Self-assembling peptides play increasingly important roles in the development of novel materials and drug delivery vehicles. Understanding mechanisms governing the assembly of nanoarchitectures is essential for the generation of peptide-based nanodevices. We find that a cone-shaped derivative of the second transmembrane domain of CXCR4 receptor, x4-2-6 self-assembles into nanospheres, while a related cylindrical peptide, x4-2-9 forms fibrils. Stronger intermolecular interactions in nanospheres than in fibrils result in slow rates of particle disassembly and protection against proteolytic degradation.