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The importance of modified peptides and proteins for applications in drug discovery, and for illuminating biological processes at the molecular level, is fueling a demand for efficient methods that facilitate the precise modification of these biomolecules. Herein, we describe the development of a photocatalytic method for the rapid and efficient dimerization and site-specific functionalization of peptide and protein diselenides. This methodology, dubbed the photocatalytic diselenide contraction, involves irradiation at 450 nm in the presence of an iridium photocatalyst and a phosphine and results in rapid and clean conversion of diselenides to reductively stable selenoethers. A mechanism for this photocatalytic transformation is proposed, which is supported by photoluminescence spectroscopy and density functional theory calculations. The utility of the photocatalytic diselenide contraction transformation is highlighted through the dimerization of selenopeptides, and by the generation of two families of protein conjugates via the site-selective modification of calmodulin containing the 21^stamino acid selenocysteine, and the C-terminal modification of a ubiquitin diselenide.

 

In modal testing, a common excitation method is a transducer in mechanical contact with the object under test. However, for some structures it is desirable to excite vibrations without physical contact. One promising excitation technique is the acoustic radiation force. However, a challenge in using this technique is that the acoustic radiation force is spread out over a finite-diameter focal region. We describe a method to directly measure the spatial distribution of this force. An ultrasound transducer emitted sine waves with frequencies of, for example f1 = 600.610 kHz and f2 = 600 kHz; the resulting radiation force had a component at the difference frequency f1-f2 = 610 Hz. A MicroAcoustic Instruments BAT6 ultrasound transducer was focused to an approximately 2 mm diameter spot on a 19.6 by 8.1 by 0.37 mm clamped-free brass cantilever with a 610 Hz fundamental frequency. A vibrometer measured the response as this focus spot traversed the edge of the cantilever. This enabled determination of the distribution of the acoustic radiation force being delivered by the transducer. This may be helpful in future studies that involve modeling the force applied to a structure using the acoustic radiation force. 

Refracto-vibrometry is a technique that uses a laser Doppler vibrometer to measure acoustic pressure fields. The vibrometer laser is directed through a medium towards a stationary retroreflective surface. Acoustic waves (density variations) for which the wavefronts pass through the laser, as the beam travels from the vibrometer to the retroreflector and back, cause variations in the integrated optical path length. This results in a time-varying modulation of the laser signal returning to the vibrometer, enabling optical detection of the acoustic wavefronts. In the current experiment, a Polytec PSV-400 scanning laser Doppler vibrometer, sampled at 100 MHz, monitored the waves emitted by a 1 MHz Panametrics V303 ultrasound transducer immersed in a water tank. The time-varying signal detected by the vibrometer at numerous scan points was used to generate videos of the time evolution of acoustic wavefronts; these videos will be presented. Refracto-vibrometry was also used for optical measurements of the time of flight of ultrasonic waves through different materials, including samples of lead and fabricated bone. This enabled determination of wave propagation speeds. The wave speeds obtained with optical detection using refracto-vibrometry were in agreement with measurements using a conventional ultrasonic transducer to detect the wavefronts.