptide product. I
> recently made a peptide amide (Rink amide MBHA resin) - sequence:
> QQYNNWPP which upon cleavage with TFA (+ water, phenol,
> triisopropylsilane scavengers) gave a rather dark colour. After the addition of
> cold ether the peptide product went a marvellous shade of bright purple.
> Following dissolution in water the product changed colour once again (a rose
> red colour). I know that colour changes can occur with certain difficult
> peptide sequences but I wondered whether anyone knows the precise reason
> for this colour change. The trp and pro residues were unprotected, the gln and
> asn residues protected with trt, and the tyr residue protected with tBu. I
> suspect that during the cleavage step one of these sidechain protecting groups
> has bound the trp sidechain but I'd like to know more.
> Best wishes
>
> Frank Ward
> King's college London
> Kensington
> UK
>
>