Novel Absorption and Fluorescence Characteristics of l-Lysine

Abstract

L-Lysine displays new absorption and fluorescence features at high concentrations (~ 0.5 M) in aqueous medium. A new absorption peak was detected around ~ 270 nm (ε = 0.34 M -1 cm -1 ). Blue fluorescence (~ 435 nm) was visible on excitation at 355 nm. The above features which were concentration dependent are attributed to likely aggregates of L-lysine. Several peptides and many proteins are known to form aggregates in the solution phase. 1 The presence of protein aggregates (amyloid) is known to be associated with diseases like Alzheimer's. 2 Consequently investigations of these com- plexes have acquired importance in the recent years. Among the amino acids, lysine has been identified as a key residue causing protein aggregation. 3 Several reports on crystalline aggregates involving lysine have also appeared. 4 However, to the best of our knowledge, there are no reports of amino acid lysine forming aggregates in the fluid phase. In this work, we report our observations on L-lysine using absorption and steady state fluorescence techniques, which indi- cate that lysine is likely to be aggregated in the aqueous phase. Figure 1 shows the absorption spectrum of L-lysine monohy- drochloride (henceforth referred to as lysine) at pH 7.0 5 for dif- ferent concentrations from 0.1 to 1.0 M. Starting from 0.1 M, we observe a gradual emergence of absorption with increasing con- centration. A significant shoulder at ~ 270 nm is noticeable at 1.0 M. The figure also displays the absorption recorded for 0.5 M glycine under the same condition. Clearly lysine at 0.5 M has appreciably more (about five fold higher at 270 nm) absorbance than glycine. Thus the side chain of lysine has a definite role to play in the absorption highlighted above and importantly the α− carboxyl and α -amino groups of lysine are much less involved. The inset in Figure 1 shows the absorbance of lysine at 270 nm plotted against concentration. A linear dependence is observed. From the slope the molar extinction coefficient was calculated to be 0.34 M -1 cm -1 at 270 nm. Both the wavelength of absorption and the extinction coefficient are difficult to explain in terms of known electronic transitions expected from lysine side chain. An interaction between individual lysine molecules seems likely as origin for the observations above. Figure 2 shows the fluorescence emission spectrum of lysine at different concentrations in aqueous medium at pH 7.0. 5 On excitation at 290 nm 6 with low concentrations (∼ 0.01 M) of lysine, we observe tiny fluorescence peaks at ~ 321 nm and ~ 334 nm. Importantly, negligible emission, if any, is observed around ~ 420 nm in this case. With gradual increase in the concentration of lysine until 1 M, we notice a concomi- tant rise in fluorescence intensity around 420 nm. A broad spectrum devoid of any vibrational features is noticeable. This emission was significantly absent under identical conditions in glycine and a few other amino acids like L-arginine, L-serine, L- glutamate and L-isoleucine, which were randomly chosen (data not shown). Hence, it is apparent that the observed lumines- cence is unique to lysine alone. From the structure of lysine, it is difficult to account for the observed fluorescence since it lacks a conjugated system or an aromatic moiety. 7 The inset A in Figure 2 depicts the plot showing the varia- tion of the fluorescence intensity at 435 nm with lysine concen-