The Bacteriostatic Action of Basic Di- and Triphenylmethane Derivatives Part I. The Inhibition of Bacteriostatic Action by Inactive Compounds of Similar Structure

Abstract

DURING our earlier experimental work on the bacteriostatic activity of certain di-and triphenylmethane dyes1~2~3~4 we have observed some facts suggesting that, among methane carbon atom derivatives of these dyes, only those with a "potential quinoid structure" were able to inhibit the growth of Staphylococci and Streptococci. We have noted112 that the leucobases (R3CH), the leucoamines (R,C.NHa and the leyocyanides (R,CCN) of basic triphenylmethane dyes were practically inactive,while the corresponding dye cation (R,C = R), carbinol bases (R,C.OH), leucosulphonic (bisulphite) and leucosulphinic (hydrosulphite) derivatives (R,C.SO,Na, R,C.SO,Na) exerted a strong bacteriostatic effect of equal degree. It is a well known fact that carbinol bases possess a tautomeric quinoid ionised form whose cation appears in their dye salts. Bisulphite and hydrosulphite derivatives, for their part, are unstable compounds, transformed more or less rapidly, in contact with air, into coloured dye salts (vat dyes).Still more conclusive have been the results of experiments with diphenylmethaneThe bacteriostatic activity of these is far weaker than that of the triphenylmethane homologues, but the stability of their methane carbon atom derivatives allows a more clear cut separation between "essentially non-quinoid" and "potentially quinoid" structures. The non-quinoid compounds R,CH, (bis [dimethylaminophenyl] methane) and R,CH.NH, (leuco-auramine) were practically negative, whereas both the compounds R,CHOH (the "potentially quinoid" Michler's hydrol) and R-C(NH,)=R (the quinoid auramine dye) showed bacteriostatic effects of the same degree. The sulphonic derivative (R,CH.SO,H), which is, contrasting with its unstable triphenylmethane homologues, a stable nonquinoid compound, did not exert, correspondingly, any noticeable bacteriostatic activity. The non-quinoid Michler's ketone (R,C = 0) was also inactive.The most important common feature of active di-and triphenylmethane derivatives, which distinguish them from inactive compounds of similar structure, is constituted in our opinion by their "potentially quinoid" character. An actually quinoid structure, however, cannot be looked upon as a requisite for the bacteriostatic action, since quinoid dye cations and actually non-quinoid carbinol bases have a quantitatively equal effect. Especially noteworthy is, from this point of view, the similarity of the bacteriostatic action of Michler's hydrol, non-quinoid at the pH of the bacteriological medium (7.0) and that of auramine, quinoid cation at the 262