RESUMO
We recently introduced a Dynamically Weighted Complete Active Space Self-Consistent Field (DW-CASSCF) electronic structure for excited-state dynamics. In this Communication, we reformulate analytical gradients at this level of theory using a Lagrangian approach, thereby reducing the required number of coupled-perturbed CASSCF calculations to one per state gradient. In addition, we derive and implement derivative couplings at the DW-CASSCF level for the first time. We demonstrate the new formulation of DW-CASSCF gradients by optimizing a conical intersection for the p-hydroxybenzylidene-imidazolinone anion, the green fluorescent protein chromophore, to shed light on its observed radiationless decay dynamics in the ultraviolet region.
RESUMO
Bacteriorhodopsin (bR) and halorhodopsin (hR) are both membrane proteins that transport ions across the cell membrane in halobacteria. Their ion transport function is triggered by photoactivated isomerization of the retinal protonated Schiff base (RPSB) chromophore. In spite of their similar structures, bR and hR exhibit widely differing RPSB isomerization rates and quantum yields (with bR being both faster and more efficient than hR). Previous simulations of photoisomerization in bR and hR using ab initio multiple spawning (AIMS) with QM/MM have successfully reproduced the experimentally observed ordering of quantum yields and isomerization rates, but the origin of these differences remains elusive. Here we investigate the role of electrostatic interactions in the protein pocket surrounding RPSB. We probe the influence of protein electrostatics by modifying the charge of the complex counterion in bR/hR to be more/less negative than the native state. We find that such modifications lead to bR-like behavior in hR and vice versa. This demonstrates the crucial role of electrostatic interactions in controlling the outcome of RPSB photoisomerization.
