  Nonaromatic amino acids are generally believed to be nonemissive, owing to
their lack of apparently remarkable conjugation within individual molecules.
Here we report the intrinsic visible emission of nonaromatic amino acids and
poly(amino acids) in concentrated solutions and solid powders. This unique and
widespread luminescent characteristic can be well rationalized by the
clustering-triggered emission (CTE) mechanism, namely the clustering of
nonconventional chromophores (i.e. amino, carbonyl, and hydroxyl) and
subsequent electron cloud overlap with simultaneously conformation
rigidification. Such CTE mechanism is further supported by the single crystal
structure analysis. Besides prompt fluorescence, room temperature
phosphorescence (RTP) are also detected from the solids. Moreover, persistent
RTP is observed in the powders of exampled poly(amino acid) of
{\epsilon}-poly-L-lysine ({\epsilon}-PLL) after ceasing UV irradiation. These
results not only illustrate the feasibility of employing the building blocks of
nonaromatic amino acids in the exploration of new luminescent biomolecules, but
also provide significant implications for the RTP of peptides and proteins at
aggregated or crystalline states.
