We synthesized analogues of spermine and studied the effects of chemical structure, ionic strength, and temperature on lambda-DNA nanoparticle formation. Effective concentration of polyamines for DNA condensation (EC50) was lowest for hexamines (0.2 mu M) and highest for spermine (tetramine, 4.2 mu M). The EC50 value increased with [Na+]. Dynamic light scattering showed nanoparticles with hydrodynamic radii (R-h) of 4050 nm. Effect of temperature on R-h was measured between 20 and 70 degrees C. For spermine, R-h remained relatively stable until 50 degrees C and increased significantly at > 60 degrees C. In contrast, the hexa- and penta-valent analogues exhibited a gradual increase in R-h, between 20 and 70 degrees C. The nanoparticles were mainly toroidal, as revealed by electron microscopy (EM). EM studies showed changes in morphology and size of condensed structures with an increase in temperature. A possible mechanism for the differential effects of temperature on DNA nanoparticles might involve different modes of DNA-polyamine interactions.