Ibrahimi, F. | Search bioRxiv

bioRxiv2023

Bio-Heat Transfer and Monte Carlo Measurement of Near-Infrared Transcranial Stimulation of Human Brain

Transcranial photobiomodulation is an optical method for non-invasive brain stimulation. The method projects red and near-infrared light through the scalp within 600-1100 nm and low energy within the 1-20 J/cm2 range. Recent studies have been optimistic about replacing this method with pharmacotherapy and invasive brain stimulation. However, concerns and ambiguities exist regarding the light penetration depth and possible thermal side effects. While the literature survey indicates that the skin temperature rises after experimental optical brain stimulation, inadequate evidence supports a safe increase in temperature or the amount of light penetration in the cortex. Therefore, we aimed to conduct a comprehensive study on the heat transfer of near-infrared stimulation for the human brain. Our research considers the transcranial photobiomodulation over the human brain model by projecting 810 nm light with 100 mW/cm2 power density to evaluate its thermal and optical effects using bioheat transfer and radiative transfer equation. Our results confirm that the near-infrared light spectrum has a small incremental impact on temperature and approximately penetrates 1 cm, reaching the cortex. A time-variant study of the heat transfer was also carried out to measure the temperature changes during optical stimulation.

bioengineering↗

bioRxiv2021

Upstream charged and hydrophobic residues impact the timing of membrane insertion of transmembrane helices

During SecYEG-mediated cotranslational insertion of membrane proteins, transmembrane helices (TMHs) first make contact with the membrane when their N-terminal end is ~45 residues away from the peptidyl transferase center. However, we recently uncovered instances where the first contact is delayed by up to ~10 residues. Here, we recapitulate these effects using a model TMH fused to two short segments from the BtuC protein: a positively charged loop and a re-entrant loop. We show that the critical residues are two Arg residues in the positively charged loop and four hydrophobic residues in the re-entrant loop. Thus, both electrostatic and hydrophobic interactions involving sequence elements that are not part of a TMH can impact the way the latter behaves during membrane insertion.

biochemistry↗