Fluorescence and quantum mechanical approach on the interaction of amides and their role on the stability and coexistence of the rotamer conformations of L-tryptophan in aqueous solution.

Photophysical investigation on the fluorescence decay characteristics of L-tryptophan and a derivative N-acetyl-L-tryptophanamide (NATA) with alkyl amides were carried out in water. L-tryptophan exists in the zwitterionic form and exhibits a biexponential lifetime which is correlated to the existence of rotamer structures. Addition of formamide (F) and dimethylformamide (DMF) results in a decrease in the fluorescence lifetime and its proportion of the most stable structure of L-tryptophan wherein acetamide (ACM) results in an increase of the same. Interestingly, all the amides result in the formation of the lifetime of the rotamer whose lifetime doesn't exist initially and the lifetime and its distribution increases irrespective of the nature of amide. The interaction between L-tryptophan and amide is attributed to hydrogen-bonding such that these interactions influence the relative proportion of the existence of individual rotamers in the presence of amides.Strikingly, in the case of NATA that does not exhibit rotamer structures; the fluorescence lifetime is quenched in the presence of F, whereas ACM and DMF result in a larger fold of enhancement resulting in two different lifetimes. The variation in the fluorescence lifetime and amplitude of the various conformers of L-tryptophan and of NATA is completely governed by the concentration of the amides in solution such that the microenvironment surrounding the fluorophores are completely reorganised. The hydrogen-bonding functional groups in amides that are responsible for the coexistence of rotamers are elucidated and well supported by quantum mechanical (QM) studies. Time-correlated single-photon counting(TCSPC) technique is used as a probe as well as marker in establishing the variation in the lifetime properties of L-tryptophan and NATA with non-fluorescent hydrogen-bonding solutes in water which promotes this as fascinating field of research in the context of fluorescence properties of a complicated amino acid-like tryptophan.