Additives have a multifaceted impact on nucleation and crystal growth mechanisms, which still require further charting.

This contribution maps out the infl uence of a typical organic amino acid additive, L-phenylalanine, on the crystallization of

a model amino acid, L-glutamic acid. The nucleation and growth processes are strongly modulated under the eff ect of this

additive, which ultimately signifi cantly infl uences polymorphic selection, phase transformation, shape, size, and size distribution

of the crystalline products. A mixture of α-glu and β-glu nuclei is formed at low additive concentrations, while only

α-glu nuclei are found at high additive concentrations. Thus, this additive molecule preferably induced α-glu conformation,

thereby triggering the nucleation of α-glu instead of β-glu, which resulted in polymorphic selection of α-glu. Moreover, this

additive favorably adsorbs on the facets of α-glu crystals, thereby inhibiting heterogeneous nucleation of β-glu, resulting

in impeding a critical phase transformation pathway of α-glu to β-glu. This additive also remarkably impacted crystalline

products’ shape, size, and size distribution.