. 66, 836 (1988). Temperature-dependent uv spectroscopy has been used to determine the differential salt effect of tetramethylammonium chloride (Me4NC1) and sodium chloride (NaCl) on the thermal stabilities of a series of deoxyoligonucleotide duplexes that sy'stematically differ in base composition and sequence. This approach has allowed us to resolve the AT-specific effect of Me4N+ from the more general, non-specific cation effects of Me4Nf versus Na'. Our results reveal that in the absence of AT-specific effects, Me4N' (relative to Na') significantly reduces the thermal stability of duplex structures. We offer an interpretation of this differential salt effect in terms of the influence of the condensed cation on water structure at the surface of the duplex. We also observe an AT-specific effect of Me4N+ which results in an increased thermal stability for AT-containing duplexes in Me4NC1 relative to NaC1. This specific effect which is governed by mass action is found to be uninfluenced by base sequence and to be a linear function of the percent AT base pairs in the duplex.Luis A. MARKY, KENNETH S. BLUMENFELD et KENNETH J. BRESLAUER. Can. J. Chem. 66, 836 (1988). On a utilise la spectroscopie uv a tempkrature variable pour determiner l'effet diffkrentiel de sel des chlorures de tetramCthylammonium (Me4NC1) et de sodium (NaCl) sur les stabilites thermiques d'une skrie de duplex de d~soxyoligonuclko-tides qui diffkrent d'une f a~o n systematique dans la composition de la base ainsi que dans la skquence. Cette approche nous a permis de differencier entre l'effet spkcifique au AT du cation Me4N+ des effets cationiques plus gknkral mais non specifiques du Me4N' vs Na'. Nos resultats revklent que, en l'absence d'effets spkcifiques au AT, le Me4N' (par rapportau Na') reduit d'une f a~o n importante la stabilite thermique des structures des duplex. On offre une interprktation de cet effet differentiel de sel en fonction de l'influence du cation condense sur la structure de l'eau la surface du duplex. On a aussi observe un effet du Me4N+ qui est specifique au AT et qui provient d'une augmentation, dans le Me4NC1 par rapport au NaCL, de la stabilitk thermique des duplex contenant du AT. On a trouve que ces effet specifique, qui est soumis a la loi d'action de masse, n'est pas influence par la sequence de la base et qu'il est une fonction liniaire du pourcentage des paires de bases AT dans le duplex.[Traduit par la revue] Introduction Experimental The effect of added salt on the stability of helical polynucleotides is well documented and generally depends on the nature of the cationic species (1-4). Cation: can exert their stabilizing or destabilizing influences through specific and/or non-specific interactions with the polymer. Examples of non-specific interactions include general electrostatic effects (e.g. cation condensation and screening) as well as the resulting perturbation of local water structure surrounding the polymer. Essentially all cations in aqueous solution manifest such non-specific effects to varying degr...