Self-association of caffeine in aqueous solution. Study of dilute solutions by normal and second derivative UV absorption spectroscopy.

The concentration dependence of the spectral parameters of caffeine bands at ∼205 and 273 nm has been studied in aqueous solution by normal and second derivative spectroscopy. The concentration range was 5 x 10⁻⁶ − 5 x 10⁻³ M and thirty-five different concentrations were used.Discontinuities in parameter variation of these two bands at ∼7.5 x 10⁻⁵, ∼2 x 10⁻⁴, and ∼1 x 10⁻³M were observed as concentration was increased. These “limiting” concentrations define three quite differenciated hyper- or hipochromic effects: the first one can be explained as caffeine-water molecule interaction and the second and third as dimer and (dimer + polymer) stacking, respectively. Apparent self-association constants using the isodesmic model have been obtained K= 160 M⁻¹ (for the second hypochromic effect) and K= 13.6 M⁻¹ (for the third hypochromic effect), for the 273 nm band.It is noteworthy that the three “limiting” concentrations coincide with changes in DNA-caffeine interaction modes (H. Lang , 1976) and biological activity (I.B. Syed , 1976).