||Claassens NJ, Volpers M, dos Santos VA, van der Oost J, de Vos WM. Potential of proton-pumping rhodopsins: engineering photosystems into microorganisms. Trends Biotechnol. 2013 Nov31(11):633-42. doi: 10.1016/j.tibtech.2013.08.006. p.638 left column 2nd paragraphPubMed ID24120288
||Kashket, E.R. (1985) The proton motive force in bacteria: a critical assessment of methods. Annu. Rev. Microbiol. 39, 219–242PubMed ID2998266
||p.638 left column 2nd paragraph:"Photons of longer wavelengths still have an energy content that is higher than the thermodynamic limit to pump protons. For example, photons of 1100 nm have an energy content of 1.270 eV the thermodynamic limit to pump protons is 0.200 eV or lower, because the PMF in most bacteria is <0.200 V [primary source]. Hence, proton pumping by absorbing photons from longer wavelengths seems thermo-dynamically feasible for PPRs [proton-pumping rhodopsins]. However, absorption of these wavelengths by PPRs has not yet been found in nature. If PPRs using these wavelengths exist in nature, they will most likely be found in terrestrial environments."