Why is Al used in this phrase?

Question

Why is Al used at the beginning of this phrase? Al besar mis labios te dejaré frio esta noche.

Answer 1

Upon kissing my lips...

(literally) At the kissing of my lips....al + verb infinitive is often translated as upon ....ing.

al

Al followed by an infinitive is usually the approximate equivalent of "upon," "on" or "when" followed by the gerund (the "-ing" form of a verb) in English.

Answer 2

I agree with Quentin to a certain extent. However, in these cases, the infinitive is not a verb but a noun, so al besar is the same as a el besar, which must be in a contraction form, hence al. It´s the same as saying "al punto de besar......" (noun with adjective phrase).