The hetero-association of caffeine with 5-caffeoylquinic acid and ethidium bromide

Abstract

-cinnamic acid with (-)quinic acid)are bioactive compounds naturally available in leaves,seeds and fruits of plant species [1]. The most commonsources of CA are coffee beans, coca beans, cola nutsand tea leaves [2]; CQA is found in potatoes, coffeebeans, tobacco and wine [3]. The molecules of CA caninteract with each other in aqueous media and theseassociations have reported using various techniques suchas UV–vis spectrophotometry, FT-IR and NMR [4–10].The molecules of CA are aggregated in aqueous solutionby plane-to-plane stacking [11–13] and these kind ofstacking interactions are usually important in connexionwith the pharmacological action of caffeine and theinteraction between CA and nucleic acids. CA alsointeracts with polyphenols [14] and aromatic hydroxyacids such as methyl gallate, 3-nitrobenzic acid [15], 5-chlorosalicylic acid [16], pyrogallol [17], catechins [18],theaflavin [19], gallic acid and quercitin [20].The equilibrium constant for the complexation of CAwith the major polyphenolic constitutents of green coffeebeans were initially determined in aqueous solution by thespectrophotometry [21]. By a similar method otherresearchers also reported the complexation of CA and otherpurine alkaloids with chlorogenate compounds [22]. Yetothers described the 1:1 caffeine:chlorogenate complexationusing nuclear magnetic resonance (NMR) [23],determining the association constant and identifying thechlorogenate ion moiety involved in the complexformation. Further, they also proposed that the plane ofthe caffeine molecule is parallel to the plane of thearomatic ring of the caffeoyl ester group and that the five-and six-membered rings of the nitrogen heterocycle areequally involved in the complex formation. Apart from theabove-mentioned studies, recently there were reports onthe complexation of caffeine with 3-caffeoylquinic acidand the caffeine–chlorogenate complex in a freshlyprepared coffee brew by high resolution