The fluorescence blob model (FBM) was applied to analyze the fluorescence decays of a series of pyrene-labeled polypeptides to better understand how the amino acid (aa) composition of a polypeptide affected its dynamics. Three pyrene-labeled polypeptides were prepared by copolymerizing racemic (D,L) mixtures of different aa's, namely, glycine (Gly), alanine (Ala), and carbobenzyloxylysine (Lys(Z)) with glutamic acid (Glu) to yield Py-PGlyGlu, Py-PAlaGlu, and Py-PLys(Z)Glu, respectively. All polypeptides contained 44 (+/- 3) mol % Glu for fluorescence labeling with 1-pyrenemethylamine. The behavior of these three polypeptides was characterized in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) by fluorescence. It was then compared with the behavior of pyrene-labeled poly(D,L-glutamic acid) (Py-PGlu) to assess the effect that each comonomer had on the backbone dynamics of the polypeptides. The FBM analysis of the fluorescence decays yielded the maximum number (N-blob) of residues separating two Glu's bearing a pyrenyl group while still allowing excimer formation between an excited and a ground-state pyrene. Py-PLys(Z)Glu yielded the same blob size (N-blob = 11) as for the Py-PGlu samples. In contrast, the incorporation of similar to 56 mol % Ala and Gly resulted in an increase in N-blob from 11 for Py-PGlu and Py-PLys(Z)Glu to 16 for Py-PAlaGlu and 23 for Py-PGlyGlu in DMSO. Considering that the internal dynamics of a polymer depend strongly on the size of its structural units and keeping proline aside, whose cyclic structure prevents backbone motion, this result implied that N-blob for the 17 largest aa's of the 20 most common aa's with two or more atoms in their side chain must take a value between 11 for PGlu and 16 for PAlaGlu. This rather narrow range of N-blob values suggests that the internal dynamics of polypeptides should be much simpler to predict than their structure since only two aa's, namely, Gly and Ala, out of the 20 most common aa's, appear to contribute differently to the internal dynamics of polypeptides.