Peptides isomeric tripeptides

There are thus six possible isomers in a tripeptide that contains three different amino acids. The isomeric tripeptides containing glycine, alanine, and valine are Gly-Ala-Val, Gly-Val-Ala, Val-Ala-Gly, Ala-Gly-Val, and Ala-Val-Gly. For a peptide that contains one each of 20 different amino acids, there are 2,432,902,008,176,640,000 isomers (That is, on the order of 2 X 10 isomers.) Proteins often contain hundreds of amino acid residues, and they contain two or more amino acids of the same kind, but the number of possibilities is still astronomically large. Viewed from an evolutionary standpoint, we can say that nature has just begun to experiment. 

Haas et al.(m) have examined the fluorescence decay of tyrosine due to different Tyr-Pro conformations in small peptides to elucidate further the nature of the fluorescence change associated with Tyr-92. These peptides have acetyl groups at the amino terminus and /V-mcthylamidc groups at the carboxyl terminus. They found that whereas the dipeptide fluorescence decay requires a double-exponential fit, that of the tripeptide Tyr-Pro-Asn can be fit by a single exponential. By comparison of the average fluorescence decay time and steady-state quantum yield of the peptide to that of A-acetyltyrosine-A-methylamide, they found a relatively greater reduction in the steady-state quantum yield of the peptides. This is attributed to static quenching, which increased from 5 % in the dipeptide to 25 % in the tripeptide. The conformations of these peptides were also examined by NMR, but the results could be interpreted in terms of either cis-trans isomerization or other conformational isomerizations. 

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