The anti-AIDS drug rilpivirine undergoes conformational changes to bind HIV-1 reverse transcriptase and retain potency against drug-resistance mutations. Our discovery that water molecules play an essential role in the drug binding is reported. Femtosecond experiments and theory expose molecular level dynamics of rilpivirine bound to HIV-1 reverse transcriptase. The two nitrile substituents (-CN), one on each arm of the drug, have vibrational spectra consistent with their protein environments being similar in crystals and in solutions. Two-dimensional vibrational-echo spectroscopy reveals a dry environment for one nitrile while unexpectedly the other is hydrogen-bonded to a mobile water molecule, not identified in earlier X-ray structures. Ultrafast nitrile-water dynamics are confirmed by simulations. A higher (1.51 Å) resolution X-ray structure indeed reveals a water-drug interaction network. Maintenance of a crucial anchoring hydrogen bond, despite the enlargement and structural variation of the binding pocket, may help retain the potency of rilpivirine against the pocket mutations.

4-(Dimethylamino)benzonitrile (DMABN) has been one of the most studied photoinduced charge-transfer (CT) compounds for over 50 years, but due to the complexity of its excited electronic states and the importance of both intramolecular and solvent reorganization, the detailed microscopic mechanism of the CT is still debated. In this work, we have probed the ultrafast intramolecular CT process of DMABN in methanol using broad-band transient absorption spectroscopy from 280 to 620 nm and ultraviolet femtosecond stimulated Raman spectroscopy (FSRS) incorporating a 330 nm Raman pump pulse. Global analysis of the transient absorption kinetics revealed dynamics occurring with three distinct time constants: relaxation from the Franck-Condon L(a) state to the lower locally excited (LE) L(b) state in 0.3 ps, internal conversion in 2-2.4 ps that produces a vibrationally hot CT state, and vibrational relaxation within the CT state occurring in 6 ps. The 330 nm FSRS spectra established the dynamics along three vibrational coordinates: the ring-breathing stretch, ν(ph), at 764 cm(-1) in the CT state; the quinoidal C═C stretch, ν(CC), at 1582 cm(-1) in the CT state; and the nitrile stretch, ν(CN), at 2096 cm(-1) in the CT state. FSRS spectra collected with a 400 nm Raman pump probed the dynamics of the 1174 cm(-1) CH bending vibration, δ(CH). Spectral shifts of each of these modes occur on the 2-20 ps time scale and were analyzed in terms of the vibrational anharmonicity of the CT state, calculated using density functional theory. The frequencies of the δ(CH) and ν(CC) modes upshift with a 6-7 ps time constant, consistent with their off-diagonal anharmonic coupling to other modes that act as receiving modes during the CT process and then cool in 6-7 ps. It was found that the spectral down-shifts of the δ(CH) and ν(CN) modes are inconsistent with vibrational anharmonicity and are instead due to changes in molecular structure and hydrogen bonding that occur as the molecule relaxes within the CT state potential energy surface.

It is generally presumed that the vast majority of carriers created by chemical doping of semiconducting polymer films are coulombically trapped by the counteranion, with only a small fraction that are free and responsible for the increased conductivity essential for organic electronic applications.