The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions

Physical Chemistry Chemical Physics 17, 25793-25798 (2015)

The Soret absorption band of isolated chlorophyll a and b tagged with quaternary ammonium ions

Mark H. Stockett,Lihi Musbat,Christina Kjær,Jørgen Houmøller,Yoni Toker,Angel Rubio,Bruce F. Milne, Steen Brøndsted Nielsen

We have performed gas-phase absorption spectroscopy in the Soret-band region of chlorophyll (Chl) a and b tagged by quaternary ammonium ions together with time-dependent density functional theory (TD-DFT) calculations. This band is the strongest in the visible region of metalloporphyrins and an important reporter on the microenvironment. The cationic charge tags were tetramethylammonium, tetrabutylammonium, and acetylcholine, and the dominant dissociation channel in all cases was breakage of the complex to give neutral Chl and the charge tag as determined by photoinduced dissociation mass spectroscopy. Two photons were required to induce fragmentation on the time scale of the experiment (microseconds). Action spectra were recorded where the yield of the tag as a function of excitation wavelength was sampled. These spectra are taken to represent the corresponding absorption spectra. In the case of Chl a we find that the tag hardly influences the band maximum which for all three tags is at 403 5 nm. A smaller band with maximum at 365 10 nm was also measured for all three complexes. The spectral quality is worse in the case of Chl b due to lower ion beam currents; however, there is clear evidence for the absorption being to the red of that of Chl a (most intense peak at 409 5 nm) and also a more split band. Our results demonstrate that the change in the Soret-band spectrum when one peripheral substituent (CH3) is replaced by another (CHO) is an intrinsic effect. First principles TD-DFT calculations agree with our experiments, supporting the intrinsic nature of the difference between Chl a and b and also displaying minimal spectral changes when different charge tags are employed. The deviations between theory and experiment have allowed us to estimate that the Soret-band absorption maxima in vacuo for the neutral Chl a and Chl b should occur at 405 nm and 413 nm, respectively. Importantly, the Soret bands of the isolated species are significantly blueshifted compared to those of solvated Chl and Chl–proteins. The protein microenvironment is certainly not innocent of perturbing the electronic structure of Chls.

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http://dx.doi.org/10.1039/C5CP01513H
Notes
SBN and MHS acknowledge Lundbeckfonden and Villumfonden for support. BFM thanks the Donostia International Physics centre and the Centre de Fı´sica de Materiales, University of the Basque Country for financial support. BFM also thanks the Laboratory for Advanced Computing of the University of Coimbra, Portugal for the provision of computer resources, technical support and assistance.AR acknowledges financial support from the European Research Council Advanced Grant DYNamo (ERC-2010- AdG-267374), Spanish Grant (FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and European Community FP7 project CRONOS (Grant number 280879-2) and COST Actions CM1204 (XLIC) and MP1306 (EUSpec). LM and YT acknowledge Cost Action CM1204 (XLIC) for support (ECOST-STSM-CM1204-301114-047659).

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