A new synthetic protocol is introduced which preserves
the secondary
structure of protecting proteins encapsulating a luminescent atomically-precise
silver cluster. This was achieved by using a preformed triphenylphosphine
(TPP)-protected silver cluster as the precursor forming bovine serum
albumin (BSA)- and human serum albumin (HSA)-protected Ag18 clusters. This is the first example of the formation of luminescent
protein-protected clusters in a neutral medium, without using any
reducing agent, which results in minimal alteration of the protein
structure during cluster growth. The cluster formed showed exceptional
stability, unlike other silver clusters of this class. The formation
of these red luminescent clusters was visualized by UV–vis
and photoluminescence spectroscopy. The identification of Ag18 core was made through matrix-assisted laser desorption ionization
mass spectrometry (MALDI MS), and a plausible mechanism of the formation
was identified by monitoring the systematic growth of the cluster
core by time-dependent MALDI MS experiments and electrospray ionization
mass spectrometry (ESI MS) of the reaction mixture. The cluster was
successfully employed as a luminescent probe for cancer cell imaging.
Retention of protein conformation in the clusters was confirmed through
circular dichroism (CD) spectroscopy, and the same was reflected in
the retention of 89% of the esterase activity of BSA in the Ag18@BSA clusters synthesized by this method, compared to only
28.7% for AgQC@BSA clusters synthesized using previous
protocols, conducted in basic medium.
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MAPK/ERK-2–mediated phosphorylation regulates HIV-2/SIV Vpx interaction with human Nup153, which is critical for altering the nuclear membrane assembly for efficient transport of the lentiviral genome into the nucleus of nondividing target cells.
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