Q(y)-excitation resonance Raman spectra of chlorophyll a and related complexes. Normal mode characteristics of the low-frequency vibrations

Q(y)-excitation resonance Raman (RR) spectra are reported for film aggregates of chlorophyll (Chi) a and a series of related complexes. The latter include Mg(II) octaethylporphyrin (MgOEP), Mg(II) trans-octaethylchlorin (MgOEC), methyl-9-desoxomesopyrochlorophyllide (DMPChl) a, and methylpyrochlorophyllide (MPChl) a. These complexes represent a series in which the structural complexities of Chl a (saturated pyrrole ring, isocyclic ring, C-9-keto group, C-2-vinyl group, and C-10-carbomethoxy group) are systematically added to the basic tetrapyrrole architecture. On the basis of comparison of the RR scattering characteristics of the different complexes and the predictions of semiempirical normal coordinate calculations, a self-consistent set of vibrational assignments has been developed for all the RR active modes in the low-frequency regime (100-1000 cm(-1)). The studies indicate that the low-frequency Vibrations encompass a diverse set of motions that include both the ring skeleton and peripheral substituent groups. However, modes in the very low-frequency regime (<250 cm(-1)) are primarily due to deformations of the substituent groups. Collectively, the normal mode characteristics of Chi a and the other Mg(II) complexes provide insights into the nature of the vibrational modes that are coupled to the photophysically important, lowest energy excited states of natural photosynthetic assemblies.