Electronic structure and dynamics of torsion-locked photoactive yellow protein chromophores

Abstract

The photocycle of photoactive yellow protein (PYP) begins with small-scale torsional motions of the chromophore leading to large-scale movements of the protein scaffold triggering a biological response. The role of single-bond torsional molecular motions of the chromophore in the initial steps of the PYP photocycle are not fully understood. Here, we employ anion photoelectron spectroscopy measurements and quantum chemistry calculations to investigate the electronic relaxation dynamics following photoexcitation of four model chromophores, para-coumaric acid, its methyl ester, and two analogues with aliphatic bridges hindering torsional motions around the single bonds adjacent to the alkene group. Following direct photoexcitation of S at 400 nm, we find that both single bond rotations play a role in steering the PYP chromophore through the S/S conical intersection but that rotation around the single bond between the alkene moiety and the phenoxide group is particularly important. Following photoexcitation of higher lying electronic states in the range 346-310 nm, we find that rotation around the single bond between the alkene and phenoxide groups also plays a key role in the electronic relaxation from higher lying states to the S state. These results have potential applications in tuning the photoresponse of photoactive proteins and materials with chromophores based on PYP.

Anion photoelectron spectroscopy of protein chromophores

“…The more straightforward approaches include DFT (TD-DFT) [54] and ADC(2) [55,56] methods. We have found that the simpler TD-DFT method tends to yield VEEs that are consistently around 0.5 eV higher than those determined using higher level methods [23,25,28,29,34,35], but that the ADC(2) method compares favourably with high-level calculations, for example, with SA-CASSCF(14,12)-PT2/cc-pVDZ calculations [57] for a series of PYP chromophore anions (Table 2) [29], and XMCQDPT2/aug-cc-pVTZ level calculations [31] for the GFP chromophore anion [33]. However, due to the number of diffuse functions required in the basis set to obtain an accurate VEE, a huge number of continuum states are calculated.…”

“…The resulting small droplets then evaporate further, eventually leading to the production of isolated gas-phase ions. ESI has been combined very successfully with photoelectron spectroscopy for studying the electronic structure of a wide range of deprotonated biomolecules, including protein chromophores [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. There are several techniques for measuring photoelectron spectra, the most popular of which are based on time-of-flight (TOF) or velocity map imaging (VMI) methods.…”

“…Taken from Ref. [29]. In GFP, the chromophore is anchored both covalently and via a hydrogen-bonded network to the protein that is wrapped around it in a β-barrel structure (Fig.…”

“…PYP is an ideal system for studying biological photoreceptors as a result of its small size (125 residues), stability and ease of crystallisation [86][87][88][89][90]. Numerous spectroscopic and computational chemistry studies of the electronic structure and relaxation dynamics of isolated model PYP chromophores in the gas phase have been aimed at understanding the role of the chromophore and its environment in determining the function of this important photoreceptor [25,28,29,34…”

Photoelectron Imaging and Quantum Chemistry Study of Phenolate, Difluorophenolate, and Dimethoxyphenolate Anions

“…Broadening of photoelectron spectra following photoexcitation of resonances in the continuum has been observed in similar sized molecular anions by many other groups 14,30,31 including our own. 11,[32][33][34][35][36][37][38][39][40][41][42] This resonance in PhO − has been observed before 12,14 and assigned to indirect detachment from the S 1 (ππ * ) state of PhO − . The S 0 -S 1 (ππ * ) transition corresponds to promotion of an electron from the HOMO shown in Table 1 to the first antibonding orbital on the phenyl ring system (the LUMO of PhO − ) and therefore has shape resonance character with respect to the D 0 continuum.…”