Gramicidin A and its complexes with Cs+ and T1(+) ions in organic solvents- A study by steady state and time resolved emission spectroscopy

Gramicidin A (gr A), a linear pentadecapeptide containing four trp residues has been studied using steady state and time resolved fluorescence (at 298 K) and phosphorescence (at 77 K) in methanol (CH3OH), ethanol (C2H5OH), di methyl sulfoxide (DMSO), 1,4-dioxane, 2-methyl tetrahydrofuran (2-MeTHF), e thanol/benzene (C2H5OH/C6H6) mixed solvent. Similar studies have also been carried out in CH3OH containing monovalent cations K+, Cs+. Tl+ and divalen t cation Ca2-. lambda (max) of fluorescence is found to be a good signature of the different forms having double helical structure [dh (1) to dh (4)] (J. Struct. Biol. 121 (1998) 123-141). Steady state and time resolved quenc hing studies of gr A by KI in CH3OH and DMSO and life time of the emitting singlet states of gr A support that gr A exists as a mixture of different f orms of double helical (dh) structure [dh (1) to dh (4)] in CH3OH and as a random coil structure in DMSO. This study further indicates that emitting t rp residue in DMSO is better shielded than that in CH3OH. Phosphorescence s pectra of gr A at 77 K in CH3OH glass suggests that gr A retains a particul ar conformation dh (3) in this matrix. The phosphorescence spectra of gr A [conformation db (4)] in 2-MeTHF at 77 K is further red shifted indicating that among all the dh forms, dh (4) has the emitting trp residue in most hy drophobic environment. The hydrophobicity of the emitting tryptophan enviro nment is thus found to be in the order: dh (1)<dh (3)<dh (4). Since 2-MeTHF forms a clear glass at low temperature, it is thus possible to study the s ide chain arrangement of gr A dh (4) as a function of temperature. The phos phorescence spectra in different alcohol glassy matrix are in conformity wi th the observation of different side chain arrangement of gr A as one chang es the polarity of alcohol. Steady state and time resolved quenching studie s of gr A using Cs+ ion in CH3OH at 298 K clearly demonstrate the two bindi ng sites for the metal ions and provide the value of equilibrium constant o f the 'non-emitting' complex of gr A with Cs+ ion in the ground state. The observation of distinct red shift of the (0,0) band of the phosphorescence spectra of the complexes of gr A with K+, Cs+ and Tl+ ions at 77 K compared to that in CH3OH glass confirms the metal ion induced change of conformati on in dh (3). The result also suggests that the emitting trp residues in th e complexes are in somewhat more hydrophobic environment compared with that in the free gr A in CH3OH glass. The tripler state Life time of these comp lexes indicate that the heavy metal ions Cs+ and Tl+ are within a Van der W aal's distance of emitting trp residue in gr A in CH3OH glass at 77 K so th at they are capable of inducing increased spin-orbit coupling due to a heav y atom effect. (C) 2001 Elsevier Science B.V. All rights reserved.