Interstrand proton transfer, which is driven by the large changes in acidity or basicity that result from loss or uptake of an electron from or by a base in a base pair of DNA, can constitute a very powerful stop to the migration of holes or electrons in the direction of the DNA helix via the stacked bases. In theory, such a proton transfer could have the same rate (10(14) s[-1]) as the electron transfer. In base pair model systems, proton transfer rates up to 10(13) s(-1) have been measured. As a result of the existence of proton transfer barriers to electron transfer, the migration distance of the charge carriers in DNA is likely to be short, extending over only < or = 5 bp's (17 angstroms).