Photophysical consequences of coupling bacteriochlorophyll a with serine and its resulting solubility in water

We investigated the dependence on solvents of optical absorption and emissi on of the bacteriochlorophyll a-serine (BChl-ser), a water soluble bacterio chlorophyll (BChl) derivative. Comparison between the experimental data and those collected for BChl in nonaqueous solvents shows that only a minor in teraction takes place between serine and the macrocycle's pi-electron syste m. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S-1, In buffered aqueous solution (pH = 7.4), the Stokes shift of the BChl-ser fluorescence and its nonradiative relaxation rate are enhanced compared wit h those in nonaqueous solutions (Scherz, A., S, Katz, Y, Vakrat, V, Brumfel d, E. Gabelmann, D, Leupold, J, R, Norris, H. Scheer and Y, Salomon (1998) Photosynthesis: Mechanisms and Effects, Vol, V (Edited by G, Garab), pp, 42 07-4212. Kluwer Academic, Dordrecht.), probably as a result of a hydrogen b onding between the BChl macrocycle and the water molecules. In aprotic solv ents, without hydrogen bonds, the permanent dipole moment of the first exci ted singlet state in both BChl and BChl-ser is increased compared with the ground state by at least 2.5 Debye.