# A Highly Water-Soluble C60-Oligo-Lysine Conjugate as a Type I and Type II Photosensitizer with Enhanced ROS Generation and Photocytotoxicity

## TL;DR

Researchers developed a new water-soluble C60-peptide compound that generates more harmful reactive oxygen species for photodynamic cancer therapy.

## Contribution

A novel C60-oligo-Lys conjugate with enhanced ROS generation and photocytotoxicity for photodynamic therapy is introduced.

## Key findings

- C60-oligo-Lys generates more reactive oxygen species (ROS) than C60-PEG in aqueous solutions.
- C60-oligo-Lys shows significantly higher photocytotoxicity (IC50 < 1 μM) compared to C60-PEG (IC50 > 25 μM) in HeLa cells.
- The compound's improved performance is attributed to efficient type I and type II ROS generation and cell membrane affinity.

## Abstract

C60 has
been regarded as a suitable photosensitizer
for photodynamic therapy due to its excitation in the phototherapeutic
window (650–900 nm), high quantum yields of 1O2 generation, and low dark toxicity. However, the use of this
molecule in biomedical applications has been limited by its high aggregation
tendency in polar solvents (e.g., water), resulting in quenching of
its excited states. In this study, a C60-peptide conjugate,
C60-oligo-Lys, with a lower aggregation tendency was investigated
by chemical, physical, and photophysical methods in comparison to
a previously reported water-soluble C60-PEG conjugate.
Photoinduced 1O2 generation was evaluated by
both phosphorescence at 1274 nm and the electron spin resonance method
in an aqueous solution, with comparison to the control C60-PEG, revealing the superior capacity of the C60-oligo-Lys
conjugate. Importantly, the photoinduced type I electron transfer
reaction is occurring in C60-oligo-Lys very efficiently, even in the
absence of an e– donor, presumably due to the partially
unprotonated amines in the peptide, to form O2
•– and •OH, which are generated in a further enhanced
way by the addition of a physiological concentration of NADH. These
species are more harmful to the target cells, including hypoxic tissues
with limited oxygen concentration. Femtosecond transient absorption
spectroscopy revealed different excited state dynamics for C60-oligo-Lys and C60-PEG at short time scales in water.
By an in vitro cellular assay, significant cytotoxicity of C60-oligo-Lys was observed (IC50 < 1 μM) on HeLa
cells under visible light irradiation (527, 630, and 660 nm), while
very limited cytotoxicity was observed for C60-PEG (IC50 > 25 μM) under the same conditions. The strongly
enhanced
photocytotoxicity of C60-oligo-Lys can be ascribed to the
higher generation of both type I and type II ROS in addition to the
potential affinity of the positively charged oligo-Lys moiety for
the negatively charged cell membrane. The C60-oligo-Lys
conjugate reported in this study therefore shows high potential as
a core photosensitizer for photodynamic therapy.