Negative emission technologies, including direct air capture (DAC) of carbon dioxide, are now considered essential for mitigating climate change, but existing DAC processes tend to have excessively high energy requirements, mostly associated with sorbent regeneration. Here, we demonstrate a promising approach to DAC that combines atmospheric CO2 absorption by an aqueous oligopeptide (e.g., glycylglycine) with bicarbonate crystallization by a simple guanidine compound (e.g., glyoxal-bis-iminoguanidine). In this phase-changing system, the peptide and the guanidine compounds work in synergy, and the cyclic CO2 capacity can be maximized by matching the pKa values of the two components. Compared with glycine, the simpler amino acid congener, the cyclic CO2 capacity of the glycylglycine peptide combined with glyoxal-bis-iminoguanidine is twice as high (0.16 mol/mol). The resulting DAC process has a significantly lower regeneration energy compared with state-of-the-art solvent-based DAC technologies.