Herein we report the synthesis and structural analysis of tunable 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)derivatives bearing azidoethyl side chains. According to the number of azidoethyl side chains, the compounds are termed DOTXAZA (with X = 1-4). All derivatives retain the wellapproved DOTA scaffold for metal complexation with 8 coordinating functionalities and are applicable to Cu-catalyzed alkyne-azide cycloaddition (CuAAC). They may thus be used to assemble new Gd-based contrast agents (GBCAs) with tailored pharmacokinetic or MRI properties retaining the high stability of clinically used DOTA-derived GBCAs. The complex geometry of all DOTXAZA derivatives was investigated in solution with NMR-spectroscopy of the corresponding Eu-complexes. A correlation of complex geometry with the longitudinal relaxivity of corresponding Gd-complexes revealed an increase in relaxivity with increasing substitution number in the DOTXAZA series. Moreover, CuAAC-modified DOTAZA-derivatives with neutral, charged, and zwitterionic groups were prepared. Although of similar molecular mass, the corresponding Gdcomplexes revealed pronounced differences in relaxivities from r 1 = 3.9-7.9 mm À 1 s À 1 in water at 1.5 T and 37 °C. Relaxivities of these complexes were found to be strongly dependent on side chain charge and are most likely determined by second-sphere water effects.